PROCESS CONSIDERATIONS IN TRANSITIONING RoHS-EXEMPT ELECTRONIC ASSEMBLIES TO LEAD-FREE PROCESSES

Transcription

1 PROCESS CONSIDERATIONS IN TRANSITIONING RoHS-EXEMPT ELECTRONIC ASSEMBLIES TO LEAD-FREE PROCESSES Chrys Shea & Steve Brown Cookson Electronics Assembly Materials Woking, United Kingdom ABSTRACT Automotive electronics modules are exempt from RoHS lead-free legislation, although most manufacturers in this sector are investigating lead-free processes to help vehicle manufacturers meet the End of Life Vehicles (ELV) mandate. There are many case studies on how to transition to leadfree; the real question for exempt manufacturers is when to transition. This paper reviews the process considerations automotive manufacturers should think about when determining their transition strategy. INTRODUCTION There are a multitude of process considerations in transitioning to lead-free for any electronics manufacturer. In the past several years, the numerous initial approaches have converged and assembly processes have reached respectable yields in high volumes. Despite the established processes, however, certain processrelated reliability issues have yet to be resolved. The following discussion reviews the state of lead-free assembly processing in spring 2007 and the current investigations that may be noteworthy for manufacturers of high reliability/automotive products. COMPONENT FINISHES Often the primary driver for an exempt manufacturer to consider lead-free processing is the changes in surface finishes. As the majority of electronics assemblies migrate to lead-free, component manufacturers are transitioning the surface finishes of their devices. Newer component styles, like those used in miniaturization, are being introduced in packages that will only be available in lead-free finishes. In commercial electronics environments, multiple lead plating alloys have been introduced, and most have soldered successfully without great concern from assemblers. However, in high reliability environments, assemblers do not have the latitude to change lead platings without strenuous qualification tests, so the alloys on the components require diligent monitoring. One method of screening that has been introduced is hand-held Energy Dispersive X-ray Fluorescence (EDXRF) equipment. EDXRF gear has been miniaturized to the level of a hand-held gun that can be used when inspecting incoming components. Several are available on the market for purchase or rental. An independent study that assesses the capability of three different styles of gun was recently published. 1 It is recommended that assemblers should review the characteristics of each product and compare the equipments capabilities with their own needs prior to implementing a new incoming inspection routine. Soldering to device leads with lead-free metallisations is only one half of the component battle. The other half is ball grid array devices, which do not have lead-free plating, but have lead-free balls. It s not just a matter of coping with a different protective metallization and continuing to solder to copper; it s a matter of using a completely new alloy for the entire interconnect. The concern of lead-free BGA balls will be addressed in greater detail later on. PWB FINISHES Hot Air Solder Level Hot-Air Solder Level (HASL) PWB finishes are common in high reliability applications. Tin-lead HASL is cost effective, ensures good solderability to the underlying copper, and has an established track record of reliability. Hot air level finishes do have a downside, however. The doming effect created by the solder on pads limits it usage in finer pitch (less than 20 mil) devices. Tin-lead HASL will continue to be a final finish option in mixed metal assemblies, i.e., assemblies that may have some lead-free component finishes, but continue to use tin-lead solders for the primary interconnects. Lead-Free HASL finishes are available, and the lower wetting angle of lead-free solders create slightly less doming than tinlead. But the most commonly used lead-free soldering alloy, SAC305, is not suitable for HASL applications and is rarely used. The unsuitability of SAC305 is due to is propensity to dissolve copper very quickly. Alternative lead-free alloys with lower or no silver content are typically used for lead-free HASL, due to their lower dissolution rates. Excessive dissolution of copper traces and annular rings can create hidden defects and decrease assembly reliability. High Temperature Organic Solderability Preservatives (OSP)

