Prediction of high cycle fatigue in aluminum bond wires: A physics of failure approach combining experiments and multi-physics simulations

Abstract

Aluminum wire bonds, as used in a ceramic air cavity package for LDMOS, will intrinsically be prone to mechanical fatigue due to temperature and power cycling causing the wires to expand and shrink in a cyclical way. Under certain pulsed application conditions, the required amount of current cycles the product must survive is so high that not just low cycle fatigue, caused by cyclic plastic deformation, but also high cycle fatigue becomes a concern. This paper describes how in-situ monitored power cycling experiments, using the Joule heating of the bond wires, were performed on dedicated test structures at different stress levels with wire loop shapes and test settings critical enough to find failures within reasonable test times. Wire bond settings were varied to create different amounts of initial damage as introduced by the plastic deformation of the heel and the wedge. Finite element method was employed to calculate the stress amplitude in the heel of the bond wire in the experiments as function of current, pulse time and loop shape. This required a multi-physics approach using coupled electro-thermal and sequentially coupled thermo-mechanical simulations. The amount of initial damage was also estimated, using 2D FE simulations, in order to quantitatively take into account the initial plastic strains. With the measured failure times (Nf) and calculated stress amplitude (S) the durability or S-N curves for different amounts of initial damage could be derived and fitted with the Basquin model. These fitted models were used to predict the expected lifetime for specified field conditions. Furthermore the models can be used to derive `design for reliability rules' for wire loop shapes that will survive a specified user profile