A MICROSTRUCTURAL STUDY OF DISLOCATION SUBSTRUCTURES FORMED IN METAL FOIL SUBSTRATES DURING ULTRASONIC WIRE BONDING

A study has been conducted on the deformation mechanisms in metal subs trates subject to aluminum ultrasonic wire bonding (UWB). Aluminum wir es were bonded to copper, nickel, stainless steel, and aluminum bronze foil substrates and then removed in aqueous sodium hydroxide to permi t thin sections of bonded areas to be examined in the transmission ele ctron microscope (TEM). The results showed a variety of dislocation su bstructures formed during bonding, including dislocation cells, subgra ins, and planar arrays. Aluminum and copper showed evidence of thermal effects on microstructural evolution during bonding, such as dislocat ion annihilation at cell walls in copper and complete recrystallizatio n in aluminum. In the nickel and stainless steel substrates, which hav e higher recrystallization temperatures, thermal effects on microstruc ture were not observed. In addition, it was found that low stacking-fa ult energy (SFE) materials, such as aluminum bronze, were less likely to undergo cell formation, and only planar dislocation arrays formed. In general, it is clear that the process of UWB induces cyclic stresse s in the substrates, which exceed the yield strength of the metals exa mined. In addition, there is some heat generated during the bonding pr ocess, which can influence the resultant deformation microstructure.