2 OSP is an extremely popular final finish due to its cost effectiveness and it s flat, planar surface that enables finer pitch solder paste printing. It s got a good shelf life, and is even reworkable if applied poorly or stored beyond its recommended shelf life. The two downsides to OSPs are the limited hole fill that can be seen in wave soldering if the assemblies have experienced prior reflow excursions, and the lack of spread of lead-free solders pastes during reflow. Newer generation OSPs are now available for the higher temperatures experienced in lead-free soldering processes. These high temperature products are usually completely backwards-compatible, and work very well in tin-lead processes, exceeding the performance of OSPs designed for tin-lead. In lead-free environments, high temperature OSPs perform relatively the same or slightly better than the tin-lead OSPs in tin-lead environments. Although tinlead OSPs are occasionally used in lead-free processing, they can demonstrate poorer performance in the hotter environments. Immersion Silver Immersion Silver is emerging as a choice surface finish for lead-free assembly. Some earlier forms of immersion silver were associated with a failure mechanism known as planar microvoiding or champagne voids. These tiny voids appear in a planar form near the intermetallic region of solder joints, causing a weak spot and premature failure of the connection. There are several immersion silver options available, and planar microvoids are not associated with all immersion silver products. At this juncture it is reported that the issues of planar microvoids have been resolved in products that exhibited the problem. Immersion silver finishes provide flat, highly printable surfaces, have very good wetting and spread properties when used with SAC alloy solder pastes, and retain very good solderability for post-reflow wave soldering. Shelf life of the surface finishes can vary, however. All immersion silver finishes are susceptible to tarnish in environments with excess sulfur. These would include industrial areas where sulfur is exhausted into atmospheric environments, or even food preparation areas, where sulfur is liberated from foods as they are cut and cooked. Additionally, it has been shown that immersion silver finishes with coarser grain structures are more susceptible to tarnish. The coarser grain structures allow the underlying copper to diffuse into the silver, and as the copper gets nearer to the exposed surface, it is highly susceptible to oxidation. Electroless Nickel-Immersion Gold Electroless Nickel-Immersion Gold (ENIG) surface finishes are also relatively common in high reliability applications. The gold provides a nearly tarnish-proof surface that is ideal for long term storage and solderless contacts. Its oxidation resistance gives it a very long shelf life, and even allows for bake-out if the PWBs absorb moisture during storage. The flat topography makes it ideal for stencil printing. SAC alloys spread better on ENIG than any other finish, which assists in getting full pad coverage in the reflow cycle and topside hole fill in wave soldering. It is theorized that ENIG finishes may help mitigate blow holes due to the nickel barrier that is applied to the copper. On the downside, ENIG is the most costly final finish, black pad failures have been associated with it, and the surface being soldered to is nickel, not copper. Although black pad is now well understood, it still occurs, albeit rarely. Nickel can be more difficult to solder to than copper, and the joint construction will include a nickel layer in the intermetallic region. Intermetallic compounds comprised of tin and nickel are believed to be more brittle than tin-copper intermetallics. There is no perfect surface finish for PWBs. The assembler should consider the advantages and disadvantages of each, and make the best possible choice for the performance of their product. QUALIFICATION AND RELIABILITY TESTING Whether or not the exempt assembler is planning on implementing lead-free processes in the long term, alternative component finishes and mixed metal systems will likely need to be qualified in the short term. The economics of reliability testing may indicate longer term planning be included. A complete scope of lead-free reliability testing would include lead-free finishes with tin-lead solders, in addition to lead-free finishes with lead-free solders. The control in both scenarios would be a baseline of existing finishes with tin-lead solder. In either case, the testing should be thoughtfully planned out, and mixed metals assemblies should be carefully considered, as this scenario is likely inevitable. The test vehicle may be an off-the-shelf kit, an existing product, or a specially designed board. Off-the-shelf lead-free validation kits usually include as many different component finishes as possible and offer different PWB finishes. Unfortunately, they are typically not daisy-chained, so reliability testing is severely limited. For the high reliability assembler their efficacy may be limited, but they can provide a cost effective method of dialing in the process that will be used later on to build the more expensive test vehicles for reliability assessments. Existing products that include a variety of components may be useful test vehicles, particularly if accelerated life test equipment and protocols already exist. Minor modifications to existing designs might include the addition of Surface Insulation Resistance (SIR) coupons, simplified board I/O ports to limit the need for dedicated external connectors, or footprints of newer component packages that will be used in the future. It is now widely believed that due to the differences in creep properties between tin-lead and lead-free solders, dwell times for lead-free solder joints in thermal cycling chambers should

