Abstract
Packages of power modules are designed to show a slightly convex shape of the base-plate in order to achieve a reasonable thermal contact between the module and the heat sink when the power module is mounted. During manufacturing, soldering of materials with differences in thermal expansion coefficients will lead to bending of the “bi-material” composite, e.g. the silicon chip to substrate (usually DCB = direct copper bond Al2O3 or AlN) or the DCB-substrate to base-plate joint (Cu or MMC = metal matrix composite). Especially the large area solder joint between DCB-substrate and Cu base-plate is known to induce a certain curvature into the base-plate immediately after cooling down from soldering. The curvature will relax to a certain extent during module finishing, storage, shipping, and use which makes the prediction of the shape of the base-plate difficult. The present paper proposes a model to assess the creep behaviour of such large area solder joints in order to predict the state of bending after soldering and the amount of relaxation of bending with time.
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