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This paper presents measurements of the number of thermal cycles to failure for eutectic solder bumps in flip-chipon-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 055 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 1070-9886/98$10.00

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