Measuring Manufacturing Yield for Gold Bumping Processes Under Dynamic Variance Change

Recently, the technology of gold bumping has become more popular due to high demand for LCD driver ICs. The requirement of higher resolution application, however, will increase the difficulties for manufacturing the gold bumps due to their high pin counts. For gold bumping processes, bump height is one of the key parameters to control process yield. In reality, some inevitable process variations and shifts regarding the bump height may occur under dynamic manufacturing environment. Conventionally, manufacturing yield for gold bumping processes is calculated under the assumption that the processes are stable. In practice, however, the processes are dynamic, particularly, in the operation of Au-plating in gold bumping factories. To obtain accurate measure of the manufacturing yield, we present a capability index method for manufacturing yield calculation with dynamic variance change considerations. Using this method, the magnitude of the undetected variance change, which is function of the detection power of the S 2 chart, is incorporated into the adjusted calculation of manufacturing yield. The detection powers of the S 2 chart under various subgroup sizes are tabulated. For illustration purposes, a real application in a gold bumping factory which is located in the Science-based Industrial Park in Hsinchu, Taiwan, is presented.