Direct chip attach (DCA) microelectronic technology involves mounting an unpackaged silicon die directly onto the printed circuit board (PCB) by means of an array of solder joints  which act as mechanical and thermal interconnects between the chip and the board. These joints can be subjected to large shear stresses due to the thermal expansion coefficient mismatches between the silicon die and the board. To enhance the reliability of the solder joints, a silica-filled epoxy underfill is allowed to flow by capillary action in the space between the chip and the board. Later the underfill is cured to encapsulate the solder joints rigidly . The reliability of chip is undermined by the appearance and the growth of cracks in the epoxy due to thermal cycling and humidity. Fatigue crack growth has been recognized as an important failure mechanism in polymers  and interfaces involving polymers . Protocols for accelerated testing that evaluate the susceptibility of epoxies to such cracking and allow one to predict their performance under service conditions are thus important. Recently, we have developed a fracture specimen to study the fatigue
crack growth in epoxy due to such thermal excursions. The specimen geometry is shown schematically in Fig. 1. An epoxy ring of inner radius ri and outer radius
of ro is shrunk thermally onto the circular invar plate. Two radial pre-cracks of approximately the same length (a0 and a00) are introduced from the outer edge of the ring. The stress free temperature of the composite disk is denoted by Tsf.
Pages : 15
Size: 434 kb
Author : T. Tang , C.Y. Hui, H.G. Retsos , E.J. Kramer
Characterization of a fracture specimen for crack growth in epoxy due to thermal fatigue