High temperature storage (HTS) performance of copper ball bonding wires

Even in a fully annealed state, copper is a naturally harder material than gold. The hardness of copper, and its greater tendency to undergo strain and strain rate hardening, has created difficulties in implementing it for widespread fine pitch ball bonding because of gross cratering or, worse still, unseen damage to the chip that only `develops´ during reliability tests such as high temperature storage. It is commonly perceived that chip damage problems can be solved by using softer copper wire. However, there is little hard evidence to fully support this perception and even soft copper wires can rapidly work harden during ball bonding. In this study, the bonding performance of several copper wires with various properties is compared on 10,000Aring thick Al-1%Si-0.5%Cu pad metallisation. Attempts are made to correlate the wire properties with the bonding responses. Devices were also subjected to high temperature storage (HTS) and compared in terms of damage development (cratering)