lead-free report - hi res

8/2/2019 Lead-free Report - Hi Res

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Under the Microscope.Vision Engineering Ltd.

Lead Free Production,Under the Microscope.

How the Introduction of Lead FreeProcess will Change theAppearance & Inspection Criteriafor PCBs.

This report was commissioned by Vision Engineering, world leadersin the development and manufacture of stereo optical systems.Thousands of these systems are currently used to inspect electroniccomponents and processes. With the switch over to Lead Freeproduction, this document will lead you through the optical inspectionissues involved, explaining what will change and what needs to bedone.

About the author

Bob Willis has been a consultant and trainer to the electronicsindustry for over 15 years. Bob set up the first Lead Free workshopsin England & Europe and has developed an unrivalled knowledge forthe issues involved with the change to Lead Free production. He iscurrently the Honorary President of the SMART Group (EuropeanSurface Mount Trade Association.)

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Lead Free Production Under the Microscope.Vision Engineering Ltd.


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Introduction

The following document explains the basic assembly process for modern printedcircuit boards and where changes may occur due to use of lead-free solder alloys.Lead-free alloys are to be introduced progressively over the coming months for thescheduled ban of the use of lead based solders in July 2006. It is clear that Japan hasled the way in the implementation of lead-free soldering and progressively introducedcommercial products into the market place over the last five years.

The first section of this report deals with the assembly process stages and possiblechanges, the second part of the report focuses on defect types that may be found withvisual inspection at stages in manufacture and their probable courses. Examples of

solder joints tested destructively and then assessed by visual inspection are included.Crack testing is often used for BGA, CSP and flip chip joints. For completeness and toaid understanding some X-ray images and microsections are also included in thereport.


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Assembly Process Introduction

During modern printed board assembly there are generally three types of printed boards assembled

today:

Single sidedPlated through holeMultilayer boards

There are no specific changes to the type of boards for lead-free; the basic laminate used tofabricate the boards has not changed on current products and is not likely to change in the nearfuture. Many of the existing products produced in Japan and in Europe use existing base materialsand there has not been a big shift to materials that have a higher process temperature. The use ofdifferent flame retardant materials in the laminate may be changed in the future due to otherenvironmental legislation.

The main difference that will be apparent will be changes to the solderable coatings on the throughholes and surface mount pads. The most common finish on the surface of existing boards is tin/leadwhich is applied by solder levelling and amounts to over 60% of the market. Commonly on surfacemount boards the surface finish has been replaced by gold with a nickel barrier layer, copper withOrganic Solderable Protector (OSP), silver, tin and alternative lead-free solder levelled coatings.

Many engineers have been concerned that there will be problems with PCBs during assembly likedelamination, blistering of the board surface or cracking of the copper plating in the barrel of the

hole. It is felt that if proper guidelines are followed during assembly this should not be the case.

Blistering and delamination are surface effects on the surface of the PCB which are easy to seevisually, cracking of copper through holes are not easily seen without microsectioning the circuitwhich is obviously destructive. In each of these cases the root cause is much more likely to bemanufacturing problems at the bare board stage which become apparent during soldering.

Boards are assembled using surface mount, conventional components or a combination of bothreferred to as a mixed technology assembly. Printed boards can be assembled manually andautomatically depending on the number of boards to be produced or the number of parts per board.The basic process stages are outlined here and the possible impact on processes adopting lead-

free soldering alloys.

Conventional Through Hole

Insert through hole componentsTurn board over for manual hand solderingWave solder through hole terminationsInspection of solder joints


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Plated Through Hole Boards

Insert through hole componentsTurn board over for manual hand soldering

Wave solder through hole terminationsInspection of solder joints and through hole solder rise

Mixed Technology

Adhesive dispense or stencil printSurface mount bottom side mounted components

Adhesive cureTurn board overInsert through hole componentsWave solder through hole and surface mount

Inspection of conventional and surface mount joints and through hole solder rise

Double Sided Mixed Technology

Stencil print solder pastePlace surface mount componentsReflow solder pasteTurn board over

Adhesive dispense or stencil printSurface mount bottom side mounted components

Adhesive cureTurn board overInsert through hole componentsWave solder through hole and surface mountInspection of conventional and surface mount joints and through hole solder rise

Double Sided Surface Mount

Stencil print solder pastePlace surface mount componentsReflow solder paste

Turn board overStencil print solder paste side twoPlace surface mount componentsReflow solder pasteInspection of surface mount joints

In professional companies it is important to conduct in process inspection at each stage inmanufacture to determine the performance often quoted in Parts Per Million Defective (PPM). Thisprovides a reference to the performance of the process and can be used in small or large volumeoperations and where manual inspection can be very useful. A reference to the monitoring process

and how to determine yields is covered at www.ppm-monitoring.com


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Explanation of Assembly Processes and Potential Lead-Free Issues

Adhesive Application

Adhesive is applied to the surface of a surface mount printed board to hold surface mountcomponents in place during handling, transportation and finally the wave soldering process. Theadhesive is fundamentally only required to help locate the parts for the soldering process after whichit is not required for any other process stage.

