The reliability of a power module is of vital importance to the user, who expects his
system to work unimpaired throughout its entire service life. The elimination of the base
plate offers clear design engineering advantages that boost system reliability. In fact,
modules without a base plate can be expected to have an extended service life especially
with respect to stress induced by passive thermal cycles.
By Dr. Uwe Scheuermann, Manager Product Reliability, SEMIKRON.
Conventional power modules feature solid copper base plates. This design concept brings about a number of disadvantages with regard to reliability. In most cases, the base plates are joined to the heat sink by way of screw connections at the corners or, in the case of larger modules, along the outer edges. In these cases, to ensure optimum heat transfer between base plate and heat
sink in the centre region as well, the base plate has to be suitably deflected before mounting.
The stress resulting from the different coefficients of thermal expansion of the base plate and substrate does not, however, only play a key role in manufacturing but also when the system is in operation. In application, a power module is exposed to significant temperature fluctuations. On the one hand, these thermal cycles are caused by the power loss produced in the power electronic component. These active thermal cycles are determined by the injected power loss and the thermal resistance of the module. Generally speaking, it is possible to reduce the
temperature swing by enlarging the siliconarea of the components, thus reducing the
In the case of passive thermal cycles, byway of contrast, this is not possible. Passive
cycles result from ambient temperature changes. For a power module, such stresses
are caused by a temperature change in the heat sink which in turn may result from a
change in the coolant temperature or from power losses in other components. The load
cycling capability in the case of passive temperature swings is determined by the module design alone; it cannot be enhanced by increasing the silicon area.