Lead free alloys for flip chip bumping

In addition to the ecological and electrical aspects of area array packages, there is a need to develop new solders with similar process characteristics and usage cost to Pb/Sn solders, but which are lead free and have improved mechanical properties and microstructural stability. As operating temperatures increase, standard alloys are unable to withstand the increased fatigue caused by the thermal cycling. Current lead-tin solders lack shear strength, creep resistance and thermal mechanical fatigue resistance. A solder which exhibits enhancements of these properties is crucial in automotive and other high-reliability products and high-temperature assemblies. In electronic packaging, it is common practice to solder the various levels of the package with different solders of different melting points so that the soldering of each successive level or step does not remelt the previously soldered joints. Depending on the particular applications, the most suitable solder alloys for area array packages (flip chip, CSP, BGA) are presented in this paper. Initial reliability results are also given.