CBGA and C4 fatigue dependence on thermal cycle frequency

The fatigue life of solder bumps in flip chips increases as a function of frequency to the third power besides its dependence on shear strain and cycle peak temperature. On the other hand the tensile strain is practically negligible since both chip-carrier and chip have high elastic moduli and reasonably close coefficients of thermal expansion (CTE), inducing small and comparable bending in the chip and chip-carrier. In the case of CBGA, the card bending is more severe due to its much lower elastic modulus and much higher CTE relative to the ceramic chip-carrier. Such a structure induces tensile strains in CBGA joints (1) at the central portion of the chip-carrier during thermal cycle upswing, and (2) at the outer region during the downswing. The strain increases (and fatigue life decreases) with temperature ramp-rate and therefore with frequency, which is the opposite effect of stress relaxation as a function of frequency associated with shear strain. Therefore, the CBGA fatigue life can decrease or increase with frequency depending on whether the tensile strain becomes dominant or not. Results show that at frequencies beyond 2 cycles per hour, the tensile strain effect on fatigue starts to dominate which results in lower fatigue life.