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International Microelectronics And Packaging Society

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The International Journal of Microcircuits and Electronic Packaging, Volume 25, Number 1, First Quarter, 2002 (ISSN 1063-1674)

Mold Compound Adhesion to Bare Copper LeadFrames – Effect of Laser TexturingJoseph Fauty, James Knapp, Jay Yoder

5005 East McDowell RoadPhoenix, Arizona 85008Phone: 602-244-5022Fax: 602-244-5714e-mail: [email protected]

Abstract

This paper investigates the effect substrate preparation has on epoxy mold compound (EMC)adhesion to bare copper leads. There have been four basic strategies employed to characterizeand subsequently improve adhesion between the EMC and copper:

1. Choosing a particular copper alloy, i.e. varying alloy effects in copper to improve oxidationcontrol and thus adhesion.

2. Modifying the mold compound chemistry.

3. Modifying the surface chemistry/ topography of the copper by metal plating, organic inhibitorpriming, vacuum deposition, ion implantation, UV cleaning, chemical oxidation, and various formsof mechanical roughening including sand or bead blasting.

4. Controlling the rate of oxidation and CuO/Cu2O ratio during product assembly.

The effects of mold compound chemistry, copper alloying, control of oxidation level and surfacetopography modification through various means such as laser texturing and Electrical DischargeMachining (EDM) are investigated. It will be shown that laser surface texturing offers one possiblelow cost solution to achieving significant improvement in adhesion by changing the surfacetopography of the copper substrate.


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Key words

Epoxy Mold Compound, Copper LeadFrames, Laser Surface Texturing, ElectricalDischarge Machining

1. Introduction

Within ON Semiconductor alloy 42 (Ni/Fe) leadframes have traditionally been used with Au-Sn eutectic die attach for high power discretedevices. There is currently underway an effortto migrate to larger die in smaller packages.To achieve the necessary thermal andelectrical responses copper is being exploredas an alternative lead frame material. Thereare however problems inherent to copperwhich mitigate its universal use in plasticencapsulated packages. One issue is copper’slarge thermal expansion coefficient whichprecludes the use of eutectic die attach withlarger die. Another issue is the adhesionstrength between the epoxy moldingcompound (EMC) and copper. Still anotherissue facing plastic encapsulated metal leadframes in general is resistance to moisture-induced damage especially from catastrophicmechanical failure during solder reflow (i.e. popcorn phenomena).

This paper will investigate the effect substratepreparation has on mold compound adhesionto bare copper substrates. The effects of moldcompound chemistry, copper alloying, controlof oxidation level and surface topographymodification through various means such aslaser texturing and electrical dischargemachining are investigated. It will be shownthat a significant improvement in adhesion canbe achieved by changing the surfacetopography of the copper substrate bymechanical means. It will also be shown laserablation can be a clean low cost solution forthis process.

2. Adhesion

Ensuring good package reliability with copperlead frames in adverse conditions necessitatesthe prevention of EMC/copper delaminationand subsequent package cracking [1]. Therehave been four basic strategies employed tocharacterize and subsequently improveadhesion between the EMC and copper:

1. Choosing a particular copper alloy, i.e.varying alloy effects in copper to improveoxidation control and thus adhesion.

2. Modifying the mold compound chemistry.

3. Modifying the surface chemistry/topography of the copper by metal plating,organic inhibitor priming, vacuum deposition,ion implantation, UV cleaning, chemicaloxidation, and various forms of mechanicalroughening including sand or bead blasting.

4. Controlling the rate of oxidation and CuO/Cu2O ratio during product assembly.

2.1. Alloy Effects

Choi et al [1] performed tests to select the bestcopper alloy and determine an empirical factorthey termed “adhesion index parameter” as agauge measurement tool. Their tests showedadhesion strength was affected by alloycomposition, oxide layer thickness, and CuO/Cu2O oxide ratio. The oxide ratio proved to bethe most important factor in their study. Copperhas a strong affinity for oxygen and will readilyform an oxide even at room temperature.Within the temperatures of interest (25-3000C)both cupric (CuO) and cuprous (Cu2O) oxidewill form. Three copper alloys were testedusing Cr-Zr-Zn, Ni-Si-Mg, and Ni-Si-Zn ashardening agents. After 40 minutes of heatingin air at 2200C the alloy containing Cr-Zr-Znexhibited the best adhesion strength. Theauthors matched the alloy type with oxidethickness and CuO/Cu2O ratios as a function


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of time at temperature and came to theconclusion that the Cr-Zr-Zn alloy controlledoxide growth to a much better degree than theother two alloys. Plotting adhesion strength asa function of the CuO/Cu2O ratio showed thatregardless of alloy content when the ratio wasbetween 0.2 and 0.3 the maximum adhesionstrength was achieved. The Cr-Zr-Zn alloy bysome mechanism maintained a ratio of 0.2-0.3 for at least 40 minutes at temperature whilethe other two alloys peaked after roughly 10minutes. Ohsuga et al [2] applied a moldcompound to various copper alloys andmeasured adhesion (expanded on in SectionIII). Their results indicated adhesion strengthwas achieved in the following order fromhighest to lowest: pure copper, Cu-NiSi, Cu-Fe, Cu-Cr, and Cu-Sn. They determined thatpure copper provided the best adhesion soalloying by some mechanism actually loweredadhesion strength.

In a recent paper Y. Tomioka and J. Miyake [3]investigated copper alloy dependence on oxidefilm adhesion. Tests indicated that maximumadhesion of the oxide film to the metal existedat a certain oxide thickness and the optimumfilm thickness varied with the type of alloychemistry employed and the heatingtemperature. SEM analysis of the peeledsurfaces indicated that those with highadhesion were rough and contained small pitswhile those with low adhesion were fairlysmooth. X-Ray diffraction of the high adhesionsamples found either no CuO or very low CuO/Cu2O ratios. High ratios were found for thosealloys that exhibited low adhesion strengths.The authors concluded that if CuO forms ontop of Cu2O that would cause internal stressesto develop in the film due to the difference inthe lattice structure of the two oxides. Theauthors also stated that the alloys with thelowest adhesion strength often contained Snas an alloying element. The speculation wasthat SnO might also be forming providing alow adhesion film.

2.2. Modifying Mold CompoundChemistry

Berriche et al [4] compared ortho-cresolnovolac (OCN) to dicyclopentadiene (DCP)chemistries for their adhesion strength to a Fe-Zn-P based copper alloy. Oxidationtemperatures were 175 and 2000C, with baketimes varying from 5 minutes to 118 hours.Results showed that DCP – Cu adhesion wasvirtually unaffected by oxidation at both 175and 2000C for exposure times up to 50minutes. OCN adhesion started out lower andexhibited a sharp decrease in adhesion as bothtime and temperature increased. The authorsattributed the better performance of DCP toits lower viscosity (less than half that of OCN),which allowed better wetting, a lower thermalexpansion coefficient and a possible role ofadhesion promoters in the chemistry. Whiletheir work was concerned with alloy 42 Asai etal [5] studied the effect of adding modifiers tothe mold compound and using various phenolresins as curing agents. The authors noted thatconventional epoxy molding compoundscontaining cresol novolac epoxy resin with aphenol novolac or cresol novolac curing agentwhile providing good electrical and mechanicalproperties suffered from poor adhesion. Theirpremise was that EMC compounds fromdifunctional epoxy resins with phenol resinsas curing agents, low modulus at solderingtemperatures, low water content at equilibriumand high adhesion strength developed fewcracks at solder temperatures. Compoundsmade from multifunctional epoxy resins withhigh cross-link densities and high waterabsorption gave poor results. Their suggestionwas to use a biphenyl epoxy resin because ofits low modulus and low moisture absorption.Ohsuga et al [2] in addition to looking at alloytypes also investigated the properties requiredin a mold compound to give good adhesion tocopper. They developed a high filler typechemistry using bi-phenyl epoxy resins andelastic hardeners for alloy 42 and tried it on


