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Impact of absolute junction temperature on power cycling lifetime

The influence of the absolute junction temperature Tj* on IGBT power module lifetime was systematically investigated by means of active power cycling tests. Both the impact on the wire bond lift-off and the chip solder degradation mechanism could be determined separately by applying the concept of separating failure modes. The test results not only prove that classical lifetime models overestimate the influence of Tj*, but also show that the two dominant failure mechanisms have to be treated differently. The wire bond lift-off failure mode is weakly affected by the absolute temperature level and possesses a very small activation energy of 0.069 eV. The solder degradation mode exhibits a significantly larger activation energy of 0.159 eV, which results in a massive decrease (factor 3) in power cycling capability when increasing Tj* by 85 K. For junction temperatures above 175°C SnAg-based solder joints are no longer suitable for reliable power module designs and advanced die attach technologies such as silver sintering have to be deployed. For the description of such advanced power modules specific lifetime curves are under development which exclusively represent the pure wire bond lifetime, as for common power cycling conditions the silver sintered die attach is not subjected to ageing.