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Stress Intensity of Delamination in a Sintered-Silver Interconnection
Keywords: Reliability, Delamination, Stress
The delamination front caused by thermal cycling was quantified with a large-area, sintered-silver (Ag) interconnection of a silicon nitride substrate and copper base plate. The nominal bonded area of the interconnect was 50 x 50 mm, temperatures were cycled between -40C and 150C up to 2,500 cycles, and several samples were processed in a similar manner to examine variability. Cycling was periodically paused every 100 cycles to perform acoustic imaging of the specimens. Delamination percentages and crack growth rates were determined as a function of number of thermal cycles. Three characteristic regions were consistently observed with the delamination percentage as a function of the number of cycles. No delamination occurred within the first few hundred cycles in any of the samples. Immediately after initiation, a region of transient increase in percent delaminated area then occurred for the next few hundred cycles. Lastly, a constant or increasing rate of percent delamination occurred for the remainder of the testing. The observation of three such regions is potentially significant because it is consistent with those observed with stress-corrosion (fatigue) cracking. Given this, the results obtained from this work were interpreted in a similar manner using classical crack-velocity(V)-stress-intensity (K) analysis. The experimental observation of cohesive failure of the sintered-Ag interconnection allows us to combine the tracked ingress front of the delamination (captured with the acoustic imaging) with their modeled thermomechanical strains and stress concentrations (also linked to the size and shape of the interconnection at any given time instant) to construct and interpret the V-K response of this particular sintered-Ag material. While the authors recognize this study's interpretation may be specific to the employed sintered-Ag-substrate-base-plate system, the authors feel the analysis method may provide a logical way to interpret such damage in interconnections whose material has sustained linear elasticity. A description of the experiment, finite element modeling of the thermomechanical strains and stress concentrations, the construction of a V-K curve using the data and its interpretation are discussed. An identified V-K curve is important because it affords the possibility to pre-determine what sizes and shapes of sintered-Ag interconnects can be used without causing delamination, and this has positive implications for future reliability improvement with power electronic devices.
D. J. DeVoto, Research Engineer
National Renewable Energy National Laboratory
Golden, CO

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