Reliability Investigation of Extremely Large Ratio Fan-Out Wafer-Level Package with Low Ball Density for Ultra-Short-Range Radar

Abstract

Driven by aggressive development of electronic products with high robustness demand for automotive application to endure severe usage environment, both component-level and board-level reliabilities have to be concerned more for safety assurance. In this paper, a system-on-chip millimeter-wave ultra-short range radar (mmWave USRR) realized in complementary metal-oxide-semiconductor (CMOS) technology and assembled with fan-out wafer level packaging (FOWLP) technology was introduced, and the board-level reliability (BLR) was studied experimentally on the risk of chip-to-board interaction (CBI). The factors of solder ball material, package thickness and underfill material, thought to dominate on CBI performance, were studied experimentally. First of all, two solder materials were studied to evaluate their capabilities for this FOWLP to against board level thermal cycling and drop tests. It was found that the solder with higher elastic modulus performed much better on board-level thermal cycling (BLTC) reliability. Moreover, no difference was found in board level drop test since no failure occurred in both solder materials. Both package thicknesses of 425 µm and 580 µm were studied on the board level reliabilities, and the results revealed that the design with both thicker Si die and thicker molding material significantly improved the BLTC reliability. Both epoxy-based materials - one is low-CTE underfill material and the other is edge-bond glue, were applied to know the workability of enhancing the BLTC performance on the FOWLP. The experiment results showed that both the epoxy materials miserably decreased the BLTC performance, and severe solder crack and bulk underfill crack were found. Since vibration test is indispensable and of much concern for automotive electronics, the stringent test condition of sine-wave frequency swept from 20 Hz to 2,000 Hz and peak acceleration of either 50g or 20g, was applied to evaluate anti-vibration property of the FOWLP mTV mounted on daisy-chain PCB. From the results of 50g peak acceleration vibration test, high resistance was found in the specific daisy-chain loop which electrically connects corner solder balls. From the failure analysis it could be found that delamination existed at the interface of redistribution layer (RDL) and under-bump metallization (UBM) of component side and PCB Cu trace crack. It is noteworthy that all the failures only happened on the package located at the 5x3 array corner while subjecting to Z-axis vibration. From experience, poorly fixing the PCB on vibration platform potentially causes more bending stain on PCB during Z-direction vibration and further concentrates much higher stress singularly nearby the corner. Moreover, the board-level vibration test with 20g peak acceleration was also implemented, and there wasn't any failure found. Finally, the BLR was thoroughly studied for the extremely large area-ratio FOWLP, and the package was proved its capability of meeting AEC-Q100 compliant stringent reliability tests.