3 be extended to 2 to 3 times the dwell for the equivalent tin-lead cycles. PROCESS-RELATED RELIABILITY CONCERNS Lead-free solder joint constructions bring with them many reliability concerns, including, but not limited to: lower drop shock resistance, tin whiskers, and increased IMC growth due to thermal aging. Unresolved issues in the assembly process itself present some new reliability concerns; these include blow holes, copper erosion, and mixed metals systems. Blow Holes Figure 2. Cross section of soldered PWB that exhibited extensive voiding and blow holing. Notice the repetitive pattern of notches in the barrel wall. Figure 1. Severe condition of blow holes, or solder balloons in wave soldering with SAC305 alloy. Blow holes result from the rapid outgassing of entrapped moisture in PWB laminates and occur as the board is wave soldered. Figure 1 depicts a severe condition of blow holes. In tin-lead processes, blow holes are usually formed by the outgassing of moisture in the PWB laminate through breaches or thin spots in the barrel plating, and they look like small pinholes or small volcano peaks. It is believed that they are formed by the same outgassing mechanism in lead-free processing, but the blow holes can often look more like balloons when the solder solidifies quickly during the outgassing process. The blow hole phenomena is far more prevalent in leadfree processing than in tin-lead. The melt temperatures are higher and the contact times are longer, exposing the PWBs to a greater thermal spike at wave contact, which creates more volatilization and higher internal pressures. Figure 3. Higher magnification cross section of barrel wall. This is from a different PWB than figure 2, but both were from the same lot that exhibited voiding and blowholing problems. Notice how thin the plating is in the notch. There are many unanswered questions about blow holing in lead-free soldering, and a root cause can not be absolutely, positively assigned at this time, but several investigations have led to the drilling process in fabrication as a major, assignable factor. Figure 2 shows a cross section of the boards that produced the blow holes shown in the previous picture. 2 The repetitive uneven pattern on the barrel wall may indicate that it was not drilled smoothly, and the plating process could not properly cover the uneven areas. A higher magnification of the uneven area is shown in figure 3. The longer, hotter leadfree process is far less forgiving to plating imperfections than its faster, cooler tin-lead predecessor.

4 The occurrence of blow holes in lead-free wave soldering is widespread and nondiscriminatory. They have occurred in different laminates, all surface finishes, and with all major lead-free alloys, in all regions of the world. Early in the lead-free transition, they seemed to be associated with immersion silver finishes, but no data has been produced to verify that notion. It is more likely that more immersion silver finishes were being introduced to products that had previously used HASL or OSP finishes, and the timing of the introduction coincided with the manifestation of increased blow holes. The root cause is yet to be determined, but the situation can be mitigated by paying closer attention to the drilling and plating processes in board fabrication. Barrel wall strength that is sufficient in tin-lead processing may not be sufficient in lead-free processing. This is evidenced by the fact that, in some cases, the same lot of PWB s form blow holes in the lead-free process, but perform to expectation in the tin-lead process. Multiple cases of this occurrence have been witnessed and reported. Failure analysis experts with over 20 years experience in the field report that nearly all cases of blow holes they have diagnosed are due to the poor PTH construction. Assemblers should consult with their fabricators regarding board specifications for the higher complexity assemblies that will experience the more demanding soldering processes. Current guidance is to increase copper plating thickness callouts to 25 to 40 microns, as opposed to previous specifications of approximately microns. Copper Erosion Solid copper readily dissolves into molten tin. That s a good thing, because that s how intermetallic compounds are formed in the soldering process. The intermetallics are the glue that holds the two dissimilar metals together. Eutectic SnPb solder is 63% tin, while today s conventional SAC alloys (lead-free solders) contain in excess of 95% tin. The higher concentration of tin in the SAC 305 and 405 alloys causes the copper to dissolve faster during the soldering process. Under certain conditions, the copper can dissolve too fast, resulting in the removal of most - or even all - of it from the circuit board features. Traces and plated through holes that are exposed to wave soldering and rework processes are at a high risk for excess dissolution. The most susceptible region on a circuit board is the knee area of the PTH, which is the location where the PTH meets the annular ring. Figure 4 shows the intensified effect of copper erosion at the knee of the PTH. 4 Figure 4. Copper erosion at the knee of the PTH Different lead-free alloys have demonstrated different rates of dissolution. Studies 4,5 