Abstract
Ultrasonically bonded heavy Al wires subjected to a small junction temperature fluctuation under power cycling from 40°C to 70°C were investigated using a non-destructive three-dimensional (3-D) x-ray tomography evaluation approach. The occurrence of irreversible deformation of the microstructure and wear-out under such conditions were demonstrated. The observed microstructures consist of interfacial and inter-granular cracks concentrated in zones of stress intensity, i.e., near heels and emanating from interface precracks. Interfacial voids were also observed within the bond interior. Degradation rates of ‘first’ and ‘stitch’ bonds are compared and contrasted. A correlative microscopy study combining perspectives from optical microscopy with the x-ray tomography results clarifies the damage observed. An estimation of lifetime is made from the results and discussed in the light of existing predictions.
Highlights
Wire bond cracking and lifting-off under the thermomechanical loading experienced during electrically-driven load-cycling is extensively reported, and is perhaps the dominant life-limiting wear-out mechanism in power electronic modules
Accuracy of wire bond lifetime prediction is an essential aspect of the reliable design of power electronics components and critical to prognostics and health management.[1,2]
In this paper, supported by the x-ray Computer tomography (CT) nondestructive evaluation approach, we examine what happens under a small scale temperature fluctuation by observing the same wire bonds over time, and we weigh our observations up against existing knowledge and theories regarding degradation under small junction temperature fluctuations
Summary
Wire bond cracking and lifting-off under the thermomechanical loading experienced during electrically-driven load-cycling is extensively reported, and is perhaps the dominant life-limiting wear-out mechanism in power electronic modules. For applications which ordinarily experience ‘‘small’’ temperature fluctuations (i.e. small DTs), accelerated testing is often relied upon to provide service life estimations This approach has a major drawback, namely the existing lack of experimental evidence about degradation mechanisms under such low loads, which are at risk of being concealed by the mechanisms produced by the larger DTs often used to generate the information needed. With high enough spatial resolution, near-microstructural characteristics can be observed In cases such as wire bond degradation under small temperature fluctuations where testing ‘to failure’ may be infeasible due to time constraints, a prioi knowledge of the condition of the interface obtained through tomography can be evaluated against datasets obtained at later stages of life. In this paper, supported by the x-ray CT nondestructive evaluation approach, we examine what happens under a small scale temperature fluctuation by observing the same wire bonds over time, and we weigh our observations up against existing knowledge and theories regarding degradation under small junction temperature fluctuations
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