For many years the adhesive has been dispensed automatically onto the surface mount board and itis still the most common technique used. Adhesive deposits are single or multiple dots mostcommonly positioned directly between surface mount circuit pads. Adhesive deposit is normally notvisible under the body of the component. In recent years the use of stencil printing of adhesive hasbecome more popular as the printing process is faster that dispensing. Normally all adhesives usedfor this application are epoxy single part materials.

Generally the colour of the adhesive is either red or yellow; both colours make inspection easyagainst the green solder mask coating on most printed boards produced. Simple inspection of gluedots is conducted to make sure the adhesive is not on the surface of the pads. After cleaning off apoorly dispensed or printed deposit adhesive residues should not be visible in holes or on thesurface of the pads.

Lead-Free Process Issues

There should be no impact due to lead-free. However if the board is warped after previous reflowsteps on a double sided product the quality of the deposits will be affected. Both the definition andthe volume of adhesive will be affected. Measuring the size variation of the dots is a simple methodof inspection and quality control.

If a company uses one reflow machine for both reflow and adhesive curing problems can occur ifthey do not monitor the temperatures correctly. Reflow for lead-free will be around 240oC, adhesivecure will still be between 110-130oC for around three minutes. Often companies do not wait for theprocess temperature to drop in the oven which can result in voiding of the adhesives, lower bondstrength to the board surface and component loss. Sometimes the voids are visible to the side of thechip components if the adhesive is visible. During wave soldering this can lead in the worst case to

solder shorts under chip parts, this is only visible with X-ray or microsectioning.


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Screen Printing

The term screen printing is strictly incorrect as virtually no one uses mesh screens anymore for

printing solder paste in modern surface mount assembly. Metal stencils are used for applying thesolder paste on to the surface of the pads prior to component placement; however it is still commonfor the term screen printing to be used as opposed to stencil printing.

Normally today a 0.006-0.008 (150-200um) metal stencil is used most commonly etched or lasercut from sheet stainless steel. Alternatively nickel is used and produced by electroforming toproduce the same stencil foil of the same thickness. Normally the print apertures in the stencil arethe same shape as the pads on the surface of the board design being printed. The size of the stencilapertures are normally reduced in size by a percentage globally or a specific reduction of 0.002-0.003 to help seal the opening on the printed board pads. This also reduces the amount the solderpaste can squeeze out between the pad and the board. Photographic inspection criteria for paste

printing is available from the SMART Group PPM Project web site to download at www.ppm-monitoring.com

Both metal and rubber blades are used for printing and sealed heads are being more widely used inlarge companies, these benefit from only wasting 5-10% of the paste. In blade printing the pastewaste can be 30-40%.


Printing of solder paste should not be affected by a change to lead-free materials; they can beprinted successfully to the finest pitch currently required. Experience has shown it can be usedsuccessfully for intrusive through hole reflow, 0201 chips and Ball Grid Array (BGA). Experienceshows that lead-free alloy pastes do not wet or spread on the surface of the pad to the same degreeas tin/lead. This can lead to the base material of the pad finish being exposed after reflow. Toovercome this some companies have re-modified their stencil apertures to overcome this issue. Thisis not considered to be a reliability concern with non wetted pad corners or a picture frame of thepad visible around the joint.

Printing can be one of the process stages where operators are currently exposed to lead in paste sothere is a direct benefit to limit exposure on health grounds. The solder paste used may be slightlymore expensive than tin/lead materials in the short term as it becomes readily accepted so waste

should be kept to a minimum. With over 30% of paste scrapped after printing, a sealed head printingprocess may be very beneficial in saving costs and maintaining the pastes printing characteristics.

The printing processes can be affected by warped boards or hot or warm surfaces from previousreflow processes, all of which may occur as we move to lead-free assembly. Additionally as the pitchof the components gets smaller and the size of the process panels gets larger the tolerance of manyboards becomes an issue. The stencil is produced from the design information and is extremelyaccurate; however, the board may not be due to expansion and contraction of the base materialsused in industry. The result is that paste print on pads at opposite sides or corners of the board canbe misaligned.


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Component Placement

Component placement is mainly conducted automatically but an operator can place around 250-300components per hour in production. Machine placement rates can range from 5-100,000 placementsper hour.

Today most machines rely on camera recognition, each component is examined prior to placement,and this checks the part, its orientation, lead position and possible damage prior to placement.


Component placement machines today should be able to overcome most issues related to thechange to lead-free assembly. The move to different PCB surface finishes may require a close look

at the vision capabilities of older systems. Minor recalibration may be necessary for gold, matt tin orcopper OSP boards. This has certainly been an issue with mixed finishes where it was difficult toaccurately recognise fiducial marks as people moved away from solder levelled surfaces.

