Compliance metrics for the generalized S-bend lead design for surface-mount components

Abstract

The study reported is based on prior lead compliance analyses for a family of common SM (surface mount) component lead designs with rectangular and circular cross-sections. The directional compliance for these designs was formulated. Effective flexural and torsional spring constants were developed by elastic strain energy deformation analysis of representative structural models for common SM component lead configurations. The diagonal spring constant measures the effective stiffness of the SM component corner-most leads in the package diagonal direction; the component half-diagonal extends from the neutral center to the device corner. The long-term SM interconnection reliability during operational thermal cycling is controlled in part by the diagonal lead stiffness. Compliance evaluation has been performed for a commercial soldered S-bend lead specifically developed for high-reliability SM interconnection of LCCCs (leadless ceramic chip carriers) and MCMs (multichip modules) on epoxy-glass circuit-boards. Accelerated powered cycling of chip-leaded LCCCs on organic 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. >