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Predicting Optimal Material Combinations for Copper Wire Bond Processes Through Finite Element Analysis Models
Keywords: Finite Element Simulation, Cu Wirebond, Optimal Material Combinations
Copper wire bonding is an interconnect technology that serves as a viable cost saving alternative to gold wire. The excellent mechanical and electrical characteristics favor fine-pitch applications, and high-speed power management devices. However, there are some challenges to assembly with the Cu-Al metal system. The CuAl-intermetallic compounds (IMC) can induce a mechanical failure at the bond interface which can cause high resistance. The increased stiffness of copper wire while it benefits fine-pitch bond application(less wire sway), may not be favorable for stresses introduced during temperature cycling test where fatigue damage (inter-granular damage) to the wire may occur. This work presents results of finite element analysis (FEA) modeling that examines the two distinct package failure modes observed in copper wire bonding, wire neck and stitch-bond (2nd bond) “heel“ fractures, and lifted bonds at the pad. These failures generally do not occur during wire bond, but rather after being subjected to reliability testing, or in the customer application. The package surrounding mold material puts stress on the bond wires when it expands and contracts during test-like temperature-cycling. These failure modes can be significantly reduced or eliminated with the proper choice of package system mold compound and bond wire materials. FEA simulations of these type failure modes were performed on several package types and included varied mold compounds and bond wire. The simulations predicted what combination of mold compounds and bond wire would reduce stress levels and hence possibly eliminate these failure modes.
Aditi Mallik, Principal Engineer
ON Semiconductor
Phoenix, Az

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