Boards may not be flat due to higher process temperatures with lead-free; this is not a placementproblem but one the machines will need to deal with through effective use of board supports. Asplacement systems can overcome global and local PCB expansion and contraction issues to placeparts the dimensional effects on the laminate should not be an issue.


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Reflow Soldering

Reflow soldering is a relatively simple process. Solder in the form of solder paste is heated along

with component and printed circuit terminations, heating is conducted in a way to minimisedifferences across the surface of the board. Depending on the existing tin/lead alloys, the solderpaste particles become a liquid at either 179oC or 184oC. When the solder is in the liquid state asolder joint will form between the surfaces of the pad and the component termination.

The speed of wetting will depend on the lead and PCB coating and the solderability of that coating.It is necessary to heat up and cool down the assembly in a controlled manner. It is also necessary tomaintain the solder joints in a liquid state to eliminate voiding and form a true intermetallic bond withthe base materials normally 1-3um thick.

Reflow soldering may be conducted with convection or vapour phase although convection has the

majority of the market place world wide. There are companies who use VP for lead-free in Japandue to the complexity and thermal mass of the products being soldered. There are three suppliers ofVP systems in Europe which may have a small impact on the market in the future.

Convection reflow is conducted currently at 220-225oC with a time above liquidus 30-60 second,above 183oC for tin/lead alloys. Boards are generally passed through a reflow system with 4-7process zones on either a meshed conveyor or supported on two linked/pin chains. There arecurrently advantages to using nitrogen to displace the oxygen in convection reflow ovens but veryfew companies use nitrogen. It may be the case that nitrogen may be more widely used for lead-freein the future. In VP processes the process excludes oxygen so the process benefits from an inertsoldering environment.


Many companies feel that reflow machines will need to change for lead-free but it depends on thetechnology level of the system and the throughput speed required for your product. A professionaloven with four or five process zones and a process length of 3-4 metres should provide satisfactoryresults with higher temperatures. The only issue then is the exit temperature of the boards for therequired throughput speed and the outer skin temperature of the oven from a health and safety pointof view. If you have a higher reflow temperature then the exit temperature will be higher for the sameconveyor speed unless additional cooling is provided.

In terms of solder joint inspection the surface of the joint may look slightly grainy in an appearancedue to the temperature, the alloy type or flux left on the joint surface during cooling. It is envisagedthat the basic inspection criteria for solder joints will not change. Standards may, however, usesome reference photographs of lead-free joints for reference to the surface texture.

Experience has shown that the lead free alloys do not wet or spread as well as tin/lead, on surfacemount termination solder rise is not as apparent as with tin/lead. Of course when solders arereflowed and joints formed on non tin/lead surfaces or on surfaces that do not become liquid duringnormal soldering temperatures a demarcation line will be visible.There will probably be more incidence of component damage where companies have not

considered the peak temperature requirement of the components. Surface distortion, cracking,delamination of parts may occur, most of which can be seen with visual inspection.


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Cooling can be very important to double sided boards with lead-free. It is inevitable that the exittemperature of the assembly will be higher; it may be 10-20oC higher depending on the thermalmass of the assembly and its heat retention. If the boards are going to be sent directly into a printer

for second side assembly with no buffer the boards can still be hot which can lead to printingdefects. Although not proven yet there may well be a relationship between cooling rates and theincidence of fillet lifting on through hole reflow parts. Just like with wave soldering, lifting of thesolder fillet occurs when the solder joint interface is still in a liquid state.

It is also inevitable that machines will require more frequent maintenance of moving parts, conveyorlinks and convection fans due to higher temperatures. It is not clear yet what impact this will have inthe future.


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Wave Soldering

Wave soldering consists of pasting a board assembly first through a fluxing station which applies a

liquid flux on the base of the board and into the through holes. This helps the soldering operation bycleaning the surface oxides from all metal surfaces on leads and PCB pads. The board then passesthrough a pre-heat section to raise the printed board and component termination temperature.Normally the top side board temperature will reach 100-120oC. The pre-heat helps by activating theflux to improve wetting during contact with the solder it also helps to displace most of the volatilealcohol or demineralised water in the flux, this is used as a carrier for the fluxing agents.

Depending on the design of the board it then passes through either one or two waves of moltensolder. The solder will wet all solderable areas on the board and the solder will also rise in the platedthrough hole to form solder fillets on the top side of the board. Nitrogen may also be used in thisprocess to aid soldering but mainly to reduce dross and oxide formation on the surface of the solder

bath normally operating for tin/lead between 245-255oC. Nitrogen is used to displace the oxygenfrom around the wave area.


Lead-free will probably have more impact on the wave soldering process than any other process,many people were just not prepared for the issues which have become apparent. Alternative high tincontent solders have a tendency to attack the stainless steel used in different parts of the ductingand impellers used to form and drive the wave shape. This means that existing solder machines willneed to be replaced or parts of the bath replaced. Fortunately most suppliers do have solutions tothis equipment problem.

