Reliability and microstructure of lead-free solder die attach interface in silicon power devices

The use of lead in electronic consumer products is being phased out in exchange for lead-free alternatives. Understanding the physics of failure mechanisms within lead-free solder alloys will facilitate the replacement of lead bearing alloys for silicon device bonding in power semiconductor modules. Japan´s legislation for reduction in lead bearing solders is presently in place while European legislation is slated for 2006. Industry has identified an urgent need for engineering information about reliability and performance of lead-free alloys. Thermal stress and sustained high temperature work to deform the solder in the die-attach layer of power semiconductors. Solder is subjected to these stresses because it is sandwiched between silicon, which is a brittle material, and copper, which is harder than solder. Deformation is therefore driven by the difference in coefficients of thermal expansion of these layers. Solder deformation occurs under three different strain mechanisms: elastic, plastic and creep. In a thermal cycle, all three work together to fatigue the solder layer. Existing cracks and voids in the solder will increase in size resulting in reduced thermal performance.