Assembly and reliability performance of flip chip with no-flow underfills

The objective of this paper is to investigate the feasibility of using no-flow underfill and to understand the effect of die size, filter size, filler loading and solder mask design on assembly and reliability performance. Flip chip on board (FCOB) test vehicles at 200 μm pitch with 5×5 mm² and 10×10 mm² die were used in this study along with 11 no-flow underfill materials with varying filler sizes and loadings. Daisy-chain resistance measurement was used as a criterion for the assembly yield characterization and reliability performance assessment. In general, no-flow underfills with silica fillers ≥1 μm were observed to have poor wettability of the solder joints due to filler entrapment between the solder bump and pad on the test board. The 5×5 mm² die was found to have better performance in temperature cycling and moisture sensitivity testing than the 10×10 mm² die. For good temperature cycling performance, a small die with a low CTE no-flow underfill is recommended.