Could compliant external leads reduce the strength of a surface-mounted device?

A simplified analytical model of hybrid-integrated-circuit (HIC)/printed-wire-board (PWB) assembly bowing is developed to explain a paradoxical situation, observed by T. F. Marinis, R. C. Reinert and W. M. Sherry (Proc. 34th Electronic Components Conference, 1984) during testing of compliant-leaded HIC's: the bending moments applied to the printed wiring board (PWB) and causing fracture of the hybrid integrated circuit (HIC) turned out in some tests to be smaller when leads with greater compliance were installed. It is shown that such a result is due to the redistribution of the lead reactions at certain combinations of the HIC length, HIC and PWB flexural rigidity, and lead spring-constant. The analysis indicates that only sufficiently compliant leads can reduce the stress in the HIC, while leads of moderate compliance can result in even greater stresses than stiff leads. An easy-to-calculate governing parameter that characterizes the mechanical behavior of HIC/PWB assemblies with compliant leads is presented. The results can be of help in physical design of compliant-leaded surface-mounted devices.<<ETX>>