Underfill of flip-chip: The effect of contact angle and solder bump arrangement

An underfill encapsulant was used to fill the gap between the chip and the substrate around the solder joints to improve the long-term reliability of a flip-chip interconnecting system. The underfill encapsulant was filled by the capillary flow. As a part of the series studies of understanding and analysis of the underfill mechanism, this study was devoted to investigate the effects of the contact angle and the bump arrangement on the underfill flow. The Hele - Shaw flow model, that considered the flow resistance in both the thickness direction between the chip and substrate and the plane direction between solder bumps, was assumed. A modified capillary force model including the effects of the contact angle and the bump arrangement was proposed. The capillary force was formulated based on either quadrilateral or hexagonal bump arrangements. A capillary force parameter was used as an index for comparison of different bump design, including bump diameter, gap height, bump pitch, contact angle, and arrangement. A hexagonal arrangement of solder bumps was found to have a higher capillary force and better underfill efficient.