Electromigration Effect on the Pd Coated Cu Wirebond

Wirebonding with bare Cu and palladium-coated copper (PCC) wires have reemerged as mainstream packaging technologies. This paper investigates the failure mechanisms in the PCC wirebond packages under electromigration (EM) tests. First, the directionality of failures at high current density and temperature conditions is discussed. Results illustrated that the intermetallic compound (IMC) growth rate is different when the EM flux direction is in the opposite direction of the chemical flux. Energy dispersive X-Ray spectroscopy (EDX) results paved the way for a fundamental understanding of wirebonding failure. Intermetallic compounds of CuAl 2 and Cu 9 Al 4 are observed after interfacial focused ion beam (FIB) cuts are made to obtain clear images of the failure site through scanning electron microscopy (SEM) and EDX. The Pd coating relocates from the Cu wire surface towards the ball area during the initial bonding process. Therefore, a small concentration of Pd alloys with the bulk Cu within the wire, which is small enough to go unnoticed during the manufacturing process. However, the Pd concentration makes a large enough difference to reduce the contact area between the ball bond and the Al pad. This is due to the solid-solution hardening that causes the ball to undergo a greater amount of work hardening during the bonding process. The smaller contact area yields a greater current density and resistance, enhancing the EM effects in PCC wirebonds. Throughout the EM test, Pd atoms migrate toward the Al pad, creating a higher concentration(∼6at%) of Pd near the interface between the ball bond and the Al pad. Interestingly, Pd has a minor effect on the failure mechanism and crack propagation owing to its concentration and resultant increased hardness at the interface. These results can lead to future studies that may lead to improvements in PCC wirebonding reliability.