Numerical and experimental correlation of high temperature reliability of gold wire bonding to intermetallics (Au/Al) uniformity

Abstract

A study of the Au/Al intermetallics compound (IMC) in gold wire bonds on aluminum pads is presented. A test vehicle has been deliberately built for high temperature storage (HTS) test. There are two kinds of samples, i.e., one is with uniform intermetallics in the ball bond at time zero, and another one is with non-uniform intermetallics. It is found that the package with initial uniform intermetallics passed the HTS test, while the package with initial non-uniform intermetallics failed with ball lift failure after HTS storage. Unique thermo-mechanical simulation is performed to understand the failure mechanism for HTS. Two types of intermetallics model have been established correspondingly, i.e., one with uniform intermetallics and another one with non-uniform intermetallics. The volume-induced stress under HTS has been simulated. It is found that the difference between IMC stresses in uniform and non-uniform models is not obvious, while there is a large difference between stresses in gold area of uniform and non-uniform model. For the non-uniform model, high normal tensile stress exists in the inner convex areas of gold bond, which connect two pieces of separate intermetallics, while for the uniform model, there is no such high stress area. The interfaces between Au and IMC in non-uniform model are the critical interfaces with high stress. It is indicated from these results that HTS life is much longer for the bond with uniform IMC than that with non-uniform IMC. The thermo-mechanical stress model correlates well with the test results.