Generally the alternative lead-free alloys do tend to cause more shorts, this is due to the poordrainage of the solder from the base of the board. It can also be due to the learning curve whichengineers are now going through to optimise processes. With the higher process temperatures andthe common use of low residue fluxes there is a tendency for the flux to be removed from the boardduring preheat and initial wave contact which again can lead to shorts. This is another area whichprocess engineering and suppliers will need to focus on.

Normally a solder bath will operate at 245-255oC for tin/lead; in the case of common lead-free alloysthese will be run at 260-270oC which will increase the degree to which the boards will sag during

solder wave contact. The use of board supports in the wave should be considered during boarddesign, this requires a 3-4mm no go area in the centre of the board. An area is defined by thedesign engineer where no encroachment of surface mount or through hole terminations are allowed.

A tensioned wire can then be positioned in the wave which supports the board as it passes throughthe wave. The support performance is far superior to the use of pallets or titanium clips.


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Visually the solder joint difference can appear different but just like reflow the grainy surface isaffected by the alloy, cooling rates and the amount of flux on the surface when it cools. Theinspection of joints will not be a major problem for staff with selected reference images of joints.

When using lead-free the challenge for wave soldering will probably be hole fill and evidence ofwetting on the top side of the board. This will be due to a combination of factors like:

o PCB surface finisho Correct process temperatureso Thickness and layer count of multilayer boardso Fluidity of the alloy usedo Correct flux application.

It should be borne in mind that the minimum requirement of the IPC level 3 inspection criteria is 75%fill of the plated through hole which can easily be achieved with lead-free processes.


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Manual Soldering

In the case of manual or automated soldering a tinned soldering iron tip comes into contact with the

component termination and the pad surface at the start of the soldering operation. The tinned tip isimportant as the tip should not contact the joint areas it is the solder on the tip that transfers the heatto the joint. When the two surface wet cored solder wire is fed into the joint area, the amount will bedependent on the type of joint and if a plated through hole needs to be filled.

The flux comes from the core of the wire; the wire size and the soldering tip size are dependent onthe type of connections to be produced. In the case of de-soldering the same principle is used; asoldering iron tip with a hole through the tip is tinned and placed over the through hole termination.When the solder is in a liquid state a vacuum is applied which will suck all the solder from the jointarea. Again the correct wetting of the tip is important to transfer the heat as quickly as possible.

Lead-free manual soldering and de-soldering will continue to use the same tools as currently used inindustry today. The professional range of tools will be able to meet the requirements of solderingand de-soldering with lead-free alloys. Most suppliers have already changed the plating/coating ofthe tips away from tin/lead, however many have not conducted studies into the life expectancy of thetips in production. With high process temperatures and the high tin content there may be issues withplating erosion of the barrier plating, failure of which will lead to erosion of the base material. Anyattack on the barrier layer will prevent wetting of the tip which in turn will make soldering or de-soldering difficult. Two companies have already experienced problems in this area; unfortunately noinformation is available on the tips or their plating specification.


To date the only issue highlighted with hand soldering is the life of the solder bits. Some evidencesuggests that when using lead-free alloys the bit plating is attacked leading to increased wear.Based on the corrosive nature of the high tin content alloys this may be the case but little testing hasbeen conducted so far.

When using tips incorrectly without tinning them, leaving the soldering irons on for hours and usingtoo higher temperature will of course have a similar effect. The same situation may occur with de-soldering tools which basically have the same metallurgy and plating and need to be reviewed withyour supplier. The problem of poor wetting of tips is well known as it has been experienced during

the introduction of no-clean materials, less flux, heavy oxide formation caused problems in iron use.

De-soldering with copper braid/wick still works for removing solder shorts on surface mount andconventional through hole terminations. Care should be taken with copper braids to make sure it isnot stored for long periods as the solderability does deteriorate, this then impacts on the ability towick solder quickly from the surface of the joints.

Soldering with lead-free alloys uses fundamentally the same process steps but care needs to betaken with the tips with regular inspection to look for wear and poor wetting.


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Manual Inspection

Manual inspection of lead-free joints can be conducted in the same way as with conventional alloys.

The same magnification, lighting and reference standards can be used for assessment of joints. Thedifficulty may be the interpretation of the solder joints in the transition period from tin/lead to lead-free. As long as the management team allows time for staff to become acquainted with the subtotalchange in surface mount and through hole joints there should not be many issues.

Existing standards and magnification should be used and in most cases these will be satisfactory topick up any issues in the process. Manual inspection also has a key role to play for in-processinspection where companies are determining the manufacturing capability of a process and the yieldfrom a particular product.


Lead-free alloys may have a different surface appearance; generally they do not seem to wet aswell as their tin/lead counterparts. On surface mount pads solder paste does not spread as wellduring reflow, also in terms of wetting rise on surface mount terminations lead-free alloys do notseem to rise as high on terminations. The joints can still be judged based on existing internationalstandards like IPC610 but it will be useful to have example lead-free alloy joints as reference tointroduce inspectors to the different surface appearance.

