Effect Of Minor Aluminum On The Microstructure And Mechanical Behavior Of Low Silver Solder Alloy

Solder interconnects play the role of mechanical supports, signal transmission,and thermal conduction. Heat production of chips increases with increasing level ofintegration, leading to concern of solder interconnect reliability. Due to the advantagessuch as low melting point and good wettability, Sn-Ag-Cu ternary alloys become oneof the most promising replacements for traditional SnPb solder alloys. However,eutectic Sn-Ag-Cu solder is not very suitable for large scale production because of itshigh cost and weak resistance to mechanical shock. Low silver solder has goodresistance to mechanical shock but poor performance in thermal cycling due to thesparsely distributed intermetallic compounds in solder bulk. Furthermore, cracks werefrequently observed in the neck region near the upper interfaces of the BGA solderinterconnects. So a small amount of Al was added to low-silver solder to form Cu-Alintermetallic compounds, which have low density to float to the upper interfaces of thesolder interconnects. In this way, localized strengthening effect can be realized byadding aluminum to low-silver solder. This modified solder alloys can improve theperformance of solder interconnects in thermal cycling and keep the ductility oflow-silver solder alloys at the same time.This dissertation studies the methods of smelting a small amount of aluminumwith Sn0.3Ag0.7Cu solder to get fine intermetallic compoundswith good shape. Inaddition, the relationship between the shape and amount of Cu-Al intermetalliccompounds and cooling speed was studied, and the optimum addition amount wasdetermined.BGA solder balls were fabricated from modified bulk solder material and gooddistribution of intermetallic compounds in BGA balls was achieved. Additionally, theintermetallic compounds were dispersed evenly on theupper interface of the solderinterconnects. Also, the wettability and melting points of the modified solder withdifferent aluminum amounts were characterized by wetting balance and DSC, and thehardness of different regions in solder interconnects was characterized by Vicker’smicro-hardness testing machine. The hardness of localized strengthening region withfine Cu-Al particles inside increased dramatically, while other regions in the rest partof the solder interconnect was not significantly affected.