Research Papers

Void Formation Mechanism of Flip Chip in Package Using No-Flow Underfill

[+] Author and Article Information
Sangil Lee

The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332gtg647p@mail.gatech.edu

M. J. Yim, Daniel Baldwin

The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332

J. Electron. Packag 131(3), 031014 (Jul 31, 2009) (5 pages) doi:10.1115/1.3153369 History: Received March 25, 2009; Revised April 18, 2009; Published July 31, 2009

This paper investigates the void formation mechanism induced by chemical interaction between eutectic solder (Sn63/Pb37) wetting and no-flow underfill material curing during flip chip in package assembly. During the process, low weight molecular components, such as fluxing agents and water molecules, could be induced by the chemical interaction between solder wetting and underfill curing when these components are heated to melt and cure, respectively. The low weight molecular components become volatile with exposure to temperatures above their boiling points; this was found to be the main source of the extensively formed underfill voiding. This mechanism of chemically and thermally induced voids was explained using void formation study, differential scanning calorimetry thermogram comparison, and gas chromatography and mass spectroscopy chemical composition identification on the suggested chemical reaction formula. This finding can enhance understanding of the mechanism that drives no-flow underfill voiding and can develop a void-free flip chip assembly process using no-flow underfill material for cost effective and high performance electronics packaging applications. Furthermore, this study provides the design guideline to develop an advanced no-flow underfill having high performance at high temperature range for the lead-free application.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

(a) No-flow underfill voiding for organic acid-based fluxes and (b) their reaction with oxirane of epoxy resins (8)

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Figure 2

Typical micrographs of C-SAM analysis: (a) failed part in electrical interconnection and (b) yielded part in electrical interconnection

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Figure 3

A typical test vehicle in void formation study: (a) schematic illustrations of TV-1 and TV-2, (b) TV-1 without solder before heating, and (c) TV-2 with solder bumps before heating (8)

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Figure 4

(a) Sample A: no-flow underfill in vial; (b) sample B: no-flow underfill and lead-tin solders

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Figure 5

Typical micrographs of void formation study: (a) TV-1 without solder after heating and (b) TV-2 with solder bumps after heating (8)

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Figure 6

DSC measured heat flow diagrams of sample A (without solders) and sample B (with solders)

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Figure 7

Spectrums of evolved gases in underfill reflow process with eutectic solders and without eutectic solders using GC-MS



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