A modelling approach to assess the creep behaviour of large-area solder joints

Packages of power modules are designed to show a slightly convex shape of t he base-plate in order to achieve a reasonable thermal contact between the module and the heat sink when the power module is mounted. During manufactu ring, soldering of materials with differences in thermal expansion coeffici ents 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 DC B-substrate to baseplate joint (Cu or MMC = metal matrix composite). Especi ally 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 d uring module finishing, storage, shipping, and use which makes the predicti on of the shape of the base-plate difficult. The present paper proposes a m odel to assess the creep behaviour of such large area solder joints in orde r to predict the state of bending after soldering and the amount of relaxat ion of bending with time. (C) 2000 Elsevier Science Ltd. All rights reserve d.