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copper. They determined that the propertiesof mold compounds required for goodadhesion consisted of low water absorption,low thermal expansion, low modulus and highflexural strength at high temperatures. Tadaand Fujioka [6] looked at modifying the glasstransition temperature (Tg) of moldingcompounds to improve adhesion. They notedthat a common practice to lower Tg bydecreasing the cross-linking density of thecompound lead to increased resistance topackage cracking but at the same timedecreased resistance to moisture loading.Tada and Fujioka determined that a low cross-linking density was necessary for resistanceto cracking but a reasonably high Tg wasnecessary to pass high temperature/ highhumidity testing. The authors introduced amethod to lower the crosslinking densitywithout affecting Tg by introducing rigidstructural elements by mixing a naphthalenestructure into the matrix resin. The results wereimpressive. Sauber et al. [7] and later Saitohet al. [8] used linear fracture mechanics toinvestigate mold compound properties andpackage geometry effects on delamination andcracking of EMC on both alloy 42 and copperlead frames. It was found for copper leadframes that an EMC with a small Young’smodulus and a specific coefficient of thermalexpansion (for the lead frame geometriesstudied – 12 ppm) was recommended forpreventing delamination between the bottomsurface of the die pad flag and the EMC. Theyalso discovered that within the range of dieflag thickness studied a thinner package wasless susceptible to delamination at the bottomsurface of the die flag independent of the sizeof the chip. In contrast once delamination startson the top surface of the die flag, lower valuesof CTE for the mold compound, thinnerpackages, and larger chips will enhance ratherthan alleviate delamination along the bottomsurface of the die flag.

2.3 Modifying Surface Chemistry/Topography

Chemical adhesion between two dissimilarmaterials is possible because of weakintermolecular forces known as Van der Waalsforces. The two primary attractive forcesoperating are London dispersion andHydrogen bonding forces. Both forces ofattraction are possible because of dipole -induced dipole or dipole - dipole interactions.Though hydrogen bonding is the strongest ofthe intermolecular forces dispersion bondingtends to dominate and it is much weaker thanan interatomic force such as ionic or covalent.Therefore to improve adhesion to the EMCeither the substrate surface chemistry must bemodified or a mechanical component added.Asai et al [5] looked at surface treatment forits effect on adhesion strength. They exposedalloy 42 lead frames to a vacuum depositionand rf sputtering at 13.52 Mhz to modify thesurface. Vacuum deposition was used tochange the surface to Al or SiO and rfsputtering to change the SiO into SiO2 or Si3N4.Layer thicknesses were measured in the100nm range. All surface treatments resultedin near zero delamination in the as-cured state.Lap shear tests at 2150C after moisture loadingshowed significant improvement over controlsamples. In two back-to-back papers Evansand Packham [9 and 10] investigated thecause of enhanced adhesion between apolyethylene polymer and a purposely-oxidized copper substrate. Previous studiesshowed that adhesion of polyethylene to metalsubstrates fell into two categories; onedependent upon polymer oxidation and theother independent of polymer oxidation butdependent on surface topography. Studies ofadhesion of polyethylene to other metals suchas iron/steel found good adhesion associatedwith the metal’s ability to oxidize the polymer.In a series of experiments the authors showed


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that in the case of polymer adhesion to copperoxide good adhesion was a consequence ofthe surface roughness of the oxide. Love andPackman [11] later investigated changes insurface chemistry on the peel strength ofcopper/polymer interfaces. After chemicallycleaning Oxygen Free High Conductivity(OFHC) copper foils they deliberately createdtwo different surface morphologies by chemicaletching and mechanical sanding. All foils werethen chemically oxidized. The chemicallyetched samples were further broken down intothree groups, one “as is” after oxidation, oneexposed to Cr+ ion implantation and the otherimmersed in a known copper complexingsolution of benzotriazole/ ethylene glycol. Peelstrength tests as a function of time attemperature (1500C) showed surfacemorphology making no difference at time zerobut diverging dramatically with time attemperature. The authors attributed thedifference to a mechanical locking effect of theetched units. In addition samples of the etchedmorphology exposed to ion implantationresulted in peel strengths less than that for the“as is” samples while an improvement wasnoted for the samples immersed in the primersolution. Although not mentioned as a causethe authors did note that ion implantationchanged the oxide from CuO to Cu2O.Improved adhesion with the primer wasattributed to a chemical interaction betweenCuO and the azole-type primer. S. Kim [12]looked at controlled chemical oxidation ofcopper. He noted that growing oxideschemically produced structures more uniformand “dendritic” then the globular morphologyof normally heat generated oxides. Whileadhesion strength was higher than for normalheat-treated samples it was still too low toresist delamination from the copper.

J. Kim et al [13] performed a study on theeffects of chemically dimpling the surface ofcopper lead frames with various metal plating

materials. Surface metal plating materialsstudied included bare C-194 copper,microetched copper for surface roughening,Ag, Au, Ni, Pd and CuO. The authors showedthat dimples etched into the lead frame surfaceincreased interface adhesion between the leadframe and epoxy molding compound. Forsome of the plating schemes in particular barecopper and microetched copper the adhesionstrength increased linearly with the number ofdimples. All dimples (dimple size being about8.0 mils in diameter by 3.0 mils deep) wereetched into the copper. The authors attributedthe increase in adhesion to mechanicallyinterlocking effect. They stressed that theshape of the dimple was critical for theinterlocking. Square or round form factors wereeffective while pyramid-shaped was not. It wasalso noted in the paper that others have hadpositive experimental results with adding holesto the lead frames in various locations.

2.4 Controlling Rate of Oxidation

Cho et al [14] investigated oxidation effectson a Ni-Si-Mg-P copper alloy. The moldcompound they used was a cresol novolacepoxy resin with 84 wt% SiO2 filler. Aftercleaning lead frames were exposed to 150,200 and 3000C in an air-circulating oven.Evidence of CuO was apparent even as lowas 10 minutes at 1500C. When plotted againstoxide thickness or time at temperature theaverage adhesion strength exhibited aphenomenon observed by many otherinvestigators; namely an increase in adhesionto a maximum point then a decrease as oxidethickness increased. The authors showed thatthe oxidation time to reach maximum adhesionbecame shorter as the temperature increasedbut plotting adhesion strength vs. oxidethickness showed pull strength was always afunction of the thickness and not the heattreatment temperature. The kinds of oxides


