Reliability Evaluation of BOAC and Normal Pad Stacked-Chip Packaging Using 90nm Low-K Wafers

This work evaluates the bondability and the reliability tests for the stacked-chip TFBGA wire bond packaging with the Sn-4.0Ag-0.5Cu lead-free solder ball. The bonding-over-active-circuit (BOAC) pad is the top test chip and the normal pad is the bottom test chip and are combined in the stacked-chip packaging. Both test chips are 90 nm low-K dielectric with six copper layers and one layer aluminum pad and a background ranging from 775 mum to 150 mum. To compare the bonding pad strength for the BOAC and normal pad low-K wafers, this study uses depth-sensing indentation method to identify the effective Young's modulus and hardness. The effective average Young's modulus of the normal pad and the BOAC pad are 86 GPa and 69 GPa, respectively. The test results indicate that the effective Young's modulus of the normal pad exceeds that of the BOAC pad by 17 GPa. The average hardness of both test pad structures is very similar, and measures 0.67-0.7 GPa. The wire bonding parameters for both test pads in this study are the same as the normal and the non-low-K pad structure. The bondability test of the ball shear and the wire pull test results are superior to the criteria by 80% and 83.75%, respectively. All stacked-chip TFBGA packaging samples underwent reliability tests, including HAST, TCT, and HTST All the bondability and reliability tests passed the criteria for the BOAC pad and the normal pad low-K structures. Accordingly, this work shows that the proposed stacked-chip TFBGA packaging passes the bondability and the reliability tests. The proposed packaging improves the electrical performance, enhances the utility of the active chip area and saves chip area through the use of low-K and BOAC chips. Furthermore, the results show that the effective Young's modulus can be the index for the pad structure strength, which is obtained from the depth-sensing indentation test.