High-power
substrates for hybrid circuits which combine aluminium with an anodic coating are highly efficient, the heat generated from the chip mounted on the coating being rapidly dissipated. Unfortunately because of the large difference in thermal expansion coefficients between the two materials, large thermal tensile stresses are set up in the coating. Such stresses can cause cracking in the coating and subsequent loss of electrical integrity. A finite element model of the substrate has been developed and validated theoretically and through equilibrium balances. The model has been used in wide-ranging sensitivity studies in order to determine the desirable coating material and geometrical properties necessary to attenuate the coating peak tensile stresses. It has been found that coating thickness, thermal expansion coefficient and elastic modulus have the greatest effect upon maximum stress values and that the thickness and expansion coefficient should be as great as possible whilst the elastic modulus should be as small as possible for minimum peak stress. The model is currently being used to predict stresses under transient conditions, thermal cycling, multipoint heat inputs and for crack development studies.