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formed and their layer structure were almostidentical in the temperature range 150-3000C.Maximum adhesion strength was alwaysobtained at an oxide thickness of 21-25 nm.The adhesion failure between the EMC andlead frame with an oxide film was caused by aweakness in the CuO morphology or theinstability of CuO due to its inherent brittlenessand density difference from Cu2O. Cu2O isknown to grow with a specific epitaxialrelationship with the copper lead frame surfaceat initial stages of oxidation while randomgrowth is favored for CuO. Therefore asoxidation continues the weak link becomes theCuO/Cu2O interface. The authors postulatedmicrovoid formation at the CuO/Cu2O interfaceas oxidation proceeded as the cause of thedecrease in adhesion strength with time attemperature. Two explanations were given forthe initial increase in adhesion. One was thatoxidation of the copper gave rise to changesin the surface chemistry and topography whichmay be beneficial to adhesion. A rougherlooking surface was noted as oxidation timeincreased in the early stages. The secondexplanation was surface wettability – contactangle measurements showed a decrease incontact angle during the early stages ofoxidation. Takano et al [15] investigated oxidefilm properties and the effect of the film onreliability performance of a packaged part. Theauthors noted that growth of the oxide film wassuppressed when the oxygen concentrationwas less than 5%. They also noted inagreement with Cho that adhesion strength tooxidized copper drastically decreased whenthe film thickness exceeded 20 nm (~200Å).The authors developed two equations for oxidegrowth that mimic both die attach and wirebond. Both equations involved heating in airso the difference in the two was related to theheat transfer mechanism; natural convectionfor die cure in an oven versus thermalconduction for a substrate sitting on a wirebond pedestal. The authors also stressed thatprecise control of oxide thickness was required

in order to avoid problems with adhesion tocopper. Chong et al [16] postulated thepresence of voids at the oxide/metal interfaceas the cause of poor adhesion and subsequentdelamination. The degree of voiding increasedas the degree of oxidation increased duringnormal assembly of product. They also foundthat post mold cure had a negative impact oninterfacial integrity of an oxidized surface. Theirpaper focused on oxidation as a result of wirebonding conditions. Oxide thickness and ratewas measured after various temperature andheating time combinations in open air.Oxidation of copper at higher wire bondtemperatures (2800C) showed very rapid initialrate then a reduction as time durationincreased. An oxide thickness of 250 nm waspossible in less than 200 seconds at 2800C.As the temperature was lowered through2000C the oxidation rate decreased until at2000C it was low enough to remain constantout to 5 minutes. CuO/Cu2O ratios weremeasured as a function of temperature. It wasfound that the ratio increased (i.e. CuO grows)as temperature increased. The Cu2O alwaysappeared first with CuO growing in intensityas temperature increased. All units tested hadno observable delamination since those withdelamination were excluded from the tests. Soeven with good wetting adhesion strengthsuffered as temperature and time increased.In fact lead frames heated at 2000C for 400seconds produced better adhesion results thanclean copper. J. Kim et al [13] deliberately grewa thick black copper oxide plating (most likelyCuO) which they reported as being at leasttwice as thick as the ideal thickness reportedby others. Their results indicated excellentadhesion of this thick oxide in apparentcontradiction of previous reported results.

There are issues with all the above-describedmethods for improving adhesion. Seeking outspecific copper alloys limits one in the choiceof metals to use plus results in added cost forspecial formulations. Modifying EMC chemistry


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has the same limitations, namely narrow rangeof choice and added cost for the addition ofspecial adhesion promoters. The third methodfor improving adhesion is arguably the bestchoice – i.e. modifying the lead frame surfaceeither chemically or mechanically. However,almost all methods described add extraprocessing and almost certainly involve addedcleaning steps before the lead frame is suitablefor use. The last method, controlling theoxidation of the copper surface also has goodresults but suffers from extreme difficulty inmaintaining sufficient control in the variousassembly processes that use heat. All themethods described add costly processing and/or do not lend themselves to easyimplementation on the manufacturing floor.What is needed is a simple cost effectiveprocess that can easily be implemented intothe assembly process. One purpose of thispaper is to describe a very simple cost effectivemethod of providing enhanced EMC/copperadhesion strengths using basically any copperalloy and low cost mold compound chemistry.This method enlists the use of laser ablationto cleanly texture the lead frame surface priorto molding. Implementation of a laser ablationsystem onto the assembly floor is as easy assetting up a traditional laser-marking machine.As shown below the ablation process is fastenough for production volumes, needs no postprocess cleaning operations and as long asthe standard precautions are taken for anytemperature processing steps the ablationprocess can be employed at any point in theassembly flow prior to mold though the closerto molding it is the higher the adhesion strengthachieved.

3. Objective of Research Work

As mentioned earlier Au-Sn eutectic die attachis a standard process for Alloy 42 lead frames.With the switch to copper Au-Sn becomes

untenable when die sizes increase not muchabove 0.030 inches. For larger die sizes epoxydie attach becomes necessary. Nishimura etal [17] compared alloy 42 with copper leadframes and determined differences in packagecracking mechanisms existed under conditionsof temperature cycling. In alloy 42 crackingusually occurred at the interface between theEMC and the bottom of the die bond flag(metal) due to the large thermal mismatchbetween the two (5 ppm vs. 20 ppm). In thecase of copper cracking was found to occuron the top of the die flag along the sides. Sincecopper has a thermal expansion coefficientclose to the EMC (17 ppm vs. 20 ppm) themold compound is not the issue; rather the useof compliant adhesives to relieve thermalstress on the die becomes the issue. Thehypothesis was that the adhesive allowed thedie flag to slide under the silicon die (17 ppmvs. 3 ppm) during temperature excursionsallowing large tensile stresses to build up atthe edge of the die flag. Due to relatively pooradhesion strength of the EMC to copperdelamination occurs which leads to cracks.The authors stressed the fact that if theadhesion of the EMC to copper were ofsufficient strength the stress developed wouldbe so small cracking would not likely occur.Takano et al [15] determined that copper leadframe package cracking during solder reflowwas due poor adhesion of the mold compoundto the copper. In addition to this the advent oflead free soldering means exposure to highertemperatures (2600C) during surface mount.All this adds up to increased demands on thequality of the adhesive strength between themold compound and lead frame.

As mentioned previously an objective of thisinvestigation is to describe the laser texturingprocess and through experimental resultsshow it to be a simple and robust method forimproving EMC adhesion to copper. By wayof contrast laser ablation will be compared to


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Electrical Discharge Machining (EDM), anothermethod investigated for mechanicalroughening. While EDM significantly improvedadhesion it suffered from the following:

1. The need to expose lead frames to eitherwater or oil during the process.

2. A tendency to leave carbon deposits onthe lead frame surface.

3. Difficulty in implementing a large bulky andexpensive machine in a productionmanufacturing flow.

4. A post machining cleaning step is required.

To confirm viability of laser texturing the abilityof packages to withstand moisture loading(850C/85%RH for 168 hours followed by solderreflow) was used as the reliability gate tomeasure the success of this process.

4. Sample Preparation / Test Setup

4.1. Materials

Copper Alloys

A previous study carried out by Ohsuga et al[2] determined mold compound adhesion tocopper varied with the type of alloy used. Foralloys receiving no heat treatment prior to moldin order starting with the highest adhesion waspure copper, Cu-NiSi, Cu-Fe, Cu-Cr, and thenCu-Sn. When subjected to heat treatment(2000C for 40 minutes) the order changed toCu-Fe, Cu-NiSi, Cu-Cr, pure Cu and then Cu-Sn. Their hypothesis was that alloying affectedadhesion in two ways:

1. Pure copper provided the best adhesionso alloying by some unknown mechanismlowers adhesion strength.

2. Alloying controls the oxidation process insome manner so that at a specific time attemperature some copper alloys act betterthan others.

They measured adhesion strength as afunction of oxide thickness and showed asothers have that adhesion increased at first,went through a maximum then decreased withfurther oxidation. However in contrast to Cho[14] and others they found that maximumadhesion was reached at different thicknessfor each alloy type. Good adhesion wasevidenced by separation of the moldcompound from the oxide while poor adhesionwas evidenced by separation of the oxide fromthe copper.