Most of the defects in conventional and surface mount assembly processes can be seen with thealternative alloys and there will be an increase in solder shorts on the wave soldering process. Thesolder alloy does not seem to drain as well from the board during exit from the wave. Some of thecommon process defects that may be encountered are illustrated in the defects section.

New process defects are fillet lifting seen in reflow of through hole and wave soldering but have notas yet been shown to cause reliability problems. Incomplete reflow where joints are formed but thesurface of the joint seems to have spots on the surface of the joint which are in fact solder particlesfrom the paste.


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BGA manual inspection

BGA inspection is conducted either with X-ray or with an endoscope system which allows solder ball

terminations formed between the device and the board surface. The outer rows of terminations caneasily be viewed but less so the inner rows. Inspection of joints is based on complete reflow, noevidence of paste particles remaining and the complete wetting of surface pads.

The benefit to this method of inspection is to show the surface texture of the joints and subtlewetting variations on pads which are not visible with X-ray. Although the same criteria can be usedfor joints in IPC example joints with different alloys will be required for reference with lead-free. Thedisadvantages of this process are that voids are not visible which is why X-ray was used for areaarray devices.


The inspection of BGA and chip scale packages is slightly easier visually with this technique due tothe fact that the solder joint surface is less reflective with lead-free alloys. Visual inspection, whensetting up a lead-free process, is probably more revealing than X-ray inspection due to the fact thatsubtle variations in reflow and non-reflow can easily be detected. In the case of reflowing BGA andCSPs with tin/lead terminations in combination with lead-free alloys, it is perfectly feasible to formsolder joints during reflow with the tin/lead from the terminations and the flux from the solder pastemaking reliable joints. However, the lead-free particles can remain separated from the bulk of the

joint if the reflow profile is not conducted correctly.


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Automatic Inspection

Automatic inspection has become more and more popular in recent years but still only amounts to a

small percentage in the electronics industry. Mainly automatic inspection employs a single snapshotof the board and then it is compared with a golden board or stored image. Alternatively it is done ona joint by joint basis again compared with pre-defined criteria held in the systems computer.

Initially either all the criteria has been defined by the supplier and modified where necessary by thecustomer or it is learnt from boards in the process.


Lead-free joints do look different in some respects to tin/lead so it is inevitable that modificationsmay be necessary in the set-up criteria for automatic systems. There is no reason to believe that

systems in the market place will not be able to cope with lead-free joints. It may be necessary tohave a wider tolerance on the acceptability criteria if both tin/lead and lead free joints are to beinspected with the same system


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X-Ray Inspection

X-ray has always been used in the industry for inspection of solder joints but it was not till the

introduction of area array devices that X-ray became common place. Manual and semi-automaticsystems remain the most popular in the industry. Far more systems are used off line and online forprocess monitoring. Only high volume and high reliability products can demand 100% inspection.


X-ray is being increasing used for BGA, flip chip and through hole reflow. Some have debated theability of the X-ray to work with lead-free alloys. It fair to say that you have to make minor changes tosetting but that should not be a problem. Many trials have been conducted on most of the popularlead-free alternatives and X-ray works as well as with traditional tin/lead alloys.


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Rework and Repair

Surface mount rework and repair is relatively straight forward. Components to be removed require

the solder joints to be reflowed. When all joints are in a liquid state the component can be lifted fromthe surface of the pads. Normally to prevent spikes on the pads flux is applied prior to componentremoval. The correct flux reduces the amount of solder taken from the pad, often just flux and a newcomponent are required to make a re-connection.

There are three types of rework tool:

Hot airContact toolsIR tools

Each can be used successfully with lead-free some are not suitable for all component types.


Surface mount rework is not difficult; the key is correct preparation and the use of the correct tools.Control of temperature is very important on small and large components which will mean moretemperature profiling of boards as the process tolerances will be much tighter. Existing equipmentwill still be effective for lead-free rework; however, the temperature set points may rise and someproduction supervisors may have to be a little more patient during component removal andreplacement.

There may be a move to more contact tools, modified soldering tips and heated tweezers forremoval of components. Contact tools speed up component removal as opposed to hot air tools, thekey to many rework situations will be pre heat to reduce the total rework cycle time and possibleboard and component damage.

On joint reliability, concerns have been voiced on the use of mixed alloys during rework, if acombination of alloys was used would the joints be less reliable? Based on the work to date thisdoes not seem to be an issue.


