Reliability Study of Large Fan-Out BGA Solution on FinFET Process

Abstract

Driven by aggressive product roadmap of high performance and low power IC, the complexity of both design and interconnection has significantly increased for field requirements. Unlike single-die FCBGA area limitation (generally within 600 mm2), fan-out technology significantly extends the die scale far beyond and largely improves both SI and PI requirements. To name a few, networking product is one of the specific applications; wired-ASIC would be another. However, fan-out structure inherently exhibits weaker mechanical properties due to its substrate-less process. As a consequence, study on the high latent risk of chip-to-package interaction (CPI) becomes important for the success of this technology. In this work, the reliability of CPI was experimentally investigated at 16nm FinFET process node on a large scale die size (~860mm2) and FOBGA package size (67.5mm*67.5mm). The result shows no CPI induced defect was found owing to specific RDL pattern was designed. Also the heat spreader type (including ring, lid, and without spreader) were studied, and the result shows that the stiffener ring could help on reducing overall package warpage by 20% as compared with the one without ring. And the lid type heat spreader having highest stiffness performed the best warpage behavior. Furthermore, it was revealed that optimizing ring type spreader width effectively reduces the deformation variation in the temperature range from 25 °C to 150 °C. Moreover, the board level reliability, including temperature cycling and drop tests, for the FOBGA was evaluated experimentally with daisy-chain PCB. Despite the low risk exhibited by board-level mechanical stress tests, e.g. shock and monotonic bend tests, on die bump and fan-out RDL, the BGA ball lifetime seemed to be inevitably getting worse under thermal gradient stress (temperature cycling). This was majorly due to the warpage behavior induced by large package size. It has been well known that PCB design variation causes dynamic and fatigue failure discrepancies. In this paper, through-hole design (Cu plating or Cu paste filling) and core material were studied on the effects of PCB design variants on board level thermal stress. The result shows the PCB with high Tg core or with Cu-paste filled through hole, has much better temperature cycling reliability than the one with Cu-plated-through-hole PCB. And there was no failure which relevant to CPI issue was found even after 2500-cycle TCT. Moreover, the board level dropping results reveal that the large FOBGA passed the drop test.