Predicted Residual Bow of Thin Plastic Packages of Integrated Circuit Devices

Thin plastic packages are prone to residual bows caused by the thermal contraction mismatch of the constituent materials: silicon (chip), metal (leadframe), and epoxy (encapsulant). Since excessive bow can make normal mounting of a plastic package on a printed wiring board difficult, it is important that such a bow can be predicted, minimized, and, if possible, avoided. Accordingly, in this analysis we develop a simple and easy-to-apply calculation method for the prediction of the residual thermally-induced bow in a thin elongated plastic package. We use the obtained formula for the curvature to determine whether the chip/leadframe assembly can be positioned within the epoxy encapsulant in such a way that no residual bow occurs. We show that employment of epoxy encapsulants with elevated coefficients of thermal expansion can be an effective means to reduce the bow, and that application of thin and/or low expansion leadframes is also desirable. The calculated bow, obtained for a 1mm thick, 14mm long package, agrees satisfactorily with the finite-element solution. The results of our analysis can be utilized as a guidance in the evaluation of the expected bow, as well as for a rational physical design of thin plastic packages.