UFI (UBM-Free Integration) Fan-In WLCSP Technology Enables Large Die Fine Pitch Packages

A low-cost, highly reliable UFI (UBM-free integration) wafer level chip scale packaging (WLCSP) for high performance, large die (>100 mm2) with fine solder ball pitch (350 µm) and high I/O (>800) has been developed. Reliability issues including solder cracking and high chip warpage are known to be the main challenges for extending the die size of conventional WLCSP to more than 5x5 mm2 with ball pitch smaller than 350 µm. In this paper, we present the novel WLCSP designed with a stress-mitigation structure which can significantly enhance the reliability of the package and enables die size extension. By controlling the maximum strain location and optimizing the materials using the stress-mitigation structure, the chip warpage and the stress between Si and PCB can be effectively reduced to enhance both component and board-level reliabilities of the package. The UFI WLCSP passed all component-level tests and exhibited board-level thermal cycle life that is 1.4 and 2.3 times longer than that of the conventional WLCSP in terms of the first failure and the Weibull distribution (t63), respectively. As a result, highly reliable packages with various die sizes, including 5.2x5.2, 7.2x7.2 and 10.3x10.3 mm2 and with both 400 and 350 µm ball pitches have been developed. The RDL within the package of UFI WLCSP can be utilized to form inductor structure which exhibits quality-factor nearly 50% higher than that of an on-chip inductor. Besides, in WLCSP process, die sawing typically causes delamination and cracking of packages which results in process failure in the WLCSP. The UFI unique structure consists of sidewall protection which is capable of preventing chipping and therefore effectively improves the process controls and the reliability margins. In addition, to fulfill the requirement of high I/O products and to enhance the design flexibility, we embedded multiple RDL in UFI WLCSP with die size of 10.3x10.3 mm2 and ball pitch of 350 µm, which provides a fan-in package solution with I/O >800. The large die UFI WLCSP has passed the component-level reliability tests of TCB1000, uHAST96 and HTS1000, and board-level reliability tests of TCG500 and drop tests.