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Investigation on Copper Wire Bonding Reliability on TQFP Package
Keywords: Copper wire bonding , green mold compound, Pressure Cooker Test (PCT)
Copper wire bonding is gaining popularity and moving to production mode as the technology matures. Besides bonding challenges, the reliability of the copper wire bond is beginning to gain its focus as it warms itself up for production readiness. For gold wire bonded package, its reliability focusing point is mainly on its intermetallic growth between the Au-Al system as Kirkendall void will develop over time under high temperature baking. Compared to Au-Al system, the Cu-Al system has a much slower growth rate. Hence, this is deemed to be a more reliable system. In this study, a daisy-chain die was copper wire bonded for reliability study of the TQFP package with two different green mold compounds. Phase 1 study, using five common type of reliability tests, screened out that both PCT and HAST tests should be the focus instead of the HTS test, which is the common focus in gold wire bonded package. Phases 2 and 3 tests explored different wire bonding conditions with two green mold compounds, focusing on PCT and HAST tests. The tests suggested that wire bonding condition can greatly enhanced the probability of passing PCT test. The key lies in how well is the bond between the Cu-Al interface, indicated by the Shear Strength per Unit Area of the bonded ball, as shown in Figure 1. The heavier wire bonding parameters had resulted in larger bonded ball diameter. This could have a constraint in fine pitch package. This is however, not universal. In package with the other mold compound, there is no visible co-relation for the PCT test and there were HAST test failure, whereas there were no HAST test failure with the previous mold compound. Therefore the strong interaction between the mold compound and wire bonding condition cannot be overlooked. From the interaction of the mold compound, it can be seen that the hydro-thermal stresses induced by the package plays an very important role in the PCT performance of the leadframe package.
Hong Meng Ho, Member of Technical Staff
Kulicke & Soffa (S.E.A.) Pte. Ltd.
Singapore 569871,
Singapore


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