Lead-free Solder Implementation for Automotive Electronics

Gordon Whitten Delphi Delco Electronics Systems

1 Corporate Center, Kokomo, IN 46901 gordon.c.whitten @delphiauto.com

Abstract Lead-free solders for electronics have been actively

pursued since the early 1990's here and abroad for environmental, legislative, and competitive reasons. The National Center for Manufacturing Sciences (NCMS-US)', the International Tin Research Institute (ITRI-UK)', Swedish Institute of Production Engineering Research ( IvF-S~eden)~ , Japan Institute of Electronics Packaging (JIEP -Japan)4, Improved Design Life and Environmentally Aware Manufacture of Electronics Assemblies by Lead-free Soldering ( I D E A L S - E U ~ O ~ ~ ) ~ , and, more recently, the National Electronics Manufacturing Initiative (NEMI-US)6 have been aggressively seeking lead-free solutions.

The automotive environment is one of the toughest since it includes chemicals both solvents and salt spray, wide temperature ranges, vibration, and humidity. A change in alloy requires extensive testing and durable materials.

Tests of PWB surface finishes, Tg, and thickness will be described as they relate to Lead-free solder implementation. Requirements for boards and components will also be discussed.

Introduction Delphi Delco Electronics Systems has been working on

Lead-free solder since 1993. At that time there was great concern that a rapid move away from SnPb eutectic solder would have grave consequences for the automotive industry. All along, the intent was to methodically select a reliable lead- free alloy, characterize the process window to ensure a capable process, and work with suppliers who can provide reliable parts with lead-free surface finishes. The automotive environment is one of the most difficult for electronics. Vibration is present continuously, with wide thermal excursions, many solvents, salt spray, and high current. The mission life is also quite long and increasing. While short- lived consumer products may be able to convert quickly, there is real concern that converting away from SnPb eutectic will be extremely risky. For that reason, Delphi Delco Electronics Systems has been, working aggressively to develop and validate Lead-free Solder.

NCMS Project Delphi Delco Electronics Systems experience began in

1993 with participation in the National Center for Manufacturing Sciences Lead Free Solder Effort. That $10 million dollar effort which extended over several years, identified several candidate alloys, but also identified several significant problems. It has been described in detail elsewhere so only the major aspects will be reviewed here.'

Figure 1 NCMS Downselection

Figure 2 Bathtub Curve

An initial list of 79 possible alloys was down-selected to 7 for in-depth reliability testing. The reliability tests explored the rising portion of the classical bathtub failure curve for the alloys under condideration.

The 3-Parameter Weibull Function describes solder wearout failure very well in this region as shown in the accompanying fit.

where CDF(t) is the Cumulative Device Failures normalized to 1, t is the time, sometimes expressed in cycles, p is the shape parameter which measures the width of the transition region, y is the first failure time, and q is the weibull life, the time where 1-l/e or about 63% of the devices have failed. It's important to note that a 2-parameter weibull function does not fit solder wearout properly since it assumes a useful life of 0, i.e, y = 0.

0-7803-5908-9/00/$10.00 02000 IEEE 1410 2000 Electronic Components and Technology Conference

..., C p r r . ~ J ~ + l ; o c : o l M . ~ I M i l h l

Figure 3 Three Parameter Weibull Fit to Data

There were three different test vehicles used by the NCMS team. The Solderability Test Vehicle (STV) was used to assess manufacturability early in the program. The Surface Mount Reliability Test Vehicle(SMRTV) and Through Hole Reliability Test Vehicles(RTV-TH) were used later in the program. The alloys were tested at both 0 to lOOC with a 30 minute thermal cycle, and at -55 to +125C with a 72 minutes cycle.

SMRTV - Each 11 x 17" board contained: 6 - 132 Pin BQFP 6 - 84 pin PLCC (J-Lead)

60 - 1206 Chip Resistors (Alumina) 60 - 1206 Chip Capacitors (Barium Titanate) There were five boards tested for each alloy, so that there

were 60 LCCC per alloy, 300 resistors/alloy, etc. Like devices were daisy-chained together with by-pass pads that allowed a failed device to be bypassed thereby enabling further testing of the remaining devices.

