Measurements of deformations and strains of a Si-epoxy-FR4 structure during thermal testing

Deformations and strains of a Si-epoxy-FR4 structure were evaluated during thermal testing using high sensitivity, real-time Moiré interferometry. The specimen studied was a sandwich structure consisting of a silicon chip, epoxy underfill and FR4 substrate. The deformations and strains of the FR4-underfill and silicon-underfill interfaces of the specimen under certain thermal loading were examined. The results show that the shear strain increases significantly along the interfaces, with the maximum shear strain occurring at the intersection of the specimen edge and the silicon-underfill interface. The shear strain at the silicon-underfill interface experienced a 2% increase after heated for two hours at 100 °C, but the shear strain at the FR4-underfill interface showed a 12% increase. This is an indication that the creep effect is more dominant in the FR4-underfill interface. The interfaces of the specimen experience partial strain recovery after one hour of the holding time at 20 °C.