Mechanical and Electrical Properties of Au-Al and Cu-Al Intermetallics Layer at Wire Bonding Interface

Mechanical and electrical properties of the Au-Al intermetallics layer were compared to the Cu-Al intermetallics layer. A thermal aging test was used to expedite the formation and growth of the intermetallics layer. Dynamic hardness, surface hardness, and intermetallic thickness of both bonding systems was compared. Unencapsulated samples were subjected to a ball shear test while encapsulated samples were electrically tested. Results showed that the Cu-Al intermetallics layer is relatively harder than Au-Al. Both intermetallic thicknesses grow linearly at about 18.0 and 0.07 nm/h, respectively, but the Au-Al intermetallic layer is 20 times thicker than Cu-Al before the aging test. The 1.5-μm growth of Au-Al intermetallic thickness acquired an additional shear force of 160 gf. The threshold value of the intermetallics layer thickness for the Au-Al and Cu-Al bonding system is 3.8 and 0.14 μm, respectively, before it shows degradation in shear force. Electrical resistance for the Au-Al system is higher than the Cu-Al system, especially at higher aging readout points.