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Research Papers

Interactions Between Flip Chip Underfill and Solder Alloy

[+] Author and Article Information
D. Blass1

 Universal Instruments Corporation, Conklin, NY 13904dan.blass@lmco.com

P. Borgesen

Department of Systems Science and Industrial Engineering, Binghamton University, P.O. Box 6000 Binghamton, NY 13902-6000pborgese@binghamgton.edu

1

Present address: Lockheed Martin, Owego, NY.

J. Electron. Packag 132(3), 031008 (Sep 09, 2010) (5 pages) doi:10.1115/1.4002010 History: Received October 17, 2009; Revised June 06, 2010; Published September 09, 2010; Online September 09, 2010

The effects of underfill selection on flip chip reliability were always a complex issue. Mechanical optimization of the underfill performance, achieved by the addition of appropriate fillers, is invariably a tradeoff between the adhesion and the coefficient of thermal expansion (CTE) and, thus, also between in-plane and out-of-plane stresses. Another critical concern is the degradation of the underfill in processing and/or long term exposure to operating temperatures and ambient humidity. This is strongly affected by the chemical compatibility with combinations of solder mask, chip passivation, and flux residues. The latter is believed to be responsible for our observation of interactions with the solder alloy, too. As for the effects of glass transition temperatures and CTE, we find materials that were close to optimum for eutectic SnPb to be very far from the best options for lead free joints. We report on two sets of systematic experiments. The first addressed the performance of combinations of underfills, no-clean fluxes, and solder alloys in a JEDEC level 3 moisture sensitivity test. The second one involved thermal shock testing of flip chip assemblies underfilled with one of five different materials after soldering with SnCu, SAC305, and SnPb.

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Copyright © 2010 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

(a) Daisy-chained flip chip and (b) contact pads on PCB

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Figure 2

Reflow profiles used for (a) SnPb assembly and (b) LF-2 assembly

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Figure 3

Outline of first set of flip chip assemblies (see Fig. 1)

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Figure 4

Outline of first set of flip chip assemblies (chip layout reflected in Fig. 7)

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Figure 5

C-SAM images showing examples of delamination. On the left, underfill R has started delamination from solder joints near the corner. On the right, the same underfill has popcorned.

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Figure 6

Weibull plots of cumulative failure distributions for flip chip assemblies underfilled with five different underfills. For four underfills, results for different solder alloys are combined. For underfill U, results for lead free and SnPb solders are shown separately.

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Figure 7

C-SAM images of assembly soldered with SnPb and underfilled with underfill G shown before and after exposure to 2000 LLTS cycles

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Figure 8

C-SAM images of assembly soldered with SAC305 and underfilled with underfill E before and after exposure to 1000 LLTS cycles

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Figure 9

Assembly soldered with eutectic SnPb and underfilled with underfill U before and after exposure to 2000 LLTS cycles

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Figure 10

Assembly soldered with SAC305 and underfilled with underfill U before and after exposure to 500 LLTS cycles

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