IEEE Transactions on Components and Packaging Technologies, Vol.27, No. 3, pp. 493-498, September 2004

Fill Pattern and Particle Distribution of Underfill Material

Y. Hang, D. Bigio and M. Pecht
University of Maryland
College Park, MD 20742


Underfills can dramatically improve flip chip reliability. However, the fillers used in some underfills can be dispersed unevenly, causing less than optimal reliability. In this study, underfill dispensing was conducted using various fill patterns. Experimental results show that particle settling occurs during the curing process, rather than during dispensing, and is affected by the difference between filler and matrix densities and underfill viscosity. Particle migration is a secondary mechanism, which causes uneven filler distribution. A vertically oriented transfer molding machine could help to mitigate settling.

Index Terms - Coefficient of thermal expansion (CTE), curing process, flip chip, particle migration, printed circuit board (PCB), underfill dispensing.

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