FAILURE PREDICTION OF WIRE BONDS DUE TO FLEXURE

Abstract

Solid state power modules are subjected to harsh environmental and operational loads. Identifying the potential design weakness and dominant failure mechanisms associated with the application is very critical to designing such power modules. Failure of the wedge-bonded wires is one of the most commonly identified causes of failures in power modules. This can occur when wires flex in response to a thermal cycling load. Since the heel of the wire is already weakened due to the ultrasonic bonding process, the flexing motion is enough to initiate a crack in the heel of the wire. Owing to the prevalence of this failure mechanism in power modules, a generalized first-order physics-of-failure based model has been developed to quantify these flexural/bending stresses. A variational calculus approach has been employed to determine the minimum energy wire profiles. The difference in curvatures corresponding to the wire profiles before and after thermal cycling provide the flexural stresses. The stresses/strains determined from the load transformation model are then used in a damage model to determine the cycles to failure. The model has been validated against temperature cycling test results. The effects of residual stresses, that are introduced during the loop formation, (on the thermal cycling life) of these wires also has been studied.

It is believed that the ultrasonic wirebonding process renders the wires weaker at the heel. Efforts have been made to simulate the wirebonding mechanism using Finite element analysis. The key parameters that influence the wirebonding process are identified. Flexural stresses are determined for various heel cross-sectional profiles that correspond to different bond forces.

Additional design constraints may prevent some of the wedge-bonded wires from being aligned parallel to the bond pads. The influence of having the bond pads with a non-zero width offset has been studied through finite element simulations. The 3D minimum energy wire profiles used in the modeling has been obtained through a new energy minimization based model.