Three different copper alloys were chosen forthis study in order to determine whetheralloying had a significant impact on adhesionstrength of a laser prepared surface. Thechemical makeup of each alloy along withindustry advertised mechanical properties aregiven in Table 1.

C110 copper is essentially pure with amaximum impurity level of 0.05 wt% oxygen.Alloys C151 and 194 use a dispersionstrengthening mechanism to reach higherstrength levels. In dispersion strengtheningatoms of the alloying species form smallparticles in the pure copper matrix. It is theseparticles that directly impact the strength of thealloy. In C151 a fine dispersion of CuZr3particles form within the copper matrix whilein C194 small particles of iron andphosphorous form within the matrix. Becauseof their strength electrical and thermalconductivity both alloys find wide use inintegrated circuit lead frame applications.

Samples of each copper alloy pull tab (seeSection D for pull tab details) were submittedfor tensile testing without any mold compoundto derive a baseline for subsequent mechanical


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Designation Chemical Make Up (wt%) Hardness

Tensile Strength (kgf/mm2)

Yield Strength (kgf/mm2)

Elong (%)

C110

Electrolytic Tough Pitch (ETP)

Pure copper with .05 wt% O2 max.

Full hard (89 RF) 30-37 32 9

C151 99.9 Cu, 0.05-0.15 Zr, Al/Mn/Fe 0.005

max.

Half hard (37 RB) 30-36 29 16

C194

97.0 Cu min., 2.1-2.6 Fe, 0.05-0.20

Zn, 0.03 Pb, 0.015-0.150 P

Half hard (59RB) 37-44 32 17

Table 1. Copper Lead Frame Alloy

Designation

Stress at Max

Load - Cold

(kgf/mm2

)

Max Load at Break -

Cold (kgf)

Stress at Max Load

- PMC

(kgf/mm2)

Max Load at Break -

PMC (kgf)

Stress at Max Load

– 260C Reflow

(kgf/mm2)

Max Load at Break – 260C Reflow

(kgf)

C110 26.7 + 0.8

50.8 + 0.2

26.3 +0.1

50.0 +0.2

22.4 +0.1

42.5 + 0.3

C151 27.4 + 0.1

52.0 + 0.1

27.3 +0.2

51.8 +0.4

23.4 +0.4

44.4 +0.4

C194 40.2 + 0.4

76.3 + 0.7

40.2 +0.1

76.4 +0.1

38.1 +0.1

72.4 +0.1

Table 2. Measured Data


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testing. Machine set up is discussed in anothersection of this paper. Samples of each alloywere tensile tested as is (cold) without anyprocessing, after a simulated post mold cureof 175C for 3 hours and again after a simulatedsolder reflow at 260C (maximum furnacetemperature set at 360C). The results are givenin Table 2. Comparing this data to that givenin Table 1 alloy C194 cold stress at maximumload measured roughly in the middle of industryadvertised range while both C110 and C151measured lower than the advertised range.While post mold cure did not appear to affectmechanical strength there was a noted dropin value after exposure to solder reflowtemperatures.

4.2 Mold Compound Chemistry

A crucial part of this experiment was to provethat no special mold compound chemistry isneeded to enhance adhesion when lasertexturing is used. With this in mind threedifferent mold chemistries were chosen torepresent standard inexpensive compounds inwidespread use in the industry versus arelatively expensive compound specificallyformulated for high adhesion strength tocopper. A fourth compound was added lateron to access the true nature of the interfacestrength between copper and mold compound.Properties of the compounds chosen for thisstudy are given in Table 3.

Mold compound A represents a standard lowcost compound that is used on almost 80% ofthe product manufactured in ONSemiconductor. This compound has previouslybeen shown not to adhere too well to copperafter temperature excursions. The compounddesignated as B represents ‘green’ chemistryspecifically formulated for adhesion to copperand rated as MSL level 1; therefore fairlyexpensive. Compound C represents a low coststandard mold resin chemistry that has been

cited as having good adhesion to copper butnot rated as MSL 1. Compound D was chosenbecause it develops a lower than usual bondstrength to copper and tends to degrade rapidlyonce past the supplier’s recommended postmold cure time.

5. Description of Laser System

A Lumonics Light Writer SPe Nd-YAG laserwith a 0.060 inch aperture was the principallaser system used. It has an output power of50 watts with a maximum 40 amp powersupply, a wavelength of 1064 nanometers anda pulse frequency that is selectable fromcontinuous wave (CW) to 64 kHz.

6. Description of Electrical DischargeMachining (EDM)

The EDM process depends upon an electricaldischarge between two electrodes to producea plasma field that basically erodes bothelectrode surfaces via a strictly controlledcavitation-erosion type mechanism. Erosion issymmetrical and depends on many factorsincluding polarity, electrical discharge intensityand duration, thermal conductivity, electricalproperties of the intervening dielectric materialetc. An EDM machine can use either a wire ora solid metal piece called a “sinker” electrode.Wire and sinker electrode material will varydepending on the particular process, materialmakeup of the work piece etc. Typical materialsused for both include brass, tungsten, gold,beryllium, or copper to name a few. The wireor sinker electrode forms one of the twoelectrodes in the circuit. The work piece (thepart to be machined) forms the secondelectrode. An electric field develops betweenthe machine electrode and work piece. This


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Resin Type

Cure Agent

Tg (0C)

CTE [1]

Flexural Modulus

[2]

Flexural Strength

[3]

Water Absorption

[4]

A Epoxide Cresol Novolac Anhydride 190 24 155

124 ~0.50

B Biphenyl Proprietary 130 12 190 170 0.15

C Dicyclo-Pentadienyl / Biphenyl

Phenol-Novolac 150 8 270 170 0.12

D Epoxide Cresol Novolac

Phenol-Novolac 160 15.6 117 114 ~0.30

Note 1: x10-6 (ppm) Note 2: x 102 N/mm2 Note 3: N/mm2 Note 4: % in boiling water

Table 3. Mold Compound Properties

field accelerates free positive ions andelectrons to high speeds forming an ionizedconductive channel that rapidly reaches veryhigh temperatures (8,000 to 12,0000C). Thehigh temperature causes localized melting ofmaterial from both the work piece andelectrode and also causes gas bubbles to form.When the current is turned off the sudden dropin temperature causes the bubbles to implodeforcing previously molten material away fromthe electrode surfaces. The molten materialresolidifies in the dielectric and is washedaway. Wire EDM usually uses water as thedielectric while sinker electrode EDM uses oil.The particular machine used for this analysiswas a Charmilles Technologies model with asinker electrode and oil dielectric. The sinkermaterial used was graphite.

7. Tensile Pull Sample / InstronMachine Setup

Figure 1 shows a photo of a tensile strengthtest sample once it is molded and separatedinto individual units. The pull tab is specificallydesigned so that only adhesion of the moldcompound to the tab is tested. No mechanicalinterlocks are present to confound the results.An Instron Model 5566 test system with a 10kNload cell was used to measure the strength ofthe copper-encapsulant interface. One end ofthe package was held rigid while an externalforce was applied the other end. All tests werecarried out at room temperature with acrosshead speed of 4.0 mm/min. The machinewas set up to automatically record load as afunction of time. Maximum values of load atfailure were automatically recorded from theload versus time curves.


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Figure 1. Photo showing a tensile strength test sample once it is molded andseparated into individual units. The encased end of the pull tab is

shown in Figure 5.