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Examples of Lead-Free solder alloys

Examples of lead-free solder alloys melting below 180oC

Alloy System Composition (wt%) Melting range (deg C)

Sn-Bi Sn-58Bi 138Sn-In Sn-52In 118

Sn-50In 118-125Bi-In Bi-33In 109

Example of lead-free solder alloys melting 180-200oC

Alloy System Composition(wt%) Melting range (oC)

Sn-Zn Sn-9Zn 198.5Sn-Bi-Zn Sn-8Zn-3Bi 189-199Sn-Bi-In Sn-20Bi-10In 143-193

Examples of lead-free solder alloys melting 200-230oC

Alloy System Composition Melting range (oC)

Sn-Ag Sn-3.5Ag 221Sn-2Ag 221-226

Sn-Cu Sn-0.7C 227Sn-Ag-Bi Sn-3.5Ag-3Bi 206-213

Sn-7.5Bi-2Ag 207-212

Sn-Ag-Cu Sn-3.8Ag-0.7Cu 217Sn-Ag-Cu-Sb Sn-2Ag-0.8Cu-0.5Sb 216-222


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Example Lead-Free Joints from Reflow and Wave Soldering

The following are example solder joints produced with lead-free solder alloys. The most common

solder alloy which will be used in industry will be from the Tin/Silver/Copper family but somecompanies will use Tin/Copper and Tin/Nickel in manufacture. For consumer products other lowertemperature products will be used. Ideally everyone wants one alloy and when issues of componentcompatibility have been overcome the Tin/Silver/Copper will probably be the alloy of choice.

Example Wave Solder Joints

Solder Joints produced with Sn/Ag/Cu Tin/Silver/Copper

Solder Joints produced with Sn/Cu/Ni Tin/Copper/Nickel


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Example Reflow Solder Joints

Solder Joints produced with Sn/Ag/Cu Tin/Silver/Copper


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Lead Free Process Defects

The process defects illustrated are typical of what has been seen in recent years during the

introduction of lead-free materials and processes. If is inevitable that as more production processesare introduced and we gain more experience many more issues will come to light. It is hoped thatmany of the process indicators and functional defects can be easily detected.

Printed Circuit Boards

Non Wetting of Pad:

This is a classic example of solder wicking where the solder has all reflowed and wicked up thecomponent leads. In this case it is clearly the fault of the printed board solderability; the OSP coating

has failed to protect the surface of the pad. In any reflow process if one surface is slow to wet thesolder can be transferred directly to that surface. It is important that both PCB coating and leadfinish are equally solderable.

Don't be too quick to blame the PCB supplier; it may also be an assembly problem. Copper surfacecoatings like OSPs can be degraded by washing the board, long storage times in excess of sixmonths and high cure or reflow temperature. Make sure you confirm the true cause of the problembefore looking for someone else to blame. The same effect can be seen on other PCB coatings likethe gold board below.


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Solder Wicking:

In this example of solder wicking it is clearly caused by the printed board surface finish. Thesolderability of the gold is poor due to a plating problem and the paste has not wetted the pads

during reflow. Close examination of the leads shows a slightly bulbous appearance on the leads.The solder has tended to wet the lead rather than the pad. Often in this situation the printing processis blamed for not printing paste successfully on to the pad. This would not be the case as there isclear evidence of flux residues around the pads on the surface of the laminate. The solderability ofthe printed board should be tested using a wetting balance.

PCB Delamination:

The two light areas on the surface of the PCB are delamination. They are effectively blisters of airinside the board. During reflow, wave soldering or rework, moisture in the board expands causingthe blisters. The root cause is not moisture as this is in all PCBs, it is a fault in the bonding, this gaphas allowed moisture to accumulate and when it expands the blister is visible and will often causeproduct failure.


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Components

Poor Solderability:

Non wetting of the toe of leaded surface mount devices is common and will continue to occur withthe greater use of no clean low residue pastes. The lead tip when formed and cut during componentmanufacture is left exposed with no protective coating. It oxidises during storage and is very difficultto solder. The toe of the lead is not covered in the component manufacturer's specification or in thesolderability standards. The inspection standard should not reject joints where no wetting is visible atthis point. This fault is common when soldering with traditional tin/lead alloys and will still be adiscussion point when adopting lead-free soldering. The main strength of the joint comes from theback heel of the joint not the toe.

Plastic Cracking:

Component cracking is normally due to the incorrect use or specification of the component. Initiallycheck the supplier's maximum soldering temperature and duration. Also consider any other specialrequirements for component storage. Cracking can be found on the top and bottom of the part andon plastic in high stress areas.

The most common cause, as in the case of the example, is excess heat. If the component supplierspecifies the maximum temperature for reflow of


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Screen Printing

Paste Alignment:

Minor paste misalignment of the solder paste is shown on these 0201 pads. This was seen on thesurface of boards which also included fine pitch areas. These alignment problems are becoming areal issue with larger boards. The pad to pad dimension from opposite edges of the board isdifferent to the design data which is used to make the stencil.

There is a limit to standard board materials and we all need to appreciate the capabilities of thebasic materials. The problem is not related to lead-free processing but an issue for small partprocessing in the future.

Paste Alignment:

The example shows slight misalignment of the paste on the surface of the fine pitch pads. Theexample board was being printed with lead-free paste and also featured 0201 chip components onthe opposite edge of the board, which proved difficult with the alignment of the board. Although theprinting may be satisfactory the board material tolerance can make the alignment of a stencil to thepads difficult. Modern printers can provide correction to minimise the offsets but it does mean therewill be a compromise on the exact position of the paste on the pad.

