FAILURE PREDICTION OF WIRE BONDS DUE TO FLEXURE

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

InterPack2003 -35136 WIRE FLEXURE FATIGUE MODEL FOR ASYMMETRIC BOND HEIGHT

ABSTRACT This traditional qualification test procedure has several shortcomings. First, the selection of the temperature cycle magnitude and duration is often arbitrary, and the results of the testing are not properly correlated to field life. Second, the procedure is costly and time consuming and is therefore undesirable in today's product development environment of shortened design cycles and quick time-to-market. It is no longer considered best practice to make a prototype, subject it to a series of standardized tests, analyze the failures, fix the design, and test again. A fundamental model that can be used before testing to assess the susceptibility of module designs to wire flexural fatigue is therefore extremely desirable both to minimize testing and to aid in the proper interpretation of the test results. The use of such models to qualify assemblies for field use is known as virtual qualification. This paper presents such a model that can be used to assess the likelihood of wire failure due to cumulative damage resulting from repeated flexure during thermal cycling. This paper presents the first physics-of-failure based life prediction model for flexural failure of wires ultrasonically wedge bonded to pads at different heights. The life prediction model consists of a load transformation model and a damage model. The load transformation model determines the cyclic strain at the heel of the wire during temperature cycling. This cyclic strain is created by a change in wire curvature at the heel of the wire resulting from expansion of the wire and displacement of the frame. The damage model calculates the life based on the strain cycle magnitude and the elastic-plastic fatigue response of the wire. The model supports virtual qualification of power modules where wire flexural fatigue is a dominant failure mechanism. The model has been validated using temperature cycling test results, and can be used to derive design guidelines and establish a relation between accelerated test results and field life