DEVELOPMENT OF NANO-TUNGSTEN-COPPER POWDER AND PM PROCESSES 1 Seong Lee, 1 Joon-Woong Noh, 2 Young-Sam Kwon, 2 Seong Taek Chung, 3 John L. Johnson, 4 Seong Jin Park and 4 Randall M. German 1 Agency for Defense Development Yuseong, P.O.Box 35-5, Daejon 305-600, Korea 2 CetaTech, Inc., TIC 296-3 Seonjin-ri, Yonghyeon-myon, Sacheon, Kyungnam, 664-953, S. Korea 3 Breakthrough Technology, Kennametal Inc. 1600 Technology Way, Latrobe, PA 15650, USA 4 Center for Advanced Vehicular Systems Mississippi State University P. O. Box 5405, Mississippi State, MS 39762-5405, USA ABSTRACT Heat dissipation technology in electronics packaging is a critical element in nearly all new chip generations, caused by a power multiplication combined with a size reduction. In order to guarantee their high end performance the generated thermal energy has to be removed out of packages. A heat sink or heat spreader, mounted on a base plate, requires the use of special materials possessing both high thermal conductivity (TC) and a coefficient of thermal expansion (CTE) matching semiconductor materials like Si or GaAs as well as certain packaging ceramics. In this study, nano tungsten coated copper powder has been developed with a range of compositions from 90W-10Cu to 10W-90Cu in weight percentage. Powder injection molding and die compaction were used to make samples to evaluate TC and CTE with density for two compositions of tungsten coated copper powder. The measured TC lies among theoretical values predicted by several existing models. INTRODUCTION Increasing power requirements and decreasing size of high-performance microprocessors present heat dissipation challenges to microelectronic package designers. These factors require the management of significant amounts of power over a small physical area, leading to high power densities. The lack of solubility between W and Cu enables composites of these materials to be processed with useful combinations of properties for electrical contacts and thermal management materials [1-4]. Composites with 10-20 wt.% (19.3-35.1 vol.%) Cu are most commonly used for thermal management due to a good thermal expansion match with silicon. In this study, the W-Cu alloy made from a new nano tungsten coated copper powder was evaluated in terms of density, TC, and CTE for thermal management applications. 9-53
EXPERIMENTAL AND RESULTS Powder Preparation Conventional ingot metallurgy can not be used due to the large melting point difference between W and Cu. Infiltration or sintering are available, but pores remain because there is no mutual solubility between W and Cu, as shown in Figure 1. (a) (b) Figure 1. SEM image with remained pores between W and Cu; (a) W-Cu infiltration and (b) W-Cu mixture. In this study, newly developed nano W-Cu composite powders are used. The method to manufacture the powder is shortly explained as follows; tungsten trioxide (WO 3 ) and cupric oxide (CuO) powders were mixed in a ball mill. Compositions used in this study were 85W-15Cu, 80W-20Cu, 65W-35Cu, and 20W- 80Cu in weight percent. The mixed oxide powder was reduced in a hydrogen atmosphere to form W-Cu composite powder. Figure 2 shows a schematic diagram for producing W-Cu pseudo-alloy powder [5]. Figure 2. Composite powder fabrication for pore-free W-Cu pseudo-alloy Figure 3 shows a comparison of the scanning electron micrographs (SEMs) of three powders with composition of 65W-35Cu and corresponding theoretical density of 13.7 g/cm 3. The composite powder shows that tungsten has been coated on the surface of copper particles. In addition, compositions of nano tungsten coated copper powder can be controlled with a range of from 90W-10Cu to 10W-90Cu in weight percentage. Figure 4 shows two powders with different composition. 9-54
