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The Maturing of Lead-free Assembly
December 31, 1969 |Estimated reading time: 5 minutes
From Implementation to OptimizationBy Jim Hall, ITM Consulting
When we observe all the new materials being offered and all of the product and process data that is being published, it appears clear that lead-free (LF) assembly is transitioning from initial implementation to an optimization phase. Or from "How do I build an acceptable LF product?" to "What can I do to my materials and processes to make my LF products less expensive, easier to assemble, and more reliable?" Certainly, many electronics manufacturers (especially in the U.S.) have yet to produce a LF assembly, but with the exception of high-reliability (HiRel) products, LF versions of most types of electronic products are being successfully manufactured. For example, Sony and Panasonic have been marketing LF consumer products for over 10 years and Motorola has been supplying LF cell phones since 2000.
The tin/silver/copper (SAC) alloys, especially SAC 305, have been successfully used as LF solders in both reflow and wave processes for a wide variety of products. Although it does not wet to PCB and component terminations as well as tin/lead, the solder joints produced with SAC 305 have been judged to be acceptable in a wide range of applications. In fact, thermal-cycling-based accelerated life tests have shown these LF solder joints to be more reliable than tin/lead in a large number of (not all) applications. These results have been extensively chronicled quantitatively in the published works of JeanPaul Clech of ESPI Inc. Because SAC 305 has been accepted and implemented in many production facilities, statistically valid data is available on costs, yields, reliability, etc. This allows the industry to approach, scientifically, the optimization of LF products, materials, and processes.
There remain long-term reliability questions for some Hi-Rel products. "Several scientists at the IPC/Soldertec conference in San Sebastian agreed that nearly 90% of all electronic products will not be affected by RoHS's (i.e. LF's) impact on reliability and quality, but the remaining 10% will," reported the IPC Review for March-April 2007. Extensive accelerated life testing is ongoing in military, aerospace, telecommunications, and other affected industries. Testing and analysis techniques are being refined and expanded, especially for failure models necessary in determining the acceleration factors that correlate test results to real-life conditions. The results of these efforts will provide a better understanding of reliability parameters for design of future products. Of course, as Dave Hillman of Rockwell/Collins advises, "we will never be 100% certain of product reliability until we build some and put them in the field for 10 or 20 years." But current testing efforts should minimize the risks.
Many current optimizations are based on more in-depth evaluations and modifications to LF solder alloys. All optimizations are ultimately aimed at reducing costs, but specific efforts include further utilizing LF's material advantages over tin/lead, such as increased strength, while minimizing or even overcoming weakness such as brittleness and reduced wetting. It appears possible to not only equal, but exceed the performance of tin/lead for many applications. Extensive research and testing was performed in the selections of the SAC alloy and this information is providing valuable input for optimization efforts.
Early research on the SAC alloy family demonstrated consistent material properties (liquidus, wetting, strength, ductility, etc.) over formulation ranges of 15% silver (Ag) and 0.31.0% copper (Cu). Many combinations other than 305 have been utilized in LF products, but current investigations are choosing very specific Ag/Cu percentages to address related solder process and product performance issues. Examples of this are the evaluation by IBM Canada of SAC 310 balls (Ag 3.0%, Cu 1.0%) for BGA packages to minimize the dissolution of copper from PCB pads during reflow, and the use of SAC 105 to reduce failures in drop tests of handheld electronics.
Another area of optimization is the large number of efforts underway to improve specific characteristics of SAC by adding small percentages of additional chemical dopants. The addition of bismuth (Bi) has been evaluated by many sources for several years with widely varied results. More recently small additions of nickel (Ni), cerium (Ce), manganese (Mn), titanium (Ti), and other metals are being evaluated for improved properties. Improvement in the durability of handheld electronic devices in drop tests has been widely investigated with these alloys, displaying promising results particularly with nickel-gold (ENIG) PCB finishes. Modifications in wetting dynamics to reduce tombstoning of 0201 and 01005 chip components are also being evaluated. All are predicated on the principle that these small additions, typically less than 0.5%, do not affect other fundamental properties such liquidus temperature or strength, and thus represent not a change from SAC but rather an optimization of the basic alloy system.
In wave soldering, for some time an increasing number of manufacturers have been moving away from SAC to tin/copper (SC) based alloys. If we go back in time to the original NEMI (now iNEMI) solder evaluations and recommendations it will be recalled that the first recommendation was SAC for reflow and SC for wave. The reason? Cost. Because of the 3% Ag, SAC can cost more than 70% above SC in bar form (the difference is much less for paste). Also, the higher liquidus temperature for SC, 227°C vs. 217°C for SAC, was found to be acceptable for wave soldering. The recommendation was later changed to the use of SAC for both wave and reflow because of process and reliability issues, such as hole-fill and fillet lifting. More recently, however, an alloy has been produced containing a small amount of nickel added to SC. This SNC alloy, often referred to as "snick," appears to solve the original problems with SC while also providing reduced copper erosion, better cosmetic appearance, and reduced dross formation. SC formulations utilizing other materials such as cobalt (Co), zinc (Zn), or silicon (Si) to provide similar or even improved performance are currently being evaluated. All of these alloys provide cost savings by eliminating silver plus additional process and product improvements (typical optimization results).
Lead-free electronic assembly is certainly becoming, if not already, main-stream. Adequate historical data now allows our industry to proceed with the natural optimization of any new material and/or process. The quantitative data and insights generated in the development and now optimization of LF alloys, soldering processes, and reliability parameters are significantly better than our understanding of tin/lead, and will benefit the development of future electronic products and the industries that build them. The change to LF solders has not be easy for many, and for some it is not over yet, but we and our industry are all better off for the effort.
Jim Hall, principal consultant, ITM Consulting, can be contacted at P.O. Box 921, Durham, N.C., 03824, jim_hall@ITMconsulting.org.