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RESEARCH PAPERS

Experimental Study of Void Formation in High-Lead Solder Joints 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 127(2), 120-126 (May 18, 2004) (7 pages) doi:10.1115/1.1876472 History: Received January 12, 2004; Revised May 18, 2004

Understanding the formation of voids in solder joints is important for predicting the long-term reliability of solder interconnects. This paper reports experimental research on the formation of void bubbles within molten solder bumps in flip-chip connections. For flip-chip-soldered electronic components, which have small solder volume, voids can be more detrimental to reliability. A previous theory based on thermocapillary flow reveals that the direction of heating influences void formation. Using different heating profiles, 480 solder joints of flip-chip assemblies were processed. A high-lead 90Pb8Sn2Ag solder was employed in the experiments. The solder samples were microsectioned to determine the actual size or diameter of the voids. A database on sizes and locations of voids was then constructed. More defective bumps, 80%, and higher void volume were found when the solder was melted from top (flip-chip side) to bottom (test board side). The observation on cases with melting direction from bottom to top had 40% defective bumps. The results show that a single big void is near the solder bump center with a few small voids near the edge. This supports the numerical study based on the thermocapillary theory. When the melting direction was reversed, many small voids appear near the edge. Big and middle-size voids tend to stay in the middle and outer regions from top towards middle layer of the bump. This experimental finding does not completely agree with the interpretation on the formation of voids by thermocapillary theory, however, the results do show that heat flux direction plays significant role in the formation and distribution of void bubbles in molten solder.

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Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Collection of bubbles near the top as a large bubble; (b) collection of bubbles in a ring near the top of the bump (11)

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Figure 2

Flip-chip-substrate assembly

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Figure 3

Reflow research oven

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Figure 4

(a) Top view of thermocouple configurations; (b) bottom view of thermocouple configurations

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Figure 5

(a) Reflow time–temperature profile; (b) temperature differential across the flip-chip-substrate assembly (heating from top and cooling from bottom)

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Figure 6

(a) Reflow time–temperature profile; (b) temperature differential across the flip-chip-substrate assembly (heating from bottom and cooling from top)

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Figure 7

Micrographs of a solder bump reflowed with melting direction from bottom to top. (a) first cut; (b) third cut; (c) seventh cut.

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Figure 8

Void volume as a percentage of total void volume as distributed by size and region for heating from bottom, cooling from top

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Figure 9

Micrographs of a solder bump reflowed with melting direction from top to bottom. (a) second cut; (b) fourth cut; (c) eighth cut.

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Figure 10

Void volume as a percentage of total void volume as distributed by size and region for heating from top, cooling from bottom

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