On a Paradoxical Phenomenon Related to Beams on Elastic Foundation: Could External Compliant Leads Reduce the Strength of a Surface-Mounted Device?

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Abstract
Compliant external electrical leads are often utilized as strain buffers in surface-mounted device technology to provide the necessary stress relief for the device, when the substrate is subjected to bending. A paradoxical situation was observed, however, during testing of compliant leaded hybrid integrated circuits (HIC): In some tests the bending moment, applied to the printed wire board (PWB) and causing HIC fracture, turned out smaller (not greater), when leads of greater compliance were installed. We show that such a paradoxical situation is due to the redistribution of lead reactions at certain combinations of HIC length, HIC and PWB flexural rigidity, and spring constant of the elastic attachment. Our analysis has indicated, that only sufficiently compliant leads can essentially reduce the stresses, while leads of moderate compliance can result in even greater stresses in the HIC than stiff leads. We suggest an easy-to-calculate governing parameter, which characterizes the mechanical behavior of HIC/PWB and similar assemblies with compliant attachments.

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