The LCCC's were the first to fail as expected. The significant difference in CTE between the organic laminate and alumina chip package results in great stress on the solder joints.

12 - 44 LCCC

The only other significant failure in SMT testing was 1206 resistors which failed well beyond 1000 cycles of -55 to +125C testing as shown below. The dark vertical bars indicate the end of testing.

aom ?ma

Figure 5 - 1206 Resistors on FR4 RTV-TH - Since there were fewer parts available for the

through hole test vehicle, the results were less conclusive. There were several important conclusions from the NCMS

work. There was not a 'drop-in' replacement for SnPb eutectic solder. Bismuth and Lead together yielded a very wide pasty range which exacerbated what was referred to as 'fillet lifting'.

Figure 6 'Fillet Lifting"

'Fillet Lifting' was really a misnomer. As the module cooled, the alloy solidified. As the module cooled further the organic laminate with it's much greater CTE pulled away from the fillet. This was not caused by a lack of wetting or lack of adhesion as shown in the next figure which is an enlargement of the previous figure. The intermetallic layer can be clearly seen on the lower pad layer which indicates that the solder did wet the surface.

Figure 4 - LCCC on FR4 for Two Different Thermal Profiles

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melting points, this introduces significant stress on through hole joints during solidification and cool-down.

I I75C

Figure 7 Fillet Lift Showing Intermetallic on

While reliability data was encouraging, there was a significant amount of work that remained to be done. The alloys of interest, in general, had higher melting points and would therefore increase thermal stress on components some of which were already operating on the edge with present SnPb reflow profiles.

Delphi Delco Electronics Systems Assembly Experience with Lead-Free Solder

Electric Vehicle PCM - During that same period, an Electric Vehicle Power Control module was hand assembled at Delphi Delco Electronics Systems using two Lead-free alloys. The assembled PCMs were fully functional, though they were not subjected to reliability testing.

Audio Receiver Board - In 1997, 200 Lead-free audio receiver boards were assembled using Lead-free solder, an existing product, and normal manufacturing tools. With normal SnPb product used as controls, 96 of these Lead-free modules were subjected to typical Product Validation Testing.

Lower Pad *

I I Figure 8 Lead-Free Audio Receiver Board The fillet lifting problem described earlier, played a major

role in the experimental design of the Audio Receiver Board build. From earlier work, the Coefficient of Thermal Expansion (CTE) of the FR4 board was known to increase significantly above the glass transition temperature, Tg as shown in Figure 9. The slope of the curve corresponds to the CTE of the material. The X and Y CTEs are roughly the same, but the Z axis CTE increases significantly above the Tg as shown in the figure. With low Tg material, and elevated

Table 1 CTE of Typical Assembly Materials

Material

FR4QY) 16 to 22 C Tg 5 to 11 FR4(Z) 53 t o 79 Tp 303 to 425 Comer

SnPb Solder

-11

Figure 9 Variation of CTE(s1ope) with Temperature above Tg for X,Y and Z Direction in FR4(from

ThermaYMechanical Analyzer)

The higher melting point of the lead-free alloys used in the assembly resulted in the alloy solidifying significantly above the T, for the most commonly used FR4 (T, - 125C). Because of this, there is significant stress on the solder joint during cool-down. Factors impacting the stress include board thickness, pad diameter, z-axis CTE for the FR4 board, through pin diameter, and modulus for the materials.

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170C 3 Board Surface Finishes

OSP / Copper OrganoSilver A m i

The good news from the assembly and subsequent testing was that there was no apparent fillet lifting observed. In fact the Lead-free assemblies out performed the associated SnPb assemblies as shown below. Figure 11 is typical of normal SnPb eutectic solder subjected to thermal shock. The second is a

Figure 11 Cracked SnPb Joint from Thermal Shock

Figure 12 Lead-free Solder Through Joint subjected to

Validation test results of the audio receiver boards were positive. There were two functional failures observed for the 96 units. Both of these were component failures. In one case a capacitor was cracked by torsional handling of the thinner than normal boards. In the other case, a semiconductor part failed. No failures were related to Lead-free solder joints.

