The effect of tungsten particle size on the processing and properties of infiltrated W-Cu compacts
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I.
INTRODUCTION
THE W-Cu and Mo-Cu compacts are frequently used as heat sinks in electronic packages. They have low thermal expansion coefficients (CTEs) to match those of the adjacent silicon or ceramics in electronic packages so that stresses arising from the differential CTEs can be avoided. These compacts also have high thermal conductivity in order to dissipate the heat generated by the power devices in the circuit.[1–6] Another important application for W-Cu and Mo-Cu materials is electrical contacts, which need a combination of good wear resistance and thermal conductivity to alleviate the erosion and arcing problem incurred during power switching.[6–9] Since W-15Cu composites are frequently chosen for use in heat sinks and electrical contact materials, they were used in this study. One of the common processes used in preparing W-Cu and Mo-Cu compacts is to mix tungsten or molybdenum powder with copper powder and then to press and sinter the admixed powder.[7,9,10] Another method, the most widely used one, is to prepare a sintered skeleton with the desired density and then to infiltrate it with copper.[7,9,10] The relative density obtained by both methods is seldom complete. To further improve the density, subsequent repressing, resintering, rolling, drawing, or hot isostatic pressing are sometimes employed.[7,9,11] Another approach is to add a small amount of Fe, Co, or Ni in the tungsten or molybdenum powder.[5,8,10,12,13] These elements help the wetting of copper on tungsten surfaces and improve the infiltration efficiency. However, the thermal conductivity of the compact is impaired by the alloying effect of these elements with the copper. To predict the CTE of a W-Cu compact, aW-C, the simplest form is aW2C 5 VW z aW 1 VC z aC
[1]
W.S. WANG, Graduate Student, and K.S. HWANG, Professor, are with the Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China. Manuscript submitted September 15, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
where aW and aC are the CTEs and VW and VC are the volume fractions of tungsten and copper, respectively. This equation was modified in several studies based on the calculations for the stress and strain at the W/Cu interface.[14,15] German recently developed new equations for thermal conductivity and CTE based on a model assuming a tungsten core surrounded by copper at eight cubic edges.[16] Thus, both tungsten and copper are interconnected, which is similar to the infiltrated W-Cu compacts. In all of the preceding studies, comparisons between predicted values and collected experimental data showed that the measured values agree relatively well with the theoretical calculations on CTE. For thermal conductivity, the measured values are more scattered and deviate significantly from calculations. This scattering could be caused by differences in the purity of copper and tungsten used, the manufacturing process, the amount of pores, the residual stresses, and the methods used for measuring the thermal conductivi
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