Determination of Thermally-Induced Stresses in Plastics by Photoelastic Techniques
01 January 1989
The technology for packaging microelectronic devices in plastics has experienced phenomenal advancement in recent years thanks in part to the introduction of new compounds that minimize undesirable high concentrations of stress in molded packages. Nevertheless, thermal stresses generated during molding remain one of the major concerns of the industry. In the mathematical prediction of thermal stresses, linear elastic models are most often used even though their use has rarely been verified by well designed experiments. We have conducted thermomechanical experiments on samples of a well characterized transparent epoxy compound which was molded around metal inserts. Both the assumption of linear elastic material behavior and the use of the stress optical law to interpret birefringence patterns were evaluated. Emphasis was placed on the effect of temperature dependent material properties, with careful measurements of the modulus and stress optical coefficient included. It was found that fortuitous agreement between linear elastic predictions and oversimplified interpretations of photoelastic measurements can be mistaken for independent verification of both results when, in fact, the actual stress levels may be overpredicted considerably. These results provide a great deal of insight into the development of stress and birefringence during the process of restrained shrinkage.