A m i had the best wetting with Organo-Silver close behind. As one might expect at the higher reflow temperatures, OSP/Cu had the poorest wettability. There were no differences in reliability

Keyfob - In 1998 there was interest in Europe at our Texton subsidiary in a lead-free keyfob. The keyfob is the

Thermal Shock

small hand-held security device which utilizes RF to open automotive door locks. A number of production units were built for test purposes using Lead-free solder.

Figure 13 Lead-free Keyfob

With the keyfob build, we discovered that in general the reflow profile used to form the solder joints of the SMT components was too cool. Upon examination of earlier NCMS cross-sections the same effect is observed.

Figure 14 SOIC with Cool Reflow

In the preceding figure voids can be seen which were created by the flux as a result of a cool reflow. After several iterations, the reflow profile temperature was increased and voiding was minimized.

Once again the product passed all validation testing. Later Development Work - There is increased concern

about the impact the higher reflow temperatures will have on components. LEDs in particular are quite temperature sensitive with internal delamination and subsequent failure.

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Some PLCCs have also been observed to delaminate because of entrapped moisture. This well known popcorn effect will be aggravated by the higher reflow temperatures and steeper ramm.

Figure 15 Silicon Die Delamination at the Paddle Interface

Figure 16 Enlarged View at the Dieaddle Interface

Clearly, the effort required to requalify components for higher reflow temperatures will be an enormous task. The Lead-free component finish problem pales by comparison.

NEMI Leadfree Effort and Reliability Plan Beginning in late 1999, the National Electronics

Manufacturing Initiative (NEMI) has facilitated an extensive Lead-free Solder Task Force. The Pbfree Reliability Team is a sub-group of that Task Force. The Objective of the Reliability Sub-Group is to identify the reliability impact of conversion to Pb-free Solders.

The task group will accomplish the objective by: 1 . Identifying current tests used to determine reliability 2. Developing information on the relative performance of

the alloys suggested by the Alloy Sub-Group along with the components including PWBs suggested by the Component Sub-Group. This may be accomplished by:

Literature Review

3.

4.

5 .

Internal Testing External Testing

Identifying new tests that may be required by the new processing and materials. Identifying industry reliability standards that may be impacted by Pbfree processing. Proposing changes for those standards identified above, to the main Lead-free group.

The current plan encompasses several different tests: Electromigration and Dendritic Growth - One of the

concerns in migrating to a new solder alloy is the introduction of a new failure mechanism. Silver and Tin are known dendritic growth elements that are present in the new alloys. The IPC B25 test coupon will be used by alloy suppliers to extensively test solder pastes containing the proposed new Tin, Silver, Copper ternary eutectic alloy.

Thermal Shock - Thermal shock testing has been defined as a thermal ramp exceeding 20C/minute. Typically the testing involves a dual chamber oven with two different thermal reservoirs, one at the high temperature and one at the low temperature of the thermal cycle. Parts are moved mechanically between the two chambers with a transition time of minutes. As a result of the dynamic nature of the transition, the temperatures of board and components may vary widely during the transition.

Thermal Cycling - Thermal cycle testing is typically done in a single chamber which is heated and cooled in a very tightly controlled manner with a typical thermal cycle as shown in Figure 1.

In contrast to Thermal Shock testing, thermal cycle testing is characterized by a thermal ramp of less than 15C/min with 10CImin typical. The thermal profile is controlled by heating or cooling the air which is rapidly circulated through the modules mounted in the chamber. While the air temperature is tightly controlled, the profile of the actual modules in the chamber is dependent on thermal load, so the profile often has to be modified in order to assure that the profile experienced by the modules is close to that shown below.

n m

Figure 17 Desired Thermal Profile

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With this process, the board and component temperatures are closer together than with the thermal shock process, though they still vary somewhat.

Its important to note that the thermal profile agreed to by the NEMI reliability team is not adequate for underhood or on-engine applications in the automotive environment. Typical thermal profiles there are significantly warmer approaching 160C in some cases.

