Impact from IMC thickness on the reliability of wire bonding

Materials to replace gold wires in wire bonding devices have been pursued for years in industries for cost reduction. The most important factors, besides the price and electric and thermal performance of the material itself, is the growth rate of the intermetallic compound (IMC) at the interface, which would severely affect the reliability of the devices. In this paper, we studied and analyzed the IMC growth for Au, Cu and Ag wires with Al pads. High temperature storage (HTS) aging experiments were employed for accelerated test. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometer (EDS) were adopted to investigate the IMC at the interface of the wires and the pads. IMC growth including Kirkendall holes and cracks were examined and compared. It is found that in the samples with gold wires, Kirkendall holes and cracks formed at the interface, which led to a high increase of electrical resistance and possible ball lift. When using copper wires instead of Au wires, although the IMC growth rate reduced, the limitation of Cu wires is due to the high hardness and yield strength at a high temperature. On the other hand, the surface of copper is easy to produce a thick oxide layer, sometimes leading to earlier failures than using Au wires. Compared with copper wires, silver wires are softer. And it is found that IMC growth rate of samples with silver wires is lower than those with gold wires. Correlations between the IMC and the reliability will be discussed.