Nonlinear elasto-plastic analysis of wire bond reliability for optical LQFP package

Nonlinear thermo-mechanical simulation is performed on optical low-profile quad flat package (OLQFP) to understand the root cause of the failure and to optimize the package design for improved wire bond reliability. 3D finite element model is established. Gold wire is considered as elasto-plastic material, which the properties are derived from the stress-strain curves measured at different temperatures. It is found that the total equivalent strain of the wire at stitch region is high during reflow process and on the other hand, the ultimate strain of gold wire at reflow temperature is low. Thus, the high strain in the stitch region during reflow is the root cause of the failure. The equivalent plastic strain and accumulated plastic strain during reflow and the following thermal cycling test are also analyzed in detail. Based on this methodology, the downset of die pad is optimized to improve the wire bond reliability during reflow. It should be noted that although the wire bond reliability may be improved during reflow by decreasing the downset, the life of the wire bond during the subsequent thermal cycling test may be compromised. Design guidelines are provided and discussed in the paper.