have shown that the high silver content alloys SAC 305 & erode copper much faster than alloys containing low or no silver content like SACX 0307 and Sn100C. It is believed that silver acts as an erosion accelerator, while certain additives in the low- or no-silver alloys act as erosion inhibitors. The existing level of copper in the solder pot also factors into erosion rates. The plated copper surface itself is suspected to play a part in the erosion process. This is indicated by the fact that two identical circuit boards from different suppliers can demonstrate vastly different erosion properties in the same soldering process. It is believed that the dissolution rate may be a function of the grain structure of the copper. Larger or columnar grain structures are perceived to erode more readily than smaller or epiaxial grain structures. Testing is currently underway to check this hypothesis. The largest impact of copper erosion occurs in PTH rework. While an assembly may only experience 4 to 7 seconds of contact on the lead-free solder wave, it can easily experience greater than 60 seconds of contact time on a solder fountain during rework. To rework a connector, the area requiring rework is fluxed and placed over the fountain, often without any preheat whatsoever. After the solder in all the barrels is completely melted, the connector is removed and the solder flow is turned off. It is likely that the holes in the circuit board did not drain completely and some solder has resolidified in the barrels. The fountain is again turned on to remelt the residual solder, which is then blown clear with air. The assembly is then fluxed, the connector inserted, and it is subjected to another soldering cycle. When molten solder can be seen at the top of the barrels, the fountain is turned off. If solder bridges are found, the operator may again turn the fountain on briefly or pulse it several times to remove the bridges. Depending on the thickness and thermal density of the circuit board, the copper features on the circuit board can be exposed for up to 120 seconds during this process.

5 Copper erosion is suspected as a factor in blow holing. Again, boards that demonstrated blow holing in a leadfree process but not in a tin-lead process were further tested. The tin-lead process was dialed up to the same parameters as the lead-free process. Boards traveled through the machine at the same conveyor speed, under the same preheat settings, with the same solder temperature and contact time as the lead-free process, but with tin-lead solder in the pot. They did not show signs of blow holing. If the increased thermal exposure of the lead-free wave soldering process were the sole assembly process factor in blow hole formation, then blow holes should have been observed during this experiment. The fact that they were not observed leads us to believe there are more factors than just the thermal excursion; perhaps copper erosion is playing a part in the phenomena. The role of erosion in blow holing has yet to be quantified. Mixed Metals Systems Many BGA packages are available only with lead-free balls. The majority of these are SAC 305 alloys, but some packages are now emerging with lower silver alloys due to the improved drop shock resistance the lower silver content provides. In either case, the assembler has two options: 1) Reball with tin-lead balls in-house or at a third party operation 2) Use the SAC alloy balls in the tin-lead soldering process Both options bring with them reliability concerns. Little work had been published on the long term effects of reballing in high performance applications, and there are still unanswered questions about the reliability effects of non-homogeneity in mixed metal systems of BGA joints. solder balls in BGAs. Test vehicles were processed with varying peak temperatures and times above liquidus in the tinlead profile window, and the joints were examined metalographically to determine levels of mixing. Cross sections of the results of different peak temperatures at fixed times above liquidus are shown in Fig 5. This study revealed that mixing is more sensitive to peak temperature than to time above liquidus. At the time of publication, these assemblies are being subjected to thermal cycling and drop shock testing. Although mixing is seen in BGA joints while the ball is still in its solidus state, the SAC 305 balls will not collapse unless they reach a minimum of 217 C, where they are in a pasty range, or 221 C where they are in a full liquidus phase. An assembler who desires complete ball collapse will need to reach a peak temperature of approximately 220 C. This confines peak temperatures to the top quartile of the tin-lead window, which brings concerns about the thermal challenges placed on the fluxes in tin-lead solder pastes. In order to assure thermal robustness at the higher peak temperatures, newer tin-lead solder pastes are emerging. They are essentially paste flux formulations designed for lead-free processing, but are blended with tin-lead solder powders. THERMALLY ROBUST TIN-LEAD SOLDER PASTES Blending high temperature fluxes with tin-lead powders resolves several transition issues on the operations side, but brings with it