Straddle-board modeling of SMT joint behaviors

Straddle-board specimens consisting of a chip carrier connected by J-leads to a printed wiring board were tested isothermally at room temperature. Thermal expansions were simulated by controlling the deflection of the bisectioned printed wiring board. Initial results are presented from using the straddle-board configuration for material response and fatigue life analysis of actual surface-mounted joints. The leads were subjected to two typical extreme modes of deformation, longitudinal and transverse deflections, to determine the behaviors and damage in each case. It was found that the transverse component of the lead deflection, which increases in magnitude toward the corner of the chip carrier, is more damaging than the longitudinal component, which does not vary from lead to lead. The fatigue data, plotted as percent load drop versus cycles, follows a linear pattern during the initial stages of damage. The effects of stress relaxation on the rate of loading show a logarithmic correlation with deflection.<
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