Development of very large Fan-In WLP/ WLCSP for volume production

WLCSP so far was used to enable low-cost manufacturing, and a high performance suitable for low I/O density for mobile and consumer products, wireless connectivity, MEMS and Sensors. Larger dies are usually packaged in Wirebond-BGA or FlipChip-BGA with a small bump pitch, applying underfill material between bumped die and FlipChip substrate to ensure the required board-level reliability. Typical Fan-In WLP/ WLCSP are ranging up to 8mm × 8mm in size, in some extreme cases up to 15mm × 15mm. Limits have been set by the board level reliability of larger chips/ packages, caused by the thermal mismatch between CSP (Chip Scale Package) and PWB (Printed Wiring Board). The paper describes the work done to overcome that limitation and develop, qualify and ramp in volume production a Fan-In WLP/ WLCSP solution beyond common practice. The WLCSP described is one order of magnitude larger in area, something which has never been accomplished in WLCSP before. Package construction and reliability test results will be presented. Daisy Chain Test vehicles have been used with different UBM diameter, UBM design, UBM thickness, dielectric material type, and solder ball alloy variants. All design options achieved minimum costumer requirement at TCoB, the most critical reliability test for such large device. With the right package design and construction, dielectric material and solder ball alloy selection, final product successfully passed more than 900 temperature cycles on board without fails. During the experiments is has been seen that number of cycles until first failure at TCoB and failure mode showed dependency on the above mentioned features.