Micro-PIV measurements of capillary underfill flows and effect of bump pitch on filling process

This study is devoted to investigate the effects of the bump pitch on the capillary underfill flow. A micro particle image velocimetry (µPIV) system was used to visualize the flows and the shape of meniscus. Transparent flip chip specimens with quadrilateral bump arrangements were fabricated by etching silicon on glass wafer. Six bump pitches from 60 to 160 µm were tested and glycerin was dispensed to fill into the flip chip specimens. From the present experiments, it is shown that the overall filling speed becomes faster at larger bump pitch and changes abruptly when the bump pitch is twice the bump diameter. The detailed meniscus movement also has different behavior if the bump pitch gets smaller and larger than twice the bump diameter. The variation of dynamic contact angle is synchronized with that of the meniscus velocity throughout the whole process. During the interaction with the flip chip bumps, the contact line of the meniscus becomes concave or convex. The curvature of the concave and convex lines is larger at the smaller bump pitch.