Prediction of equilibrium shapes and pedestal heights of solder joints for leadless chip components

The reliability of solder joints for surface mount components is closely related to the joint shape and "pedestal" (stand-off) height, i.e., the thickness of the fillet that separates the metallized surface of the component from the pad on the circuit board. In this paper, an analysis is presented to predict the profiles and pedestal heights of equilibrium solder joints that attach surface mount components to printed circuit boards. The common case of two-dimensional (2-D) joints with negligible solder density effects is considered. A criterion is also derived that represents the minimum critical volume of solder required to produce a "theoretically nonzero pedestal height", below which the model is inapplicable. The critical solder volume produces a convex joint. The criterion suggests that if a 2-D joint is concave and within the model simplifications, force equilibrium cannot exist on a component at a positive pedestal height. Extensions to cases in which solder density effects are significant are also discussed. The analysis results in a system of coupled nonlinear algebraic equations which are solved numerically. The sensitivity of joint shape and pedestal height to geometric and physical parameters is examined. Comparisons between the theory and experiment show good agreement.