Bend Testing - Handling of boards during manufacturing or during field use can lead to failure because of board flexure. This is particularly true with newer attachment methods such as BGA, CSP, and Flip Chip attachment. The rigid semiconductor structure will not flex with the organic laminate board. This can lead to solder joint failure. Testing of this mode of failure is important, since the weakness of the solder joint contributes directly to this failure mechanism.

Three and four point bend tests have been designed which explore the curvature of the board that will lead to solder joint failure.

High Temperature Soak - The purpose of high temperature soak testing is to determine whether or not intermetallic growth at the solder/substrate or solder/device boundary will be a problem. Often, at elevated temperatures, solid state migration can take place. As a result, more mobile elements will migrate rapidly leaving voids behind which can weaken the joint. This test is not planned in the current round of NEMI testing because of resource limitations.

Vibration Testing - Vibration testing is normally used with system level testing. In this case, the module is rigidly mounted to a platform which is vibrated with a prescribed frequency and amplitude spectrum. It is very important in the automotive industry where there are abundant acoustical sources capable of generating resonant modes in an electronic module. Well designed modules are expected to pass vibration testing without incident. In the military and avionics world, the parts are often tested to failure by increasing the g forces until failure occurs. There is some interest in vibration testing coupled with high temperature soak though I know of no current standard test for this. This test is not planned in the current round of NEMI testing because of resource limitations.

Conclusions The electronics industry is moving aggressively towards

lead-free solder implementation. While legislation was the early driver, market pressures are beginning, and they may be more important ultimately. The increase in market share in the European market experienced by a Japanese AudioCD player demonstrates that the market may be moving to environmentally friendly products.

Reflow soldering will probably be implemented first since it requires only a change in paste. A major hindrance at this time is the availability of components qualified at the higher temperatures required for the new SnAgCu alloy. Ultimately, lead-free component finishes will be required for a truly lead- free assembly.

Wave soldering has been less fully characterized, so it is expected that wave solder products will follow somewhat later.

r-

@ C O L O R The CD-ROM version of this paper contains

color, to assist you in interpretation. http://www.cpmt.org/proceedings/order.htd

The NEMI testing which is currently underway will provide information on newer packages that were not available at the time of the original NCMS tests. A major part of the NEMI effort will focus on updating the IC package moisture standards to reflect the higher reflow temperatures.

Opinions expressed here are solely those of the author. No approval by Delphi Delco Electronics is expressed or implied.

Acknowledgments The author gratefully acknowledges the assistance and

support given by the Lead-free Reliability Team of the National Electronics Manufacturing Initiative and the earlier Lead-Free Solder Project of the National Center for Manufacturing Sciences. The author also acknowledges significant contributions from the Lead-free team at Delphi Delco Electronics Systems. Special thanks to Pascal Bezier, Pam Sneller, Del Walls, and Matt Walsh who provided significant input for this paper.

References

National Center for Manufacturing Science, 3025 Boardwalk, Ann Arbor, MI 48108-3266

ITRI Ltd, Kingston Lane, Uxbridge, Middlesex, U38 3PJ, UK, www.itri.co.uk/index.htm

Institutet for Verkstadsteknisk, Forskning, Argongatan 30, SE-431 53 Molndal, Sweden, www.ivf.se

Japan Institute of Electronic Packaging, 3- 12-2 Nlshiogikita, Suginami-ku, Tokyo 167-0042, Japan, www.jiep.or.jp

BriteEuRam 111, Project Number BE95-1994, D.M. Jacopson and M.R. Harrison, GEC J Research, 14(2), 1997, www.cordis.lu/brite-euram/src/1994.htm

National Electronics Manufacturing Initiative, 2214 Rock Hill Road, Suite 110, Herndon, VA 20170-4214, www.nemi.org

0401RE96, National Center for Manufacturing Sciences, 3025 Boardwalk, Ann Arbor, Michigan 48108-3266

Lead-Free Solder Project Final Report, NCMS Report

* Photos courtesy of Denis OConnell, Alpha Metals,

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