Effects of bare cu wires' mechanical properties on ball bond profile

Abstract

Due to the advantages of low cost, good electrical/thermal properties and reliability performances, Cu wire had been used in the semiconductor industry for IC connections many years instead of Au wire. While Cu wire was harder and easier to be oxidized, this degraded its bond abilities and introduced some failures. During the primary evaluation of 33 microns bare Cu wire to replace 33/38 microns Au wire in one SOIC device, there were many irregular ball bonds with poor profiles after wire bonding; which may result into potential risks of quality and reliability failures. Based on the fish bone analysis and the following root cause validations, `inconsistent wire properties' and `improper EFO (electric-flame-off) parameters' were suspected as the root causes. Then the source of variance analysis confirmed that the former one was the main root cause. The comparison of cross sections found that the coarser and more inconsistent grains might result into the irregular ball bonds. Cu wires/FABs (free-air-ball) with finer grains would deformed more consistently and got less irregular ball bonds; while they were harder and had higher breaking load values, thus introducing impacting damages to bond pads. So the wire properties were controlled to get a balance between the ball bond profile and effects on bond pads, through controlling the breaking load considering its easy feasibilities. The final control limits of wire breaking load were shrunk to 17-19gf for this case based on the final confirmation run results.