Stress simulation for 2N gold wires and evaluation on the stitch bond shapes

Finite Element (FE) simulation is an effective approach to investigate the thermal stress status and the reliability trend when qualifying a new packaging material going through the reliability test. However, simulation of thermal stresses in gold wires was rarely reported in previous days due to the hardness in FE model building. This study performs 3D modeling to evaluate the thermal stresses in 2N gold wires with the full package model to identify which wires to peel for process control setup. The results from this modeling will be used to verify the second bond to file the process specification of wire peel test. The FE model in this paper describes a PBGA package with 2N gold wires surrounded by mold compound and other parts of the package. Stress evaluation is done through discussing the Von Mises stress, the equivalent plastic strain and the peeling stress. TC (temperature cycle) condition is applied in the modeling. The simulation results indicate that shorter wire(s) has higher stress than longer wire(s) and shorter corner wire(s) has the highest propensity to fail in TC. Different from the other wires, the highest peeling stress on bonding interface for the shorter center wire(s) is at the wire heel location instead of the tail end of the stitch bond. The wire heel location is usually regarded as the sensitive region related with the initial crack leading to the stitch bond failure. Thus, the shorter corner and center wire(s) should be regarded as high priority to be wire peeled and filed into wire peel specification. Besides, the relationship between stress status (reliability propensity) of stitch bond and the stitch bond shape is discussed. According to the measurement data of various stitch shapes, three typical stitch bond shapes (marked as A, B and C) are proposed and modeled. The modeling data show that the shortest stitch length (shape A) has the highest stress and plastic strain in the stitch bond comparing with the other shapes, and produces the highest peeling stress at bond interface. It might imply that the shortest stitch length could cause the higher propensity leading to bond lift in stress test. It could become a guideline for wire bond process that engineers should avoid forming the short stitch length when bonding 2N gold wires.