Copper wire off-center ball and 2nd smash bond issue resolution on XQFP package

Abstract

For the past years, acceptance and implementation of copper wire has been become popular and mandatory, especially after gold price soaring for past these years. As compared to gold wire, copper wire poses better electrical conductivity, better thermal conductivity, higher mechanical strength, lower cost and better reliability and so on. But in mass production, we may not be able to achieve desirable performance and manufacturability due to the well-known disadvantages of copper wire. One is its greater hardness, another is being easy to be oxidized. These disadvantages bring challenge to copper wire such as none stick, weak bonding, missing ball, off-center ball, cratering, short tail, smash bond etc.In this paper, off-center ball and 2nd smash bond issue were investigated. a test vehicle with 66um fine pitch and 1.2um bond pad metal thickness was selected as vehicle to study the mechanism of off-center ball and 2nd smash bond issue. First, fish bone analysis was conducted to dig out root cause. Fish bone analysis shows wire tail, EFO current setting and lead frame floating during bonding were suspected to the root cause to the off-center ball and 2nd smash bond. Second, several types of capillary with different OR, tip size and FA were screened. Based on screen result, capillary design was finalized. Second, a DOE was conducted to 2nd bond parameter to resolve off-center ball and 2nd smash bond issue. The factors are 2nd bond force and 2nd bond USG, and 2nd bond scrub offset and phase. Stitch pull strength, 2nd bond width and abnormal 2nd bond shape are responses. 2nd bond parameter was established based on DOE result. Last, EFO parameter DOE was conducted. The factors are EFO time and EFO current. FAB size, FAB roundness and abnormal FAB are responses. Besides, a DOE was conducted to verify 2nd bond parameter. 2nd bond force and 2nd bond USG, and 2nd bond scrub offset are factors, FAB size, FAB roundness, abnormal FAB and stitch pull are responses. EFO parameter window was established based on DOE result. Confirmation run was designed with optimized FAB and 2nd bond parameter. No off-center ball and 2nd smash bond issue were found in high volume production. 2nd bond parameter optimization plays a great role in resolving off-center ball and 2nd smash bond issue.