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RESEARCH PAPERS

Experimental Study of Void Formation in Eutectic and Lead-Free Solder Bumps of Flip-Chip Assemblies

[+] Author and Article Information
Daijiao Wang

Mechanical Engineering Department, The University of Texas at Austin, 1 University Station C2200, ETC 5.160, Austin, TX 78712

Ronald L. Panton

Mechanical Engineering Department, The University of Texas at Austin, 1 University Station C2200, ETC 5.160, Austin, TX 78712rpanton@mail.utexas.edu

J. Electron. Packag 128(3), 202-207 (Nov 24, 2005) (6 pages) doi:10.1115/1.2229215 History: Received September 13, 2004; Revised November 24, 2005

This paper reports the experimental findings of void formation in eutectic and lead-free solder joints of flip-chip assemblies. A previous theory indicated that the formation of voids is determined by the direction of heating. The experiments were designed to examine the size and location of voids in the solder samples subject to different heat flux directions. A lead-free solder (Sn-3.5Ag-0.75Cu) and a eutectic solder (63Sn37Pb) were employed in the experiments. Previous experiments [Wang, D., and Panton, R. L., 2005, “Experimental Study of Void Formation in High-Lead Solder Joints of Flip-Chip Assemblies  ,” ASME J. Electron. Packag., 127(2), pp. 120–126; 2005, “Effect of Reversing Heat Flux Direction During Reflow on Void Formation in High-Lead Solder Bumps  ,” ASME J. Electron. Packag., 127(4), pp. 440–445] employed a high lead solder. 288 solder bumps were processed for each solder. Both eutectic and lead-free solder have shown fewer voids and much smaller void volume than those for high-lead solder. Compared with lead-free solder, eutectic solder has a slightly lower void volume and a lower percentage of defective bumps. For both eutectic and lead-free solders, irrespective of the cooling direction, heating solder samples from the top shows fewer defective bumps and smaller void volume. No significant effect on void formation for either eutectic or lead-free solder was found via reversing the heat flux direction during cooling. Unlike high-lead solder, small voids in eutectic or lead-free solder comprised 35-88% of the total void volume. The final distribution of voids shows a moderate agreement with thermocapillary theory, indicating the significance of the temperature gradient on the formation of voids.

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Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 2

Micrographs of a eutectic solder bump reflowed with heating/cooling from bottom to top: (a) the 2nd cut, (b) the 4th cut, and (c) the 7th cut

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Figure 3

Average void volume percentages by solder type for all reflow profiles

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Figure 4

Percentage of finding a defective bump by solder type for all reflow profiles

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Figure 5

(a) Number of voids as distributed by size and region for heating from top, cooling from bottom (lead-free solder). (b) Average void volume percentages by regions for heating from top, cooling from bottom (lead-free solder). (c) Number of voids as distributed by size and region for heating from top, cooling from bottom (eutectic solder). (d) Average void volume percentages by regions for heating from top, cooling from bottom (eutectic solder).

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Figure 6

(a) Number of voids as distributed by size and region for heating from bottom, cooling from top (lead-free solder). (b) Average void volume percentages by regions for heating from bottom, cooling from top (lead-free solder). (c) Number of voids as distributed by size and region for heating from bottom, cooling from top (eutectic solder). (d) Average void volume percentages by regions for heating from bottom, cooling from top (eutectic solder).

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Figure 7

(a) Number of voids as distributed by size and region for heating from top, cooling from top (lead-free solder). (b) Average void volume percentages by regions for heating from top, cooling from top (lead-free solder). (c) Number of voids as distributed by size and region for heating from top, cooling from top (eutectic solder). (d) Average void volume percentages by regions for heating from top, cooling from top (eutectic solder).

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Figure 8

(a) Number of voids as distributed by size and region for heating from bottom, cooling from bottom (lead-free solder). (b) Average void volume percentages by regions for heating from bottom, cooling from bottom (lead-free solder). (c) Number of voids as distributed by size and region for heating from bottom, cooling from bottom (eutectic solder). (d) Average void volume percentages by regions for heating from bottom, cooling from bottom (eutectic solder).

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Figure 1

(a) Reflow time-temperature profile for lead-free solder and (b) reflow time-temperature profile for eutectic solder

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