The minor misalignment is perfectly acceptable but illustrates the issues of decreasing tolerance forthe future.


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Reflow Soldering

Popcorn Cracking:

Component cracking is normally due to the incorrect use or specification of the component. Initiallycheck the supplier's maximum soldering temperature and duration. Also consider any other specialrequirements for component storage. The two examples show cracking, the first on the edge of thepart between the PCB carrier and the moulding the second on the corner of the part.

The most common cause, as in the case of the example, is excess heat coupled with a high watercontent. If the component supplier specifies the maximum temperature for reflow of


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This can occur when there is too long a delay in the soak period of reflow prior to reaching reflowtemperature. This can exhaust the protective layer in the paste. It can of course also be due to the

joint not reaching reflow temperature, but unlikely as this is a lower reflow alloy as shown in the

lead-free alloys section.

The problem can also be related to the reflow of this paste in air. Although paste suppliers offer thisalloy for reflow in air many companies have found nitrogen necessary to get the best results. Theprofile used did meet the recommendation of the supplier. Mechanically the joints were also sound.

Cracked Joints:

Two examples of reflowed BGA joints which have been mechanically removed from the surface of a PCB.

The joints on the board were questioned after environmental testing for cracking. X-ray can find it difficult todetermine these types of faults. Using a dye penitrant fluid to detect cracks on a board prior to breaking theBGA off can be very revealing. The first example using a yellow dye shows 75% coverage, virtually all the

joint has cracked away from the pad. The red dye used here shows less than 10% crack propagation.

Lead-Free Fillet Lifting:

The example photograph shows fillet lifting on a lead-free joint after pin in hole intrusive reflow, it would justabout be visible with 30x magnification but only if you were specifically looking for the defect. The exampleshown was part of an experiment on resist-defined and non-defined pads. Normally solder mask is imaged

with a clearance of between 0.002-0.003" from the pad. The example is a standard through hole pad withstandard solder mask clearance.


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During a visit to Japan as part of the SMART Group Mission to Japan engineers discussed the benefits ofusing the solder mask to define the copper pad area. They suggested that the design modification wouldreduce or eliminate the problem in manufacture. Clearly the resist has had no effect on eliminating the

solder fillet lift, mask defined pads have not been shown in the experiments to help but more investigationwill continue on pad lift.

Grainy Solder Joints:

The image shows what appears to be grainy joints after reflow, but it is in fact the flux on the surface of thejoint which has cracks and the joint is satisfactory. When solder pastes with a high solids content are usedand overheated the flux can harden and then as the solder joint solidifies the flux flakes off. The same thingcan occur after stress testing of boards and then the joints inspected after text.

QFP Cracking (Popcorn Damage):

Large plastic components are prone to cracking during wave and reflow soldering. It is caused by moisturein the component expanding during the high temperature of the soldering operation. The cracking is eitherseen on the top of the device, which is fairly obvious, as in the example, or on the base of the device,which is not so easy to detect.

Check the component specification of the part and the required storage conditions. Many of the Quad FlatPack (QFP) and Thin Small Outline Package (TSOP) devices require storage in special low humidityenvironments to prevent moisture absorption. IPC have document guidelines on the use of these parts.The controls for higher temperature lead-free soldering will need closer attention.


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Open Circuit Joint:

The example shows an open solder joint, the solder paste has reflowed and wetted the pad but the jointhas not formed on the capacitor lead. Normally this would be associated with poor solderability of thecomponent termination and the parts would need to be tested. However, in this case the design of the padand the paste deposit extended too far under the body of the plastic base of the component. The resultwas that the lead was lifted above the surface of the solder paste as it reflowed before the lead could wet.

Non Reflow:

No wetting in this example is due to the incomplete reflow of the paste. The profile temperature was too lowand only just reached 220oC. Clearly this component termination on the board did not remain at reflowtemperature for the required time, typically 40-60 seconds above liquidus. In the case of thetin/silver/copper alloy the peak reflow temperature on the board would about 240oC.

Although most professional reflow ovens can handle lead-free reflow profiles, care needs to be takenduring reflow by correctly profiling new products and pastes. Most reflow ovens can achieve a delta t of 10-15oC across the surface of the board but to achieve this profiling needs to be done correctly.


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Wave Soldering

Fillet Lifting:

Fillet lifting is probably the only truly new defect which has been found to be associated with lead-free wavesoldering and intrusive reflow of through hole parts. Basically as the example shows the edges of thesolder fillet on the top side of the board have curled up and lifted from the pad surface.

This is mostly seen on wave soldering with lead-free alloys and there are many reasons for it occurring,mainly the thermal expansion of the board and then the contraction of the board and solder during coolingresult in the lifting. Although it does not look good and is not defined in any standards at this time it has notbeen shown to cause a joint to fail in extended environmental testing. Like fillet tearing many Japaneseengineers feel that this is a benign problem and not a reliability issue. Studies have shown no changes inresistance or joint failures after 2000 temperature cycles.

