Compliance metric for the S-bend lead design for surface mount components, with application to clip-leads

New stiffness metrics have been developed from an upgraded S-lead structural model that extends compliance evaluation capabilities to a wide variety of commercial S-lead designs. By selectively eliminating lead members, the generalized S-lead structural model can be used to represent a broad range of common lead forms for SM (surface mount) components. The S-lead directional spring constants are given in an easy-to-apply, algebraic format suitable for computer and spread-sheet evaluation. Compliance evaluation has been performed for a commercial S-bend clip-lead specifically developed for high-reliability SM interconnection of CCCs (ceramic chip carriers) on organic circuit-boards. Accelerated powered cycling of clip-leaded CCCs on epoxy-glass circuit-boards has shown that leads with diagonal stiffness in the nominal range of 10-40 lb/in provide a comparatively high margin for SM attachment reliability. The diagonal stiffness of S-bend lead designs can be tailored by using the upgraded compliance formulation to optimize the contour and critical dimensions