additional concerns on the reliability side. While these hybrid solder pastes can easily take the heat of longer, hotter profiles and are designed to wet well with SAC alloys, no-clean flux products designed for lead-free processes may not offer the same reliability properties when used in tinlead processes. No-clean products are designed to fully activate and deactivate in typical soldering cycles, and a cycle that is too short or too cool may not render the residues benign. Prior to implementing a hybrid no-clean solder paste, electrical reliability should be assessed through SIR or electromigration tests. This is typically not an issue for water washable fluxes, as the residues are removed during cleaning, and shorter, cooler reflow cycles tend to make the flux residues easier to clean than longer, hotter cycles. Figure 5. Partial mixing of SAC305 BGA balls with tin-lead solder paste. 6 Note the difference in mixing at the higher peak temperature. An inemi study 6 is underway to investigate the effects of partial mixing of tin-lead solder paste with SAC 305 Voiding is another major concern of hybrid solder pastes. By volume, solder paste is approximately 50% flux and 50% metal. Voiding is predominantly a function of the flux. If certain volatile components of the flux do not completely outgas before the metal melts, voids will be created. Once the solder deposit has begun to melt and has formed a liquidus skin around the perimeter of the deposit, the volatiles will likely not be able to break the surface tension of the molten solder to escape. Lead-free paste fluxes are designed under the premise that the metal portion of the product will not melt until 217 C, which means outgassing paths are available until the system reaches 217 C. If the same flux formulation is applied in a tin-lead process, the metal melts at 183 C and closes off the outgassing paths. Volatiles that burn off in the

6 34 degree range between 183 C and 217 C will not be allowed to escape, and can result in more voids in the joints. Regardless of whether the process chemistry is noclean or water washable, the sensitivity of the solder paste to void formation should be assessed prior to introducing a hybrid solder paste for a mixed metals process. Assuming that the electrical reliability and voiding properties of the hybrid solder paste have been deemed acceptable, there are considerable cost savings that can be realized by the assembler: streamlined qualification testing and process implementation. If the same flux is used in the hybrid solder paste as the lead-free solder paste, all electrochemical reliability assessments can be determined in the same test. If the assembler plans to implement full lead-free processes in the future, the qualification testing has been completed and should not need to be repeated. Furthermore, the assembly operations will have already optimized print parameters to ensure the best possible yields. These two advantages can yield real benefits when the time arrives to begin building fully lead-free assemblies. Conclusion Lead-free assembly processing is a reality, and every day more products make the transition successfully. Around the world, production volumes have ramped up to a level where the behaviors of the solders are now better understood, many DOEs have been run to characterize and optimize the processes, and design guidelines have been developed. Lead-free assembly processes are good, but they are not yet great. Findings from initial production launches have identified several key areas for concern and further study. These areas include blow holes, copper erosion and rework, and mixed metals BGA systems. As the assembly industry s collective knowledge base grows, these issues will be contained or resolved, and it is likely that more will be revealed as the process matures. Mass soldering processes were invented with tin-lead and refined in enormous production volumes with that alloy for fifty years. Although lead-free assembly is currently a viable process, it is in its relative infancy when compared with its predecessor. When determining a transition strategy, the exempt assembler should consider both traditional reliability concerns and the potential issues that have been identified during the lead-free transition of the RoHS-compliant assemblies. References 1) Chih, P. W., et al, Use of EDXRF for RoHS Compliance Screening in PCBA Manufacturing, Proceedings of APEX Technical Conference, ) Picchione, L., Report of Analysis, Metallographic Examination of PTH Solder Connections to Determine the Cause of Voiding, CE Analytics, ) Lee, D., Wave Solder Yield Improvement Through Design, SMTA Upper Midwest Industry Day, ) Byle, F., and Jean, D., A Study of Copper Dissolution in Pb-Free Solder Fountain Systems, Proceedings of SMTA International, ) Hamilton, C., Snugovsky, P., Kelly, M., Have High Cu Dissolution Rates of SAC305/405 Alloys Forced a Change in the Lead-Free Alloy Used During PTH Processes? Proceedings of Pan Pacific Microelectornics Symposium, ) Kinyanjui, R., and Chu, Q., inemi Technical Working Group Report, Pb-Free BGAs in SnPb Assembly Process Project, APEX, Downloadable at: tations/apex_2007/lead- Free/BGAs_SnPb_Assembly_Kinyanjui.pdf