8. Level of Oxidation

A small experiment was conducted to baselinethe level of oxidation. No attempt was madeto directly measure the thickness of the oxidefilm. Oxidation levels were scaled accordingto the amount of time the test units wereexposed to temperature. Heating was carriedout on a hot plate in air to simulate wirebonding. Test units were used ‘as is’ from thesupplier. No special effort was made to cleanthe test strips prior to the experiment. As partof the fabrication process all panels arecleaned in a dilute muratic acid bath followedby an alcohol rinse and forced air drying. Allpanels are wrapped in corrosion resistantpaper and stored in a nitrogen dry box prior touse. In this experiment all molding wasperformed using mold compound ”C”. BareC194 type copper strips were heated on a

hotplate set at 200 and 2500C for times rangingbetween 0 and 20 minutes then immediatelymolded. As part of the mold process the stripswere exposed to 170-1750C for an additional1-2 minutes prior to actual transfer of moldcompound. After mold the units were subjectedto post mold cure at 1750C for 3 hours in anitrogen atmosphere according tomanufacturers specifications. Even thoughPMC will have an effect on oxidation it wasconsidered necessary since normal productionenvironment uses PMC. In addition it has beenproven necessary for novolac cured moldcompounds in order to raise cross-link densityand Tg to an acceptable value to withstandfurther processing with the application of heat.Figure 2 shows the results of tensile strengthas a function of oxidation at 2000C. Threegroups are shown in the graph. One group with


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0 5 10 15 2020

40

60

80

Oxidation Time (min)

Adh

esio

n St

reng

th (K

g)

No Laser TreatLaser then OxidizeOxidize then Laser

Figure 2. Effects of oxidation level measured as exposure time at 2000C onadhesion strength as a function of laser treatment using C194 copper and moldcompound C. As expected lead frames laser treated after oxidation showed the

better adhesion then those laser treated before oxidation. For comparisonpurposes untreated copper controls are also shown. Data points represent the

mean value of at least five measurements.

no laser treatment was used as a control whilethe other copper strips were exposed to laserablation under two sets of conditions. Onegroup was oxidized and then laser treatedwhile the other group was laser treated thenoxidized.

Various authors have established that moldcompound adhesion initially increases withoxidation to a maximum and thereafterdecreases to a minimum value. There does

not seem to be a clear increase in strength asa function of exposure time though there is asmall spike in the data with all three conditionsin the 1.5 to 2.0 minute range. Laser treatingafter oxidation appears to be the bestalternative followed by laser treating beforeoxidation then no laser treatment. No lasertreatment shows the most dramatic drop off inadhesion strength. Laser treating afterexposure does show a drop off but then a fairlyflat response after 4-5 minutes exposure. The

Adhesion Strength as a Function of Oxidation


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0 5 10 15 20Oxidation Time (min)

20

40

60

80Pu

ll St

reng

th (K

g)

250 °C200 °C

Figure 3. Effects of oxidation level measured as exposure time at both 2000C and2500C on adhesion strength as a function of oxidation temperature for untreated

C194 copper pull tabs molded with compound C. As expected there is a rapiddrop off in adhesion with exposure at 2500C. A maximum was detected at about 30

seconds exposure time. Data points represent the mean value of at least fivemeasurements.

Adhesion Strength as a Function of Oxidation(2000C vs. 2500C)

drop off in adhesion strength may be due theinherent weakness of the oxide in the non-lasered portions overtaking the added strengthgiven by the laser pits acting as micro moldlocks. The laser pits may also not be deepenough but we were at the power limits of ourlaser equipment. Figure 3 compares data for

untreated C194 at both 200 and 2500C. In thecase of exposure to 2500C an initial increasein adhesion strength was noted at the 30-second readout. The subsequent drop instrength was much more dramatic than thatfor 2000C.


65

9. Experiment / Results

9.1. Surface Texturing

Initial Studies – EDM versus LaserTexturing

Which method of surface preparation yieldsthe best results with respect to adhesion? In aseries of experiments various methods tomodify the topography of the copper surfacewere explored. Among those tried werechemical treatments, plasma treatments withnitrogen, bead blasting, EDM, and lasertexturing. No significant results were recordedfor the chemical and plasma treatments so nofurther data will be reported in this paper. Ofthe remaining groups EDM and laser texturedunits formed the experimental groups while aset of untreated parts formed the control group.In addition to the control units bead blastedunits were used as a measure of the bestprocess for mold adhesion. Mold compoundC (Dicyclo-Pentadienyl / Biphenyl) and C194copper were used for all samples. Figure 4shows the results of the first round of tensilestrength tests performed. Six different levelsof EDM process parameters were investigatedalong with five levels of laser treatments. Alsoincluded are the bead blasted and controlgroups. A statistical analysis performed on thedata indicated all but EDM groups 4, 5, and 6to have statistically higher tensile strengthsthan the control group.

Figures 5, 6, and 7 show photos of the surfacesof the EDM and laser treated groups. Not allof the EDM test cells are shown. Figure 5Ashows the lightest while 5B shows the heaviestmachine parts. All other groups were inprogression from light to heavy texture.

An interesting observation was made for bothEDM and laser treated groups. The roughestsurfaces yielded the lowest tensile strengths

in each group. This would seem to indicate anupper limit for surface modification. Failuremode played an important part in the recordeddata. The bead blasted and laser treatedgroups with the exception of laser group 2Ahad very tight distributions compared to theother test groups. This was because almostall failures in the test groups were due tocopper breaks and not adhesion failures norplastic breaks. The standard deviations inthese cases represented the spread in coppertensile strength. Data pooling was performedon all test groups. EDM groups 1, 2, and 3were not statistically different from each otherso were pooled into one group. The same wasdone for laser treated groups 2B-2E. The bestprocess settings for each group were thencompared to each other along with the beadblasted and control groups. The results areshown in Figure 8. A Tukey-Kramer analysison the resultant groups indicated the beadblasted and laser treated groups werestatistically the same while the EDM andcontrol groups formed their own samplepopulations with statistically significant lowertensile strengths. The means and standarddeviations for each test group are listed inTable 4.

The experimental data seems to indicate a veryrobust process using laser texturing. Basedon these analysis it was decided to concentratefurther effort in characterizing the laserprocess.

9.2. Copper Alloy Effect

Pull tabs representing the three differentcopper alloys were molded with the Dicyclo-Pentadienyl / Biphenyl material (compound C).The pull tabs were organized into four groups.Half were molded as received and half moldedafter being exposed to 2000C for five minutes.Those two groups were further divided intounits that were post mold cured at 1750C for 3


66

Adhesion Strength as a Function of Various Surface TreatmentsTe

nsile

Stre

ngth

40000

45000

50000

55000

60000

65000

70000

75000

80000

Bead BlastedControl

EDM 1 EDM 2 EDM 3 EDM 4 EDM 5 EDM 6 Laser2A Laser2B Laser2C Laser2D Laser2E

Description

Control groupaverage value

Tabl

e St

reng

ht

Description

Figure 4. Box plot of showing the results of three different methods for surfacetexturing copper for improved mold compound adhesion. Compared against acontrol group in which no texturing was performed are samples that have been

bead blasted, exposed to sinker etched Electrical Discharge Machining (EDM 1-6)and those exposed to various levels of laser ablation (Laser 2A-2E). The beadblasted units were used for comparison purposes since this process has beenshown to give superior results in the past. The tight grouping (small standard

deviations) for the bead blasted and laser treated groups was due to the fact thatthe failure mode was copper breaks, therefore the only differences between the

groups is the scatter in copper tensile strength.


67

Figure 5a Figure 5b Figure 5c

Figure 5. Photos showing surface texture effects for test samples exposed tosinker etch EDM. Figure 5a is representative of group EDM 1, 5b of group EDM 6,and 5c shows a typical unetched control unit. From Figure 4 it is seen that EDMgroup 1 had better tensile strengths than EDM 6. Close ups of both etched parts

are shown in Figure 6.

