Measurements of solder bump lifetime as a function of underfill material properties

This paper presents measurements of the number of thermal cycles to failure for eutectic solder bumps in flip-chip-on-board (FCOB) circuits with and without underfill. The bump lifetimes are measured as a function of the distance to the chip centre for two different underfill materials. The results show that the lifetime of the solder bumps under thermal cycling from -55 to 145°C decreases with increasing thermal expansion coefficient (CTE) of the underfill material. For a filled epoxy underfill with CTE=28 ppm/°C, nearly matched to the CTE of solder, the mean number of cycles to failure is above 2000, compared to 50-80 cycles for devices without underfill. For a pure epoxy underfill (CTE=58 ppm/°C) the measured lifetime of the solder bumps is 500-1500 cycles. The lifetime measurements are compared to an analytical model of the number of cycles to failure based on calculations 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 failure are 1-3 times higher than the model predictions. For the underfill with the lowest CTE, the measured mean lifetime is slightly higher than the predicted values. For both types of underfill there is only a small observed variation of lifetime with the distance to the chip centre. This indicates that axial strain plays a dominant role in the mechanical wearout of the solder bumps