Study Of The Reliability Of Power Device With Lead-free Heat-sink Attachment

This paper focused on investigation of lead free soldering and reliability. And it was divided into two main parts.In the first part, the effects of voids in Sn-Ag-Cu heat-sink attachment on thermal resistance of power device and thermal mechanical properties were investigated systematically. Factors affected the formation of voids in solder attachment were discussed. The results showed that voiding percentages had significant effects on heat dissipation. Compared to voids size, the location of voids is less significant. The changes of thermal resistance were less than 1% with 10% void at different typical location. The effect of void size in heat-sink attachment on equivalent plastic strain was not significant when the voids were at the center of solder attachment. Voids location had more significant effect on maximum equivalent plastic strain compared to voiding percentages.In present experimental design and process parameter window, the effects of solder type, reflow profile, varies component and PCB pad finishes, the oxidation of surface finishes and thickness of solder attachment on voids formation were less significant. The maximum voids percent was less than 5%. The samples with big voids had more severe voids growth, interface delamination and solder fatigue crack than the samples with small voids. But the Sn-Ag-Cu heat-sink attachment with the voiding percentages 33%-48% survived thermal shock cycles above 2500, which agreed with the simulation results. The effect of voids size in heat-sink attachment on heat-sink attachment thermo-mechanical reliability was not significant.In the second part, intermetallic compound formation (IMCs) formed at different conditions and their evolution during high temperature storage and thermal shock cycling was studiedWhen the initial grains size of Cu₆Sn₅ was small, the grains grew in size very fast during 125℃ high temperature storage. But when the initial grains size was big, the changes of Cu₆Sn₅ grains size were not significant during 125℃ high temperature storage.When device side was Cu and NiAu surface finishes was used on PCB pad, the (Cu,Ni)6Sn5 grew larger at both interfaces, but its morphology did not change very much during thermal shock cycling. When HASL surface finishes was used on PCB pad, both CusSns and Cu3Sn layers grew and the morphology changed from shell type to layer type quickly during thermal shock cycling.When device side was Cu and HASL surface finishes was used on PCB pad, Kirkendall voids formed at both interface. And the concentration of Kirkendall voids at device/solder interface was the important failure mechanism. But when NiAu surface finishes was used on PCB pad, there was no Kirkendall voids been detected until 2500 thermal shock cycles.