Impact of palladium to the interfacial behavior of palladium coated copper wire on aluminium pad metallization during high temperature storage

Reliability of palladium coated copper wire is believed to have enhancement over bare copper wire. With current understanding, the main failure mode in copper wire bonding is galvanic corrosion of the less stable Cu₉Al₄ intermetallic compound (IMC) phase in the highly accelerated stress test (HAST). In gold wire bonding, palladium (Pd) is known to retard formation of the corrosive prone Au₄Al phase, which resulted in better reliability in high temperature storage (HTS) test. Recent literature on palladium coated copper wire suggested that also in copper-aluminium interfaces, IMC growth of Cu₉Al₄ is slowed down by palladium-enriched layer. However it is not fully understood on how the palladium in palladium coated copper wire interacts with the other metals within this interface. This paper covers the study of IMC phase growth rate using a palladium coated copper wire on an aluminium bond pad, palladium distribution and identification of IMC phase by means of optical microscopy, SEM and TEM. Bonded balls of palladium coated copper wire were prepared using different EFO sparking condition to achieve an obvious palladium rich phase layer at the interface and another condition without palladium rich phase. The formation and growth of the IMC phase can be achieved by subjecting the bonded ball to high temperature storage (HTS) test in an un-molded device and analyzing the IMC phase at different aging durations. The results of this IMC study could give insights as to which initial interface morphology is more robust against the common corrosion in HAST test.