Reliability study and failure analysis of fine pitch solder bumped flip chip on low-cost printed circuit board substrate

The design and proper selection of low-cost printed circuit board (PCB) is essential to the reliability and formation of solder joints for the fine-pitch solder-bumped flip-chip on board (FCOB) technology. By nature low-cost PCBs don't have the precision in patterns like the fine-pitch solder-bumped flip-chip technology. In this study, eutectic solder bumps (63Sn/37Pb) were electroplated on the test chips with various pad pitches and chip sizes. Several types of low-cost PCB designs were evaluated for the fine-pitch solder-bumped flip-chip. The effects of different PCB designs on the formation and shapes of solder joints were studied. The location and formation of voids in the underfill depend on the design and properties of the PCB. The alignment accuracy of the solder mask opening area and the copper pad on the PCB is very critical for the reliability of FCOB technology. The acceptable mis-registration on the low-cost PCB decreases as the chip size increases. Scanning acoustic microscope (SAM), X-ray imaging system, cross-sectioning, scanning electron microscopy (SEM) were used to evaluate and inspect the samples after the thermal cycling test and temperature-humidity test. The solder joint failure modes after thermal tests were studied.