MEASUREMENTS OF SOLDER BUMP LIFETIME AS A FUNCTION OF UNDERFILL MATERIAL PROPERTIES

This paper presents measurements of the number of thermal cycles to fa ilure for eutectic solder bumps in flip-chip-on-board (FCOB) circuits with and without underfill. The bump lifetimes are measured as a funct ion of the distance to the chip centre for two different underfill mat erials. The results show that the lifetime of the solder bumps under t hermal cycling from -55 to 145 degrees C decreases with increasing the rmal expansion coefficient (CTE) of the underfill material. For a fill ed epoxy underfill with CTE = 28 ppm/degrees C, nearly matched to the CTE of solder, the mean number of cycles to failure is above 2000, com pared to 50-80 cycles for devices without underfill, For a pure epoxy underfill (CTE = 58 ppm/degrees C) the measured lifetime of the solder bumps is 500-1500 cycles. The lifetime measurements are compared to a n analytical model of the number of cycles to failure based on calcula tions of axial and shear solder strain, For devices without underfill there is a good match between the model and the measurements. For the underfill with the highest CTE, the measured number of cycles to failu re are 1-3 times higher than the model predictions. For the underfill with the lowest CTE, the measured mean lifetime is slightly higher tha n the predicted values. For both types of underfill there is only a sm all observed variation of lifetime with the distance to the chip centr e, This indicates that axial strain plays a dominant role in the mecha nical wearout of the solder bumps.