Wirebond Reliability Assessments and Layout Guidelines for a TO247 Package Using FEA

Abstract

Automotive electronics design and manufacture present unique challenges for product reliability. Recent trends toward mounting electronic systems directly onto engines introduce a new level of severity in terms of thermal and vibration environments. Automotive microelectronic packages experience thermal stresses that can possibly cause components and electrical interconnection damage. The loss of electrical interconnection due to coarse Aluminium wires in electronic modules is one of the most critical reliability problems in automotive electronics packaging. The paper intends to help understand and investigate the reliability of Aluminium wire bonds that make the electrical connection between integrated chips and the leads inside a typical leaded electronic package like TO247. Finite element modeling (FEM) is used to simulate the stresses the package goes through during reliability tests. Results are compared to empirical testing. The FE model consists of the entire TO247 package including the leadframe, overmold, chip, substrate, Al wire loops and corresponding bond pads. Results from simulations provide information to map out high stress locations on the heels of the wirebonds due to package warp. In addition, the results also provide manufacturing engineers the insight to develop general bonding guidelines for Aluminium wires. As a result of this exercise, the development team is able to counter the issue and provide a design guideline to design the leadframe, place components away from high stress locations and pursue a more symmetrical layout to reduce the effect of thermal stresses.