Copper wire bonding — Elimination of pad peel

Abstract

Since starting production with Copper wire bonding for almost a decade, there are still many findings in order to achieve the most stringent quality requirements. Furthermore, some older machine platforms were also converted to Copper wire bonding in order to increase production capacity. In the cases of running extremely sensitive devices, there were many occurrences of lifted bond and pad peel defects at the same time. From the initial assessments, even those bond pad structure which was improved until reasonable robust level could still fail for pad peeling issue. On top of that, some pad peeling cases could not be simulated, rejects could not be duplicated. In order to understand the pad peeling issue, comprehensive study was conducted in order to eliminate this issue. The first set of evaluations was performed on bonding parameters, and results met most of the expectations. The results showed parameters of bonding time and bonding force did not provide much significant effect as long as they were operating within reasonable levels. As for the bonding power, its effect exhibited weird scenario. When power increased, ball shear strength was also increased accordingly. However, pad peeling defect was not able to be correlated linearly to ball shear strength, other than higher power made it becoming worse. In order to understand this issue, another new approach was initiated, equipment characterization along with process optimization. With new optimum process window, equipment characterization was performed in depth. From all the characterized machine parts, it was found that capillary installation was the most suspicious root cause. With the Copper kit installed at the firing location, normal installation jig was no longer useful. The capillary setup became inconsistent, and capillary could be slanting in any direction. The most common setup would be either capillary slanting towards operator or away from operator. One of this slant directions might interact with the transducer movement and vibration, resulted severe pad peeling rejects. In order to resolve this root cause, a breakthrough capillary installation jig was invented and deployed in production. This new jig enabled stress-free and self-center capillary setup. The proper installed capillary would allow desired ultrasonic energy transfer to the Copper ball, without any mechanical stress point. The overall improvement was combining optimum process window and consistent capillary setup. With deployment in production, pad peel issue has reduced to zero.