Predicted failure criterion (von-Mises stress) for moisture-sensitive plastic packages

Constitutive equations, based on von-Karman´s equations for large deflections of plates with consideration of thermal effects, are developed for the assessment of the propensity of moisture-sensitive plastic packages to structural failures during surface-mounting them onto printed circuit boards. The maximum value of the von-Mises stress in the molding compound is suggested to be used as a suitable failure criterion which is able to reflect the cumulative role of various geometrical and materials factors affecting the strength of the package. A situation when the molding compound is completely delaminated from the chip (or the paddle) is examined in detail. The calculated stresses are in good agreement with experimental observations. The developed calculation procedure for the prediction of the von-Mises stress can be used for the preliminary separation of the packages that need to be “baked” and “bagged” from these that supposedly do not