Development of 6mil Copper Wire Bonding for Replacing 10–15mil Aluminum Wire

In IC package assembly, copper (Cu) wire bonding is a developing alternative interconnection technology for its advantages of more cost saving and better mechanical and electrical characteristics than gold (Au) and aluminum (Al) wires. This paper discusses experimental development of 6mil Cu wire bonding on ASM wire bonder to replace 15mil Al wire in a power IC package. It covers wire and tool selection, wire bonding process development, wire bonding integrity inspection and reliability study. The wire and tool selection includes wire type, capillary and wire bonder. The process development focuses on two crucial stages, Free Air Ball (FAB) formation and bonding process window development. DOE approach is introduced into the process development. The experimental studies show that flow rate of forming gas is a key factor to form the qualified FAB and a workable process window is established. Wire pull and ball shear tests are conducted to measure wire bonding strength of the built samples based on the criteria of JEDEC. Meanwhile, crater test and Zygo’s 3-D measurement are carried out to inspect any risk of underlying metal integrity in die before reliability tests. The data discloses that sufficient Al bond pad thickness is needed to avoid underlying metal damage during bonding process. The results of the two reliability tests, High Temperature Baking (HTB) and Pre-Condition (PC) & Temperature Cycle (TC), show that Cu/AL IMC growth is slow and accordingly lifetime of product may significantly extend. The overall result of the study finally indicates that with current thermosonic ball bonder, 6mil Cu wire is feasible to replace heavy 10–15mil Al wire in power package assembly.