Stacked solder bumping technology for improved solder joint reliability

Reliability in flip chip and Ball Grid Array is a major concern. Solder joint geometry is extremely important from a reliability point of view. Commonly, the flip chip solder bump takes on the shape of a spherical segment. Mathematical calculations and finite element modeling have shown that the hourglass-shaped solder bumps experience the lowest plastic strain and have the longest lifetime. In an effort to optimize solder bump connections, we have developed a stacked solder bumping technique for fabricating triple-stacked hourglass-shaped solder bumps. This solder bumping technology can easily control the solder joint height and shape. The structure of triplestacked solder bumps consists of an inner bump, middle bump and external bump. The triple-stacked solder bumps have better compliance and are able to relax the stress caused by the coefficient of thermal expansion (CTE) mismatching between the silicon chips and substrates since it has greater height. Furthermore, the hourglass-shaped solder bump has a much lower stress/strain concentration at the interface between the solder bump and the silicon die as well as the interface between solder bump and substrate than the barrel-shaped solder bump, especially around the corners of the interfaces. In this paper, the solder bumping process is elaborated and reliability is evaluated. Mechanical tests have been carried out to analyze the strain and stress of the solder joints. The interfaces of the solder joints are investigated by SEM and EDX. Experimental results show that the triple-stacked hourglass-shaped solder bump is more reliable than the traditional solder bump.