Time-Lapse Measurements of Creep in Au-Sn Die Bonds
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0968-V04-06
Time-Lapse Measurements of Creep in Au-Sn Die Bonds Ryan Marinis1, Adam Klempner1, Peter Hefti1, Ryszard Pryputniewicz1, Thomas Marinis2, and Joseph Soucy2 1 Mechanical Engineering, Worcester Polytecnic Institute, 100 Institute Road, Worcester, MA, 01609-2280 2 Draper Laboratory, 555 Technology Square, Cambridge, MA, 02139
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 AuSn 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 sensorpackage 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 AuSn 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 AuSn has been measured interferometrically over time to quantify die distortion in a packaged application. INTRODUCTION Although the use of AuSn for braze attachment of die into packages has been used for years the properties of this interface are still relatively unknown. Residual stresses are present after brazing due to CTE mismatch between the silicon die and the ceramic substrate, causing a bowing of the die in the package. Over time, relaxation in the AuSn interface may relieve stresses, reducing the curvature in the package. Any relaxation, or change in the chip shape, may compromise the performance of a sensor. Such relaxation is certainly a possibility over a lifespan of 20 years, and may be measured quantitatively in significantly less time. Material properties of the AuSn braze material are examined by creep testing. Time lapse imaging of a loaded sample allows the determination of the activation energy and constants in the relation of strain rate as a function of stress. These properties may then be used to build a model to predict the relaxation of a component as a function of time and temperature for a brazed attachment of a die in ceramic package.
Interferometric measurements have been made on a sample substrate brazed into a ceramic package to determine the surface curvature. An initial measurement was made to determine initial
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