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Research Papers

Direct Concentration Approach of Moisture Diffusion and Whole-Field Vapor Pressure Modeling for Reflow Process—Part II: Application to 3D Ultrathin Stacked-Die Chip Scale Packages

[+] 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, Lamar University, P.O. Box 10028, 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

1

Corresponding author.

J. Electron. Packag 131(3), 031011 (Jul 31, 2009) (6 pages) doi:10.1115/1.3144154 History: Received September 18, 2008; Revised March 22, 2009; Published July 31, 2009

In the present study, the direct concentration approach (DCA) and the whole-field vapor pressure model developed in Part I of this work (Xie, 2009 “Direct Concentration Approach of Moisture Diffusion and Whole Field Vapor Pressure Modeling for Reflow Process: Part I–Theory and Numerical Implementation  ,” ASME J. Electron. Packag., 131, p. 031010) is applied to 3D ultrathin stacked-die chip scale packages to investigate wafer-level die-attach film cohesive failures during the reflow process. Oversaturation, which refers to the film that absorbs more moisture when reflow process begins, is observed using the DCA. The modeling results suggest that the moisture transport and escape through the substrate during the reflow is responsible for the film rupture. A small reduction in substrate thickness could result in a significant decrease in moisture concentration and vapor pressure in bottom layer film and therefore reduce failure rate greatly. A slight improvement in reflow profile while still meeting specification allows a significant amount of moisture loss during the reflow; hence failure rate could also be reduced greatly. The mechanism of soft film rupture at reflow due to moisture is discussed in detail. The simulation results are consistent with the published experimental data.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Storage modulus of a die-attach film with and without moisture absorption as a function of temperature

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Figure 2

(a) A TSAM image on a 6×6 array CSP panel (black regions mean failures inside packages) and (b) die-attach film cracking and voiding at the bottom layer attached to the substrate

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Figure 3

(a) A schematic diagram of a stacked-die CSP and (b) a bimaterial model for moisture diffusion analysis (Mat1: substrate; Mat2: die-attach film)

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Figure 4

Moisture diffusion history plot at the interface in Step 2

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Figure 5

Moisture concentration distributions in Step 2

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Figure 6

Two reflow loading profiles used in the present study

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Figure 7

History plots of vapor pressure and moisture concentration at the interface during the reflow

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Figure 8

Schematic diagram for Scenario I of vapor pressure buildup during the reflow

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Figure 9

Schematic diagram for Scenario II of vapor pressure buildup during the reflow

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Figure 10

Schematic structure of a 3D ultrathin stacked-die chip scale package

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Figure 11

Moisture concentration contours of a CSP at 260°C using the DCA: (a) a thinner substrate and (b) a thicker substrate

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Figure 12

Moisture concentration comparison between two substrate thicknesses

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Figure 13

Vapor pressure contours of a CSP at 250°C: (a) a thinner substrate and (b) a thicker substrate

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Figure 14

Vapor pressure comparison between two substrate thicknesses

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Figure 15

Moisture concentration contours at 250°C subjected to two different reflow profiles

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Figure 16

Vapor pressure contours at 250°C subjected to two different reflow profiles

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Figure 17

Vapor pressure comparison between two substrates

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