Component Damage:

Traditional conventional components are often not rated at high temperatures so care needs to be taken ifthey are used close to surface mount devices. In the example IC sockets have melted due to excessiveheat. The same damage can occur on the surface of the board during wave soldering. Some of the cheapconnectors and sockets on board designs will need to be re-considered for lead-free soldering. If rework isto be conducted it is possible to place a heat shield between the components and the surface mountdevice. The heat shield can be as simple as a piece of Kapton tape or a small section of thin laminate.


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Grainy Joint Surface:

In the case of lead-free the examples above for a wave soldered termination are typical of joints with

different alloys. The soldering quality is fine but the difference in the visual appearance is solely related tothe change in alloy. There tends to be a dull appearance and a grainy surface to all lead-free alloys. In thecase of wave soldering all the terminations will be fully coated with the solder alloy hence both conventionaland surface mount joint will have the same visual appearance.

Fillet Tearing:

Fillet tearing is again one of the new defects associated with lead-free and can be seen on most alloys. Itcan be seen specifically associated with lead contamination from the solder bath or the lead or PCB. Leadcontamination in the solder or at the termination interface cause changes in the local solidification rates of

the solder. This type of defect has also been seen on joints where no lead was present on any surface tobe soldered. Like fillet lifting many Japanese engineers feel that this is a benign problem and not areliability issue.


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Pad Lifting:

The microsection illustrates a typical pad lift that can occur with a lead-free process. It can be seen aftersoldering through hole terminations where the solder contracts during cooling causing the lift or peel backof the pad. Either the pad is lifted, the fillet lifts from the pad, or fillet tearing can be seen on joints of thistype. The joint image above shows fillet lifting and pad lift on the same joint.

Concern is shown for pad lifting as it could be associated with a track connection and in this case the trackand the pad could separate. The mechanism is the same as for tearing and pad lifting, mostly associatedwith the expansion and contraction of the laminate and the solder. The IPC 610 solder joint inspectioncriteria does not cover currently lead-free but does give criteria for pad lifting after soldering. The criteria isthe pad should not lift anymore than the thickness of the pad from the surface of the PCB laminate.

Insulation Inclusion:

The incomplete solder fillet has been caused by the insulation on the wire termination falling into the jointduring wave soldering. This is a common fault when wire leads are soldered directly into a circuit boardparticularly if the insulation is a low temperature material. The same problem can occur if the encapsulanton components runs down a lead during manufacture, wound coils and transformers are a classicalexample. This is often also seen on radial inserted components as the body of the part normally sits on thesurface of the board.


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In the case of coils, inductors and transformers it is common practice to pot the devices and often thevarnish or potting compound is left on the pins. These are not wave solder defects they are assemblydefects and need to be addressed by engineering.

All components should be examined for their process compatibility especially with the proposed increase intemperatures that parts may well see in wave and reflow soldering. With lead-free the solderingtemperatures will be 15-20oC higher and so will the top surface of the board during contact with the wave.During pre-heat the top side of the board may also be slightly higher.

Solder Short:

Solder shorts are becoming a major problem in wave soldering particularly as component pitches continueto decrease. When using lead-free alloys the drainage of the solder is not as good as tin/lead hence thereis a tendency to see more shorts. In the example shown shorts are seen on a standard 0.1 pitchconnector. Due to the close proximity and the number of pins the solder separation is impeded at the baseof the board. Shorting can occur due to poor fluxing, incorrect pre-heat or wave separation. With the highertemperatures flux is more likely to be exhausted during contact with the wave, this will lead to differentfluxes being made available for lead free assembly. This may mean slightly higher solids content of higheractivity. All shorting can be decreased through good design rules with reductions in pad size andcomponent lead length or using drainage pads on through hole parts.

In the case of the example shown it was necessary to change the solids content of the flux but still retain a

no clean process. Using a hot air knife did improve the soldering results on this example as did the use ofnitrogen.


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Poor Wetting:

Incomplete wetting or poor solder rise in a plated through hole will show up due to poor fluxing or pre-heat

temperature. If both are satisfactory, the defect may be associated with the surface coating of the board.

The trend in the industry is to copper surface finishes but care must be taken over their selection. Specialassembly conditions should also apply to storage, washing boards, cure and reflow temperature.


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Hand Soldering

Poor Hole Fill:

Poor or incomplete hole fill is normally a fluxing or heating issue. It is unlikely to be a printed boardproblem. In the example shown the poor hole fill is due to pre-heat settings. The solder has wetted theleads of the device but failed to wet the surface of the through hole.

As a guide the topside temperature of the printed board just before wave contact should be 100-110oC.This is generally true for double sided and multilayer boards. Single sided boards will be processed atslightly lower temperatures as no solder penetration is needed. In the case of the higher meltingtemperatures of lead free alloys premature solidification may cause this type of defect to be seen.


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