Stacked solder bumping technology for improved solder joint reliability

Solder joint failure is a serious reliability concern in flip-chip and ball grid array packages of integrated-circuit chips. In current industrial practice, the solder joints take on the shape of a spherical segment. Mathematical calculations and finite element modeling have shown that hourglass-shaped solder joints would have the lowest plastic strain and stress during a temperature cycle, thus the longest lifetime. In an effort to improve solder joint reliability, we have developed a stacked solder bumping technique for fabricating triple-stacked hourglass-shaped solder joints. This solder bumping technology can easily control the solder joint shape and height. The structure of triple-stacked solder joints consists of an inner cap, middle ball and outer cap. The triple-stacked solder joints are expected to have greater compliance than conventional solder joints and are able to relax the stresses caused by the coefficient of thermal expansion mismatching between the silicon chips and substrates since it has a greater height. Furthermore, the hourglass-shaped solder joints are to have a much lower stress/strain concentration at the interface between the solder joint and the silicon die as well as at the interface between the solder joint and substrate than barrel-shaped solder joints, especially around the corners of the interfaces. In this paper, the solder bumping process is designed and joint reliability is evaluated. Mechanical tests have been carried out to characterize the adhesion strength of the solder joints. The interfaces of the solder joints are investigated by scanning electron microscopy and energy dispersive X-ray analysis. Temperature cycling results show that the triple-stacked hourglass-shaped solder joints are more reliable than the traditional spherical-shaped solder joints. (C) 2001 Elsevier Science Ltd. All rights reserved.