Board-level reliability of coreless flip chip package assembled on a printed circuit board

The board-level reliability of a 6-layer polyimide-based coreless flip chip package assembled on a printed circuit board (PCB) under a temperature cycling condition of -55 ~ 125°C was investigated in this study. The assembly of coreless flip chip package was achieved by a 17 mm × 17 mm die with 4355 Sn37Pb solder bumps, an amine-based underfill and 1521 Sn3.0Ag0.5Cu solder balls, no defect such as gas void in underfill, cold solder joint and missing ball was found in the package. Then each test component was mounted on a printed circuit board (PCB) with using Sn3.0Ag0.5Cu Pb-free solder paste in a reflow oven, totally 10 test vehicles were assembled. Afterwards, the PCBA test vehicles were underwent temperature cycling test and the real-time resistances of the daisy-chains were recorded by Fluke measurement equipment. In the previous study, it is found that the coreless flip chip packages all failed after hundreds of temperature cycles and a failure mode of die crack is observed. However, when the component was mounted on a PCB, the defect of die crack was not longer found during the test, which was replaced by the fractures of the outmost solder balls at substrate-side, and the characteristic life of the PCBA exceeded 1500 cycles. The results represented that the component-level reliability could not completely response to the behavior under board-level test. The rigid PCB constrained the warpage of coreless flip chip and redistributed the thermal stress applied to the underfill, finally resulting in a higher life time.