MIXING METALLURGY: RELIABILITY OF SAC BALLED AREA ARRAY PACKAGES ASSEMBLED USING SnPb SOLDER
Authors: Heather McCormick, Polina Snugovsky, Zohreh Bagher Company: Celestica Inc. Date Published: 4/1/2007
Abstract: As the electronics manufacturing industry transitions towards lead free assembly, component suppliers are creating RoHS compliant versions of their packages. Some area array packages are now manufactured only with SAC solder balls, with no SnPb balled version available. As a result, it has become necessary to integrate SAC balled area array components into tin/lead assemblies. In many cases, the maximum temperature ratings on other components on these “mixed” assemblies limit the maximum temperature at which they can be reflowed. This precludes the use of a typical lead free reflow profile, and prevents the SAC component balls from melting completely. Depending on the profile, these “mixed metallurgy” joints can appear fully mixed, or may show two clear portions – one rich in SnPb, and one rich in SAC. A test vehicle was used to assess the degree of mixing of the SAC and SnPb solders during the reflow process at various temperatures and times above liquidus. Based on these results, six process conditions were used to assemble larger quantities of test vehicles for reliability testing. Interim reliability test data suggests that the failure rates under 0C to 100C accelerated thermal cycling are similar for fully mixed and unmixed solder ball microstructures.
Key words: Lead Free, Mixed Joints, Backwards Compatibility