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Time-Lapse Measurements of Creep in Au-Sn Die Bonds

  • Ryan Marinis (a1), Adam Klempner (a2), Peter Hefti (a3), Ryszard Pryputniewicz (a4), Thomas Marinis (a5) and Joseph Soucy (a6)...

Abstract

Gold-tin braze is the preferred material for attaching high-precision MEMS inertial sensors within hermetic ceramic packages. The bonds can be made at relatively low temperatures, are mechanically robust, and outgas at very low rates in vacuum sealed packages. There is one significant limitation to Au-Sn bonds, however. The thermal expansion coefficients of MEMS die and ceramic packages are not perfectly matched and temperature gradients occur when the assembly is cooled after brazing. As a result, there is considerable residual stress in the bonded assembly, which is accommodated to some extent by distortion of the sensor die. Over time, as these stresses relax, the distortion of the die changes, which causes the spacing between elements of the integral MEMS sensor to change as well. An important element of sensor-package design is insuring that stress relaxation effects do not cause the instrument to drift beyond its performance specification limits over a typical lifetime of 20 years.

Even though Au-Sn has been used for decades to attach silicon chips to ceramic substrates, there is little data available, particularly at low temperatures. An oven, with a specially designed window, allowed in-situ measurements to be made as a function of temperature, joint thickness and load stress. Additionally, a MEMS device brazed to a package with Au-Sn has been measured interferometrically over time to quantify die distortion in a packaged application.

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References

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1. Francon, M., Optical Interferometry, Academic Press, New York, 1966, p. 201.
2. Kreis, Thomas, Holographic Interferometry Principles and Methods, Akademie Verlag, Berlin, 1996, pp. 126129.
3. Dieter, George E., Mechanical Metallurgy, McGraw-Hill, New York, 1961, p. 354
4. Findley, William N., Lai, James S., and Onaran, Kasif, Creep and Relaxation of Nonlinear Viscoelastic Materials, Dover Publications, New York, 1976, p. 53
5. Yost, F. G., Karnowsky, M. M., Drotning, W. D., and Gieske, J. H., Metallurgical Transactions A, vol. 21A, July, 1990, pp. 18851889, "Thermal Expansion and Elastic Properties of High Gold-Tin Alloys"
6. Chandran, Biju I., PhD Thesis, University of Arkansas, 1996, “Determination and Utilization of AuSn Creep Properties for Bonding Devices with Large CTE Mismatches”

Keywords

Time-Lapse Measurements of Creep in Au-Sn Die Bonds

  • Ryan Marinis (a1), Adam Klempner (a2), Peter Hefti (a3), Ryszard Pryputniewicz (a4), Thomas Marinis (a5) and Joseph Soucy (a6)...

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