Improving Reliability of Fan -Out Wafer Level Package Through Doping of Lead-Free Solder Balls

Abstract

Embedded wafer level ball grid array (eWLB) or FO-WLP (Fan-out wafer-level packaging) is investigated as a package for MMICs (Monolithic Microwave Integrated Circuit) for automotive radar applications in the 77GHz range. Doping of Bi and/or Sb was applied to improve the resistance of lead-free SAC balls against thermal cycling stress. We have shown that by increasing the doping level the lifetime can be increased and determined the maximum doping level before extrinsic failure modes dominate. It is well-known from the literature that doping of Bi and/or Sb improves the resistance of lead-free SAC balls against thermal cycling stress. It was shown that lifetime of FC-BGA package can be multiplied using Bi and/or Sb doping. The lifetime gain is achieved by reduced creepage of the hardened balls caused by the doping. This reduced creepage increases the risk of causing other failures than ball cracks in the eWLB package or the PCB during thermal cycling stress. We have tested robustness of 7x8,5 mm^2 eWLB packages against thermal cycling stress T = -40 to 125 °C using SAC-balls with different Bi-doping content from 0% to 2,5% Bi concentration. As could be expected the solder ball fatigue mode is delayed more and more increasing the Bi content. We found other failure modes than solder ball fatigue emerging at doping around 1%. The fatigue modes are shifted towards cupper lines on PCB or into the package. Cu line cracks can be overcome by switching the PCB layout from NSMD (non solder-mask defined) to SMD (solder-mask defined) solder pads. In general there is a lifetime penalty when switching from NSMD to SMD, but by avoiding the Cu line cracks this lifetime loss is overcompensated using SMD pads for optimum performance. As input for simulations we have characterized the doped solder material using specimens produced in glass columns. We have also tested the components on PCBs with different top layer laminate with different CTE and elastic modulus to assess the influence of the PCB properties on the lifetime. It is a trend in automotive radar systems to use stiffer laminates which are cheaper but have negative influence on component lifetime. Solder ball shear tests were also carried out to characterize the influence of ball doping on other board-level reliability beyond thermal cycling performance. As expected ball shear force is increased with increased doping level. At increased shear-force the shearing is still done through the solder ball and does not lead to different shear-mode. Finally drop-tests were done to check influence of doping on board-level reliability performance other than thermal-cycling performance.