Comparison and mechanism of electromigration reliability between Cu wire and Au wire bonding in molding state

Electromigration (EM) reliability and failure analysis of Cu wire and Au wire bonding processes on Al substrates after molding were investigated and compared systematically. According to the resistance as a function of EM time trend, there were four stages during the EM test for wire bonding that can tentatively be referred to as the fluctuating resistance stage, stable resistance stage, rapidly increasing resistance stage and quick failure stage of the sample. When the applied current was 1.8 A on a wire with a diameter of 20 μm (at a current density of 5.7 × 10⁵ A/cm²), the lifetimes of the Cu wire and Au wire bonding processes were 1032 h and 119 h, respectively, according to Weibull analyses. Two kinds of failure modes were observed: wire fracture and first bond fracture. Almost 90% of the failed samples belonged to wire fracture category. After the EM test, the thickness of intermetallic compounds (IMCs) at the anode interface was similar to that at the cathode interface for both the Cu wire and Au wire bonding cases, which means that the normal polarity effect during electromigration did not occur here. However, the thickness of the IMCs at the Au/Al bonding interface was thicker than that at the Cu/Al interface for both the aged and EM samples. Voids and cracks were observed within the Au wire-bonding interface. It is suspected that the Cu wire bonding process had a slower thermal diffusion and smaller EM flux than those of the Au wire bonding process. Due to a decrease in vacancy flux within the Cu wire bonding region, the cracks were also suppressed, which greatly extended the lifetime of the Cu wire bonds.