Figure 6a Figure 6b

Figure 6. High magnification pictures of surface texture for samples shown inFigures 5a and b. Sample units with the surface topography of 6a had much

higher adhesion strengths than those with the surface texture of 6b.


68

Laser Group 2A Laser Group 2B Laser Group 2C

Laser Group 2D Laser Group 2E

Figure 7. Photos showing surface topography of test units from laser texturedgroups Laser 2A through 2E respectively. As with the sinker-etched parts it

appears that the less textured units possessed the higher tensile strengths. Ofthe five test cells only Laser group 2A showed lower tensile strengths. The other

four groups possessed high strengths with almost 100% copper breaks. Allphotos are the same magnification. The basic difference in surface topography is

row and column spacing for the lasered pits.

Level Number Mean Std Dev Bead Blasted 16 73794.2 574.80 EDM 123 24 69010.3 2828.04 GroupCTL 16 59213.2 7889.52 Laser2BCDE 20 73420.7 240.68

Table 4. Comparison of Processes


69

Results of Data Pooling of Adhesion Strength for Laser Treatment and EDM

Tens

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Bead Blasted EDM 123 GroupCTL Laser2BCDE

Description

All PairsTukey-Kramer 0.05

Tens

ile S

treng

th

Description

Figure 8. The best process settings for EDM and laser textured units werecompared to each other along with untreated control and bead blasted units. EDMgroups 1, 2, and 3 were not statistically different from each other so were pooled

into one group. The same was done for laser groups 2B-2E. A Tukey-Krameranalysis on the resultant groups showed that the bead blasted and laser textured

groups were statistically the same while the EDM and Control groups formedseparate populations with statistically significant lower tensile strengths.

hours and those that received no PMC. Twoquestions were to be answered in thisexperiment – does the copper alloy itself affectadhesion strength and do the alloys oxidize atrates different enough to affect adhesion.Figure 9 shows adhesion strength as a functionof copper alloy type in the as-molded state andafter post mold cure at 1750C for 3 hours.

The as-molded adhesion strength dataappeared to be alloy dependent; C194 copperclearly had the highest adhesion strengthhowever metal yielding limited the adhesionstrength of C110 and C151. All C110 samples

failed via metal break before the compound/metal adhesion could be tested. Alsoconsiderable metal strain was noted for theC151 alloy units before final failure at the moldcompound/metal interface and so may haveconfounded the results. After post mold cureall samples failed at the mold compound/metalinterface. The data seems to indicate that ifany difference in strength between the alloysexisted as-molded it disappeared once thesamples were exposed to post mold cure. Thedrop in strength for C194 is real and significanthowever no conclusions can be made withregards to C110 or C151 because of the metal


70

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4044485256606468727680

C110 C151 C194

Alloy Type

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8076726864605652484440

Adhe

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Figure 9. Adhesion strength as a function of copper alloy type as-molded andafter post mold cure at 1750C for 3 hours. As-molded adhesion strength appearsto be alloy dependent however in reality all C110 samples failed via metal breakbefore the compound/metal adhesion could be tested. Also considerable metal

strain was noted for the C151 alloy units before final failure at the moldcompound/metal interface and so may have confounded the results. The C194samples experienced true adhesion failure before and after PMC so the loss inadhesion is relevant. It is interesting to note post mold cured data indicates no

real difference between any of the alloys.

strain. For post mold cured samples theinherent weakness in adhesion strengthapparently makes the alloy type basicallyirrelevant. Figure 10 shows what happenswhen test units are exposed to 2000C for 5minutes on a hotplate in air. As-moldedstrength does not seem to differ from thosegroups not exposed to oxidation.

A Tukey-Kramer analysis performed on eachalloy type in the as-molded condition with andwithout oxidation indicated no differenceswithin each pair. This was expected with C110and C151 due to metal and not compoundadhesion failure but the result was also truefor C194, which experienced actual adhesionfailures. Post mold cure did however bring outsome real differences. Both the C110 and

C194 alloys showed significant drops inadhesion strength when compared to the as-molded condition. Only the C151 alloy unitsappeared to remained unchanged, howeverthere is no way of knowing whether metal strainaffected the adhesion strength in the as-molded condition and so how high the actualstrength may have been. In this experiment itappears that C151 (Zr alloy) reacted the bestfollowed by C110 (pure Cu) and then C194(Fe-P alloy).

The post mold cure data was analyzed todetermine the effects of oxidation without metalyielding confounding the results. Analysis ofVariance indicated both main variables alloytype and oxidation along with their interactionwas relevant. If post mold cured data with no


71

As-Molded Adhesion StrengthOxidized 5 Minutes at 2000C

Adhe

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(Kgs

)

101520253035404550556065

C110 C151 C194

Alloy Type

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gs)

101520253035404550556065

C110 C151 C194

Alloy Type

Post Mold Cured Adhesion StrengthOxidized 5 Minutes at 2000C

Alloy Type Alloy Type

Adhe

sion

Str

engt

h (K

gs)

Adh

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n St

reng

th (K

gs)

Figure 10. Box plots of adhesion strength as a function of copper alloy type inboth the as molded and post mold cured states. When exposed to 2000C for 5

minutes on a hotplate in air the as-molded strength does not seem to differ fromthat not oxidized. A Tukey-Kramer analysis performed on each alloy type in theas-molded condition with and without oxidation indicated no differences within

each pair. This was expected with C110 and C151 due to metal and not compoundadhesion failure but the result was also true for C194, which experienced actualadhesion failures. Post mold cure did however bring out some differences. Both

C110 and C194 alloys showed significant drops in adhesion strength whencompared to the as-molded condition. Only the C151 alloy units remained

unchanged, however there is no way of knowing whether metal strain affected theadhesion strength in the as-molded condition and so how high the actual

strength may have been.

oxidation is compared to that with oxidation(compare Figure 9 to Figure 10) it is apparentthat alloy type controls the rate of oxidationand therefore adhesion strength. Both C110and C194 experienced a loss of adhesionstrength in the post mold cured state whileC151 changed very little. Since all failures wereadhesion related these results are real. Thisseems to indicate that C151 is more robustwith respect to resistance to the effects ofoxidation.

9.3. Effect of Laser Treating

Does laser ablation mitigate the effects of PMCand copper alloy chemistry? To answer thisquestion another experiment was performed.Pull tabs representing the three copper alloyswere once again molded with the Dicyclo-Pentadienyl / Biphenyl material (compound C).The pull tabs were organized into four groups.Half were laser treated then molded while theother half were molded as is to act as a control


72

group and confirmation run for the previousexperiment. The groups were subdivided intounits that were post mold cured at 1750C for 3hours and those that were not. The as moldedand post mold cured data for the bare coppertabs tracked the data from the first experiment.An Analysis of Variance (ANOVA) of all datapoints indicated all three main variables(copper alloy, PMC and laser treatment) weresignificant. Two interaction terms were alsocited as significant but the inherent softnessof both the C110 and C151 alloys wereaffecting the results making the interactioneffects suspect. An important observation wasnoted. For all three alloys the failure mode forlaser treated units with no PMC was 100%metal fracture including C194 samples. Metalfracture with C194 was a new occurrence.

Concentrating on the laser treated data onlyyielded different results. Adhesion strengthbecame independent of PMC. Only the copperalloy mattered. Accepting the propositioninherent softness of C110 and C151 wasconfounding the results an analysis of theC194 data by itself was performed. Post moldcure became relevant again. The data (Figure11) showed laser treatment improved theoverall adhesion to C194, PMC tended todegrade it and the interaction between the twohad no affect.

