Calculation of Thermally Induced Mechanical Stresses in Encapsulated Assemblies

A procedure is given for the calculation of thermally induced mechanical stresses in electronic assemblies. This procedure can be used to calculate both elastic and plastic deformation during thermal cycling of multielement assemblies with complex configurations and heterogeneous material properties. It will give the strain (or stress) in each element at any temperature in the thermal cycle as the element undergoes elastic deformation, captive plastic deformation, or thermal ratchet. Several encapsulated systems were analyzed to demonstrate the applicability of the calculations to real assemblies. In one example, it was demonstrated that encapsulated electronic systems are subject to thermal ratchet. For the environmental temperatures used, it was shown that the assembly would grow (or ratchet) on each thermal cycle until one of the elements broke. It was found that thermal ratchet could be avoided by either decreasing or increasing the thickness of the encapsulant. In another example, a foamed cordwood module was analyzed to determine the effect of lead length on the stresses developed in a glass-cased component. It was shown that a small increase in the length of the component's leads significantly increased the stresses in the component.