Fragility of Pb-Free Solder Joints

Category: No-Lead Soldering
Post Date: November 8, 2004
Author(s): Peter Borgesen, Surface Mount Laboratory, Universal Instruments; and Donald W. Henderson, IBM Corporation

Description
Recent investigations have revealed that Pb-free solder joints may be fragile, prone to premature interfacial failure particularly under shock loading, as initially formed or tend to become so under moderate thermal aging. Depending on the solder pad surface finish, different mechanisms are clearly involved, but none of the commonly used surface finishes appear to be consistently immune to embrittlement processes. This is of obvious concern for products facing relatively high operating temperatures for protracted times and/or mechanical shock or strong vibrations in service.

While fragility problems and the associated embrittlement mechanisms have long been known for both electroless and electrolytically deposited Ni/Au coatings, soldering to Cu has been viewed as 'safer' as far as robustness is concerned. However, recent observations suggest the existence of two or more embrittlement mechanisms in Pb-free solder joints on Cu pad structures, each leading to brittle interfacial fracture at the pad surfaces. With risks of embrittlement associated with all the commonly used solderable surface finishes, the electronics industry is currently confronting very difficult problems. The variability in the manifestations of these embrittlement mechanisms does, however, lend hope that some of these problems may be avoidable or controllable.

While it is imperative for each company to pursue its own interests in this arena, it is also clear that there is a common interest within the electronics industry in solving the reliability problems, associated with these solder joint embrittlement problems, particularly considering the short time frames associated with the transition to Pb-free solder technology. The consortium concept may be very usefully applied to this problem, which the industry is now confronting. A consortium effort could provide insurance for the member companies against overlooking critical phenomena or understandings. A consortium could also act as a common forum for advocating the infrastructure changes which will almost certainly accompany the required solutions to these problems.

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