Thermal Stress in Epoxy Molding Compounds and Packaged Devices

Abstract

Studies on thermomechanical stresses in plastic pack aged integrated circuits after molding and in encap sulation materials, show the resulting stress distri bution on the surface of the die and clarify the fundamental mechanism in the generation of thermo mechanical stresses in molding compounds.Use was made of a piezoresistive strain gauge array to measure the stress distribution on the sur face of the die. A beam bending apparatus was used to study the importance of the thermoviscoelastic properties of the molding compound. The strain gauge allowed for the study of the effects of thermal shock testing.The largest stresses are observed as shear stresses at the corners of the die at the lowest temperature.Three commercially available epoxy -based molding compounds were studied. Two of these materials are standard packaging formulations for smaller devices. Both strain gauge and beam bending experiments showed comparable stress levels with these two materials. The third material is a rubber modified, low stress material. As expected, stress levels in devices packaged with this material, as well as stresses observed in the beam bending apparatus, were considerably lower than those for the other two materials.In integrated circuit packaging, thermosetting polymers are import ant due to cost considerations in both manufacturing and material selection. Plastic packaging (J[) has therefore become the dominant method of protecting integrated circuits from the environment. The manufacturing process involves a transfer molding operation in which an integrated circuit is encapsulated with a silica filled 1