Design study for improved solder joint reliability of VQFN packages

Solder joint reliability during temperature cycling on board (TCOB) is a critical issue not only for BGA but also for quad flat nonleaded (QFN) type packages. On the latter, only a few studies are available in the literature. In this work, Darveaux´s model for BGA solder fatigue life prediction is modified by calibrating experimentally derived model constants to TcoB test results of a 64-leaded VQFN (Very thin QFN) package. Subsequently a parameter study using full 3D FEA modeling with ANSYS® was done. The target was to identify the most sensitive parameters. The simulations predict highest plastic work at the lead side of the outer edge of a corner lead. This crack location corresponds well with experimental findings. The most remarkable result is the very high sensitivity of predicted solder fatigue life on the CTE of the PCB: just 2ppm/K variation of CTE result in 100% effect on fatigue life. Formulating parameter effects in positive direction only, other parameters with high effect (50-100%) on solder fatigue life are a high die/die-pad length ratio, a large solder contact area, and of course a reduced temperature cycling profile from -40/125°C to 0/100°C. Moderate effects of 20%-50% can be achieved with stiffness reductions on both sides of the solder joint, i.e. thinner die, thinner PCB, lower Young´s modulus of the mold compound, reduced number or thickness of Cu-layers in the PCB. An increased solder joint height and the existence of a solder fillet have also a positive effect. Almost no effect was found for alternative die attach selections, solder fillet height and package size, provided the size of die and central die pad, which was always soldered to the PCB, were increased similarly.