With laser treatment adhesion remained highregardless of PMC while significant loss wasnoted for units without laser treatment. Theremay very well be a larger rate of decrease inadhesion for the laser treated units since it wedo not really know how high the adhesionstrength could be with those units receivingno PMC. The data seems to indicate lasertreatment will increase adhesion quality ofmold compound to copper. Failure mode forall copper alloys laser treated and not exposedto PMC was 100% metal fracture. This meansthe adhesion strength was never tested sincethe copper failed first.

In an effort to make adhesion independent ofcopper yield strength and mold compoundfracture strength and so access the true natureof the adhesion interface a fourth moldcompound chemistry (compound D) was used.This compound (an epoxidized cresol novolacwith a phenol novolac curing agent) was knownfrom past experiments to have lower as-molded adhesion strength and to degrade veryrapidly when exposed to post mold cure formore than 30 minutes. This particularcompound is designed to reach maximumcross-linking with a PMC of 15 minutes at1750C and therefore not meant to go muchbeyond 20 minutes at temperature. C194copper tabs both laser treated and as-is weremolded then exposed to PMC at 1750 in anitrogen atmosphere for up to three hours. Inthis experiment all units failed via adhesion tothe copper. Figure 12 shows the results fromadhesion testing of the parts. The graphdepicts a least squares linear fit to the meanof the results at each test level. A statisticalanalysis of the data indicated both lasertreatment and PMC were significant variableshowever the interaction effect was not. Withlaser treatment the adhesion strength is higherat every PMC level but post mold cure willalways degrade strength; in effect lasertreatment improved the overall adhesion at alllevels of PMC time. The rate of decreaseappears to be the same for both laser treatedand bare copper. Therefore laser treatmentdoes improve adhesion quality. It also appearslaser treatment helps mitigate the effects ofPMC but will not make adhesion strengthindependent of it.

10. Mold Compound Chemistry

Due to the addition of adhesion promoters andproperties of specific resin chemistries somemold compounds adhere to copper better thanothers [4-6]. An experiment was designed to


73

Adh

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Mea

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No

Yes

No Yes

Post Mold Cure

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No Yes

Laser TreatLaser TreatAdhe

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LS Means Plot Laser Treatment

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No Yes

Post Mold CurePost Mold CureAdhe

sion

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Mea

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LS Means Plot PMC

Post Mold CureAdhe

sion

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engt

h LS

Mea

ns

LS Means Plot PMC- Laser

Post Mold Cure Data Pooled Laser Treated Datapooled Laser Treatment vs. PMC

Adhesion Strength as a Function of Laser Treatment and Post Mold Cure

Laser

Laser

Figure 11. Least Squares Means plots showing the effects of laser treatment andPMC on the adhesion of mold compound C to C194 copper alloy tabs. Laser

treatment improves the overall adhesion of C194. PMC tends to degrade adhesionof C194. There is no interaction between PMC and laser treat. With laser treatment

adhesion remains high regardless of PMC while significant loss was noted forunits with no laser treatment. There may still be a larger slope (rate of decrease)

for the test cell laser/no PMC since it is not known how high the adhesion may befor the test group laser treated with no PMC (100% metal fracture).

determine if laser ablation could reduce thedependency on mold compound specificchemistries. Three compounds were chosento represent resin chemistries containingepoxide cresol novolac, biphenyl, and dicyclo-pentadienyl biphenyl (compounds A-Crespectively). According to reference [5] thebiphenyl resin based compound shouldachieve the best results. Copper C194 pull tabswere organized into 12 groups. Sample tabsboth laser treated and untreated were moldedwith each compound. These six groups werefurther divided into those which received threehours of post mold cure at 1750C in nitrogenand those that did not. Figure 13 shows theresults for all three compounds. Adhesionquality for the biphenyl resin (compound B)was virtually independent of both lasertreatment and PMC. Compound B representsnew resin chemistry specially formulated for

adhesion to copper and shows excellentresults with or without PMC – a rarity for moldcompounds in general. Since the purpose ofthe experiment was to show compounds withless desirable adhesion quality but also lessexpensive could work just as well, compoundB results were excluded and the data analyzedonce more. All three main effects (lasertreatment, compound chemistry and PMC)became significant, however no interactioneffects were. As shown in Figure 13 thedicyclo-pentadienyl biphenyl compoundtended to have higher adhesion strengths thanthe epoxide cresol novolac compound at alltest levels while PMC tended to degradeadhesion. At all test levels laser treatedsamples has consistently higher adhesionstrengths than untreated units. The superiorquality of the laser treated units was borne outby an analysis of the failure modes. Thirty


74

0 1 2 3min(X) - 0.5 X max(X) - 0.5

PMC Time (hours)Bare Metal DataLeast-squares fitLaser DataLeast-squares fit

20

40

60

Adh

esio

n St

reng

th (k

g)

Figure 12. Adhesion strength to C194 copper of an epoxidized cresol novolaccompound with a phenol novolac curing agent (compound D) as a function of

post mold cure at 1750C. The graph shows a least squares linear fit to the mean ofthe data at each test level. As can be laser treatment improved the overall

adhesion at all levels of PMC time. The rate of decrease appears to be the samefor both laser treated and bare copper. In this experiment all units failed via

adhesion to the copper.

percent of the tested samples failed by metalfracture when laser treated compared to 7%when left bare. Forty-one percent of the bareunits failed for adhesion to the copper

compared to 8% of the laser treated parts.Laser treatment clearly enhances the qualityof adhesion to copper for compounds notspecifically design to do so.

Adhesion Strength as a Function of Post MoldCure Time


75

-1 0 1 2 345

52

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PMC (Hours)Compound A (Laser)Compound A (no Laser)

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PMC (Hours)Compound B (Laser)

Compound B (no Laser)

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PMC (Hours)Compound C (Laser)

Compound C (no Laser)

Adhe

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Figure 13. Adhesion strength of compounds A, B, and C versus PMC with andwithout laser treatment. Compound B is the biphenyl resin compound that was

specifically formulated for adhesion to copper and has been shown in other workto form a better quality bond than either a cresol novolac or a dicylco-pentadienylbased resin system. An important point to note here is that with laser treatmentall three compounds have approximately the same as-molded adhesion strengthand with post mold cure the loss in the cresol novolac and dicylco-pentadienyl

resin based systems is less than without laser treatment.

