Physics of failure based lifetime modelling for sintered silver die attach in power electronics: Accelerated stress testing by isothermal bending and thermal shock in comparison

Abstract

The employment of sintered silver (SAG) as die attach material is one of the most promising solutions to utilize the advantages of wide bandgap semiconductors in modern power electronic packaging. Its electrical, thermal and mechanical – i.e. reliability correlated – properties are far superior to those of time honored eutectic solders. To master the challenges of a high-quality production on an industrial level in accordance with the ever increasing application-driven reliability requirements, SAG die attach technology has to be developed, tested and lifetime-modelled in the context of a design for reliability approach, in order to be understood and thus savely put into application in the field. This requires the use of modern physics-of-failure paradigms, as today’s mostly used empirical lifetime models are meeting their limits with power electronics becoming more diverse in all relevant determinants such as materials, packaging technologies, geometric dimensions and reliability requirements. Introduced will be an isothermal bending test (ITBT) as highly accelerated, simple but very insightful test mimicking the power cycling load regime. This test will be combined with a physics of failure based approach, which will be presented including material characterization, modelling, simulation and definition of a failure parameter. This test method has the potential to produce reliability results much faster than non-mechanical tests, while the physical failure parameter still allows transferability of the resulting model. We show for the first time a physics-of-failure based lifetime model for SAG using isothermal bending in comparison with thermal shock test. For this we used the total inelastic strain (including plastic, primary and secondary creep) as local failure parameter for die attach degradation by fatigue crack growth.