Research Papers

Direct Concentration Approach of Moisture Diffusion and Whole-Field Vapor Pressure Modeling for Reflow Process—Part I: Theory and Numerical Implementation

[+] Author and Article Information
B. Xie, H. Ding

Advanced Electronic Manufacturing Center, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

X. J. Fan1

Department of Mechanical Engineering, P.O. Box 10028, Lamar University, Beaumont, TX 77710; Department of Engineering Mechanics, South China University of Technology, Guangzhou 510640, Chinaxuejun.fan@lamar.edu

X. Q. Shi

 Hong Kong Applied Science and Technology Research Institute, 2 Science Park East Avenue, Shatin, Hong Kong


Corresponding author.

J. Electron. Packag 131(3), 031010 (Jul 31, 2009) (7 pages) doi:10.1115/1.3144147 History: Received January 09, 2008; Revised March 22, 2009; Published July 31, 2009

Moisture concentration is discontinuous at interfaces when two materials, which have different saturated moisture concentrations, are joined together. In order to perform moisture diffusion modeling in a multimaterial system such as electronic packages, normalization methods have been commonly used to remove the discontinuity of moisture concentration at interfaces. However, such treatments cannot be extended to a reflow process, in which ambient temperature and/or humidity vary with time. This paper develops a direct concentration approach, with which the moisture concentration is used as a field variable directly. Constraint equations are applied to meet the interface continuity requirements. Further in this paper, a simplified vapor pressure model based on a multiscale analysis is developed. The model considers the phase change in moisture, and links the macroscopic moisture concentration to the moisture state at a microscopic level. This model yields the exact same results with the original vapor pressure model (Fan, , 2005, “A Micromechanics Based Vapor Pressure Model in Electronic Packages,” ASME J. Electron. Packag., 127(3), pp. 262–267). The new model does not need to relate to a reference temperature state. Numerical implementation procedures for calculating moisture concentration and ensuing vapor pressure, which are coupled with temperature analysis, are presented in this paper.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Schematic of moisture distribution in bulk and at interface for a bimaterial: (a) unsaturated case and (b) saturated case

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Figure 2

Saturated moisture concentration as a function of temperature of a BT sample for two different RH levels (14)

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Figure 3

Saturated moisture concentration as a function of temperature for a low-Tg die-attach film at 60%RH level (15)

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Figure 4

Special treatment and constrain equations at a bimaterial interface in the DCA

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Figure 5

Two distinct states of moisture in pores in polymer materials

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Figure 6

Implementation procedures of sequentially-coupled heat transfer/moisture diffusion and vapor pressure modeling for reflow process

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Figure 7

(a) a bimaterial two-step loading problem and (b) a single-material problem

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Figure 8

Comparison of moisture concentration between the DCA and analytical solution




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