Lead-Free Solders: A Change in the Electronics Infrastructure
C A. Handwerker
The infrastructure for electronics assembly has, from its infancy, been based on tin-lead solder (Sn/Pb), a mixture of tin and lead in the ratio of approximately 63/37 (by weight) that melts at 183 C. Since legislation to eliminate lead from electronics manufacturing was first proposed in the early 1990s, many of the major U.S. electronics manufacturers have been concerned that their ability to produce durable, reliable, safe, and affordable electronic products might be compromised with alternative solders or other interconnection materials. In the last seven years, our knowledge of die manufacturing product performance, and reliability of lead-free solders has increased to the point where we now understand the majorroadblocks in the conversion to lead-free interconnections and can formulate plans to address them. These roadblocks can be easily understood in the context of the definition of a drop-in replacement for eutectic tin-lead solders. Such an alloy would exhibit the following attributes.1. Toxicity: Lower toxicity than Pb.2. Cost and availability: Equal to or better than eutectic Sn/Pb.3. Manufacturing requirements: Melting behavior the same as Sn/Pb: wetting behavior equal to or better than Sn/Pb. This includes: A liquidus temperature very near 183 C to maintain the fluxes, temperature profiles, and assembly processes used for eutectic Sn/Pb. (The liquidus temperature is the temperature above which the alloy is completely liquid.) A pasty range close to 0 C. (Since eutectic Sn/Pb changesfrom a liquid to a solid at a single temperature, 183 , its pasty range is zero. Alloys that are not eutectic have a non-zero pasty range.) No detectable low-melting phase formed during cooling. (Some alloys have liquidus temperatures close to 185 C, but they solidify at temperatures as low as 117 C.) Wettability comparable to eutectic Sn/Pb on a variety of surface finishes with various fluxes.4. Reliability: Performance at least as good as eutectic Sn/Pb in thermal cycling and end-use environments (consumer electronics, telecommucications, military, aerospace, and automotive) for all component types. The three storage and operating temperature ranges of general interest are: -55 to +125 C for military electronics -55 to +100 C for consumer electronics and telecommunications -55 to +180 C for aerospace and automotive electronics.