Study of 6 mil Cu Wire Replacing 10–15 mil Al Wire for Maximizing Wire-Bonding Process on Power ICs

Abstract

Copper (Cu) wire-bonding with its advantage in cost, mechanical enhanced characteristic, and better electrical performance is a developing alternative interconnection technology to replace gold (Au) and aluminum (Al) wires in IC packaging manufacturing. This paper discussed the experimental study of using 6-mil Cu wire on an ASM wire bonder to replace 10-15 mil Al wire in a power IC device. It encompassed wire and tool selection, wire-bonding process development, post wire-bonding integrity inspection, and bonding reliability results. The wire and tool selection included type of wire, capillary use, and bonder capability. The process development focused on two crucial stages, Free air ball (FAB) formation and bonding process window development. Design of experiment (DOE) was extensively applied in this study. The experimental studies showed that flow rate of forming gas was a key factor to form the qualified FABs and establishment of a workable process window. Wire pull and ball shear tests were conducted per JEDEC criteria for bond strength integrity. Moreover, crater test and Zygo's 3-D measurement were used to inspect any risk of underlying metal integrity in die before reliability tests. The data showed that sufficient thickness of the Al bond pad was crucial to avoid any underlying metal damage when subjected to heavy copper wire bonding forces. High-temperature baking (HTB) and pre-condition (PC) and temperature cycle (TC) were used to evaluate the samples reliability. The results of the two reliability tests showed that Cu/AL intermetallic compound (IMC) growth was slow, which indicates potential significant product life-span extension. The study concluded that with current thermosonic ball bonder, using 6-mil Cu wire could replace heavy 10-15 mil Al wire in power IC applications.