THE EFFECT OF STENCIL PRINTING OPTIMIZATION ON RELIABILITY OF CBGA AND PBGA SOLDER JOINTS

As a follow-up and conclusion to previous work in stencil printing pro cess modeling and optimization (Li et al. 1996), we investigate the ef fect of stencil printing optimization on the reliability of the cerami c and plastic ball grid arrays. For ceramic ball grid arrays, the eute ctic solder fillet shape is calculated using a series of simple mathem atical equations. The thermal strain distributions within the solder j oints after two cycles of accelerated thermal cycling test are estimat ed using three-dimensional finite element models. The modified Coffin- Manson relationship is applied to calculate the mean fatigue lives of the solder joints. The results reveal that an optimized stencil printi ng process significantly reduces variation in the fatigue life of cera mic ball grid arrays. The results also show that the fatigue life of c eramic ball grid arrays is very sensitive to the card-side solder volu me. The maximum strain region shifts from the card-side eutectic solde r to the module side as the card-side eutectic solder volume increases . This shift in maximum strain suggests that there exists an optimum r atio between the card-side solder volume and the module-side solder vo lume for the reliability of a given ceramic ball grid array design. Th e implications of this for the package developers and users are discus sed. The calculations indicate that the fatigue life of plastic ball g rid arrays is almost insensitive to the card-side solder volume.