Adhesion Strength as a Function of Mold Compound Chemistry andLaser Treatment

Compound A Compound B Compound C

11. Moisture Loading

In order to confirm the viability of laser texturinga group of C194 copper pull tabs both lasertreated and as-is were molded withcompounds C and D. C194 copper waschosen so that the copper alloy itself wouldnot confound the experiment after exposureto temperature. Compound D was specificallychosen because of its poor adhesionperformance compared to the other compoundformulations. The purpose of the experimentwas to determine if laser treatment couldimprove the adhesion quality to the point thatany standard low cost mold compound would

become a viable candidate for adhesion tocopper. After molding and the appropriate postmold cure the four groups were exposed tomoisture loading at 850C / 85% RH for 0, 48and 168 hours. They were then submitted forsimulated solder reflow in a forming gaspurged furnace. The units were reflowed threetimes at a peak temperature of 2300C thensubmitted for tensile testing. The resultant datais shown in Figure 14. Mean values ofadhesion strength are plotted as a function ofmoisture loading for each compoundformulation. Statistical analysis of the dataindicated that both main variables lasertreatment and moisture loading were


76

10

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-10 28 66 104 142 180Moisture Time (hr)

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-10 28 66 104 142 180Moisture Time (hr)

Adhe

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Laser treatedBare

Adhesion Strength as a Function of MoistureLoading and Laser Treatment

Compound C Compound D

Figure 14. Mean values of adhesion strength as a function of moisture loading formold compounds C and D. Samples of both mold compounds were exposed to85% RH / 850C for 0, 48, and 168 hours then reflowed in an inert gas atmosphere

furnace at a peak temperature of 2300C three times. Laser treatment created asignificant improvement in both compounds to the extent that laser treated unitsexposed to 168 hours of moisture then reflowed had higher adhesion levels than

those not exposed to moisture (but still reflowed).

significant while the interaction effect wasmixed. Compound C had no interaction effectwhile compound D showed a strong effect. Thismay indicate a reliance on mold compoundchemistry but more compound formulationswould have to be tested to prove this. Someimportant points can be noted from the plotsin Figure 14. Laser treatment represents adefinite improvement in adhesion quality. Upto the limit of the test at 168 hours laser treatedparts outperformed un-treated units even thoseexposed to no moisture. For compound D lasertreatment almost made adhesion independentof moisture loading and reflow within the testedrange.

12. Alloy Recommendation

Because of the inherent softness of C110 thechoice is really between C151 and C194. Whiletemperature induced softening of C151 limitedits use for some experiments in thisinvestigation an opinion can still be made. AlloyC194 consistently shows the highest adhesionstrength in the as-molded state but usuallyexperiences a large drop after post mold cure.In contrast the adhesion strength for C151changed very little as a function of both PMCand exposure to oxidation (2000 for 5 minutes).The post mold cure adhesion strength for C151


77

seems to be every bit as good as that for C194.Taking the effects of oxidation into accountC151 appears to be more robust than C194 inthe post mold cured state. While adhesionstrengths for both C151 and C194 are roughlythe same absolute value the C194 losses alarge portion of its strength after post moldcure. This loss of strength may be reflected inthermal mismatches that could cause otherissues such as delamination and cracking. Ifthe data for C151 is accurate the delaminationand cracking issues may be reduced. Thelimited softening of the alloy during post moldcure may actually be a benefit for stress reliefbetween the metal and mold compound.

While adhesion properties of C151 aresuperior to those of C194, softening of the alloymay cause issues for wire bonding after hightemperature reflow for thinner substrates (<0.010 inches) or substrate designs with longslender bond fingers. Lower limits for tensilestrength or hardness may have to be specifiedso the alloy does not soften appreciably duringsolder die attach or PMC.

13. Does Laser Treatment Mitigate theUse of Specially Formulated Expensive Compounds?

Laser treatment definitely improves the qualityof adhesion to copper but does it do so to theextent that any low cost formulation can beused? The basic answer to this question wouldbe qualified yes but is clearly dependent onthe application and quality level one is tryingto achieve. There are a lot of factors that mustbe considered when choosing an appropriatecompound. Copper alloy, lead frame design(single sided QFN’s versus double sidedPDIPs) which governs the amount of copperavailable for laser texturing, the quality levelone is trying to achieve and compound costare a few. Most quality test failures for MSL

Level 1 are for delamination of the moldcompound from the die flag or leads. Lasertreatment can cure this and allow the use ofless expensive compound formulations butperformance after PMC and moisture testingmust be accessed.

14. Conclusions

Laser texturing has been proven to be a veryrobust process yielding significantly higheradhesion strengths for all epoxy moldcompounds tested.

Laser texturing improved adhesion quality ofall compound chemistries tested but somechemistries such as the novolac basedcompounds may be too weak to withstandsome quality requirements even with the aidof laser ablation. Whenever possible biphenylor dicyclo-pentadiene resin chemistry shouldbe chosen over a novolac based one for betteradhesion to copper. With the aid of lasertexturing a specially formulated therefore highcost compound may not be necessary forsome applications.

References cited in this paper establishedmold compound chemistry affected adhesionto copper in the following order from highestto lowest – biphenyl, dicyclo-pentadiene andcresol-novolac. Our studies confirmed this.Correlating as-molded and PMC adhesionstrength to mechanical properties of thevarious mold compounds seems to suggestthe combination of low modulus, waterabsorption and CTE gives the best results (i.e.biphenyl compound). The two cresol-novolacsfaired the worst.

Laser treatment helps mitigate the effects ofPMC but will not make adhesion independentof it.


78

Laser texturing improved the performance ofall three copper alloys however the inherentsoftness of C110 limits it use for electronicapplications. While both C151 and C194 willwork C151 seems to offer a better solution thanC194. C194 appears to interact adversely withoxidation. Of the three alloys only C151retained its adhesion quality after exposure to2000 C for 5 minutes in air.

Lower limits for tensile strength or hardnessfor C151 may have to be specified so the alloydoes not soften appreciably during solder dieattach or PMC.

Laser texturing can be performed at the startof assembly or just before mold, however theoptimal place to insert is just prior to mold tooffset any uncontrolled oxidation effects.

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[4] R. Berriche, S. Vahey, et al, “Effect ofOxidation on Mold Compound – Copper Leadframe Adhesion”, 1999 InternationalSymposium on Advanced Packaging Materials, p.77-82.

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[9] J. Evans and D. Packham, “Adhesion ofPolyethylene to Copper: Reactions betweenCopper Oxides and the Polymer”, Journal ofAdhesion, Vol. 9, 1978, p. 267-277.

[10] J. Evans and D. Packham, “Adhesion ofPolyethylene to Copper: Importance ofSubstrate Topography”, Journal of Adhesion, Vol. 10, 1979, p. 39-47.

[11] B. Love and P. Packman, “Effects ofSurface Modifications on the Peel Strength ofCopper Based Polymer/Metal Interfaces withCharacteristic Morphologies”, Journal ofAdhesion, Vol. 40, 1993, p.139-150.


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[11] S. Kim, “The Role of Plastic PackageAdhesion in Performance”, IEEE Transactionson Components, Hybrids, and ManufacturingTechnology, Vol. 14, No. 4, Dec. 1991, p. 809-817.

[12] J-K Kim, et al., “Interface AdhesionBetween Copper Lead Frame and EpoxyMolding Compound: Effects of Surface Finish,Oxidation and Dimples”, 50th ElectronicComponents and Technology Conference, 2000, p.601-608.

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Biographies

Joseph Fauty received his M.S. degree inMaterials Science from the State University ofNew York at Stony Brook in 1975. He is aSenior Principal Staff Engineer with ONSemiconductor working in the PackageTechnology Development Lab. Mr. Fauty has25 years experience in hybrid and MCMprocess technology. He is a member of IMAPSand IEEE.

Jay Yoder is a Manufacturing Processtechnician with ON Semiconductor’s CoreTechnologies Packaging Lab. He has received2600 hours of advanced Electronics trainingwhile serving in the US Navy and is currentlypursuing a degree in Electro-MechanicalAutomation. Mr. Yoder has approximately 11years of experience in the SemiconductorManufacturing Industry. His currentresponsibilities include processcharacterization and optimization for newproduct development, tooling design andequipment maintenance for related back /front-end manufacturing.

James Knapp is the Package TechnologyDevelopment Lab manager. Mr. Knapp is anindustry recognized expert in plasticencapsulation with over 20 patents in the field.Mr. Knapp’s primary focus for the last year hasbeen power QFN packaging and packagewithin a package concepts.

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