Compliant Wafer Level Package for Enhanced Reliability

Wafer level package (WLP) volumes are steadily increasing due to their small package size and low manufacturing cost. However, applications to date have been mostly limited to die smaller than 5mm x 5mm. Solder joint fatigue due to stresses generated by the CTE mismatch between the die and the printed circuit board (PCB) limits adoption of WLP for large dies. Tessera's new compliant WLP technology greatly enhances thermal fatigue reliability of the package. A compliant layer under the solder joints effectively dissipates thermomechanical stress between the die and the PCB. FEA analysis was carried out to optimize compliant layer shape and mechanical properties, several materials were evaluated for this application, and prototype units were built. The prototype 9mm x 14mm packages exceeded 1600 cycles of temperature cycling from -40 degC to 125 degC. A compliant WLP package version with copper pins was also developed and tested for continuity at the wafer level. This structure has the potential to reduce wafer level test and burn-in (WLBT) costs substantially by eliminating the need for expensive die contact on the whole wafer contactor.