(a) (b) (c) Figure 3. SEMs of three 65W-35Cu powders; (a) W-Cu mixture, (b) W-Cu composite (CetaTech), and (c) W-Cu composite (H.C.Starck). (a) 85W-15Cu (b) 65W-35Cu Figure 4. SEMs of two W-Cu composite powders with different compositions. Sample Preparation W-Cu composite powder was pressed into a cylindrical shape of 12.7 mm in diameter and 8 mm in height with compaction pressure of 150 MPa. We used three different compositions, 20W-80Cu, 65W-35Cu, and 85W-15Cu. For 85W-15Cu, we used two different oxide powders. One was as-received powder and the other one was attrition milled to reduce its particle size. Figure 5 shows dilatometry results by Netzsch Dil 402E with 5 o C/min to 1400 o C for mixed and composite powders along with their corresponding microstructures. The composite powder shows much better sintering behavior than the mixed powder. The compacts were sintered a hydrogen atmosphere at temperatures ranging from 1110-1350 C, which depended on their composition and were determined from the dilatometry results by Netzsch Dil 402E with 5 o C/min to 1400 o C, as shown in Figure 6. 9-55
Figure 5. Dilatometry for composite powder and mixed powder Figure 6. Dilatometry for three different compositions Mechanical Properties Average densities, measured by Archimedes' technique, and microhardness of the sintered compacts are listed in Table 1. Measured densities are very close to theoretical values. The sintered microstructures of W-Cu composite powders were homogeneous without pores, as shown in Figure 7. From compression tests at room temperature, we found a strength of 820 MPa and a compressive strain of 33 % for 65W-35Cu. Table 1. Density and microhardness for various compositions. composition (wt.%) density (g/cm 3 ) relative density (%) hardness (HRB) 20W-80Cu 9.88 99.0 51 65W-35Cu 13.60 99.3 101 80W-20Cu 15.35 98.0 103 85W-15Cu (fine copper) 16.16 98.3 107 9-56
Figure 7. SEMs for sintered compacts with compositions and sintering conditions. Table 2. Thermal properties for various composition composition (wt.%) density (g/cm 3 ) thermal conductivity (W/m K) heat capacity (J/kg K) thermal diffusivity (cm 2 /s) 65W-35Cu 13.60 297.8 301.7 0.7263 80W-20Cu 15.35 241.0 228.0 0.6893 85W-15Cu 16.00 209.8 228.5 0.5747 85W-15Cu (fine copper) 16.16 222.1 231.3 0.5933 Thermal Properties The room temperature thermal conductivity was determined. For disk-shaped samples, the thermal conductivity was calculated from the thermal diffusivity, which was measured according to ASTM E1461 using an Anter Flashline 5000 thermal diffusivity laser flash analysis system. Heat capacity was also measured by the same machine. The samples were machined to a thickness of 2 mm. From the thermal diffusivity, the thermal conductivity was calculated according to the following relationship C (1) where C p is the specific heat and is the density of the sample. Table 2 shows the measured thermal conductivity values along with thermal diffusivity and heat capacity. Figure 8 plots the measured thermal p 9-57
conductivity values for several compositions in comparison to 3 existing models for thermal conductivity of composites [6]. Figure 8. Thermal conductivity vs. copper content in comparison to model predictions. We also measured the coefficient of thermal expansion (CTE) by dilatometer, NETZSCH DIL 402 C. 65W-35Cu alloy has a CTE of 11.6 10-6 K -1 with linear behavior in range of 36-200 o C and 85W-15Cu alloy has a CTE of 8.3 10-6 K -1 with linear behavior in range of 38-200 o C, as shown in Figure 9. (a) 65W-35Cu (b) 85W-15Cu Figure 9. Coefficient of thermal expansion measurements. Electrical Properties We measured two electrical properties, electrical resistivity and electrical conductivity by ASTM 4-probe method with cubic brick sample of 5 mm 5 mm 40 mm. 65W-35Cu alloy has an electrical resistivity of 3.3 cm and 85W-15Cu alloy has an electrical resistivity of 4.8 cm. As for electrical conductivity, 65W-35Cu alloy has 52% IACS and 85W-15Cu alloy has 36% IACS. 9-58
CONCLUSION In this work, a novel process to fabricate W-Cu composite powders has been presented. The microstructures of sintered product from this powder were pore-free and homogeneous. Thermal conductivities and coefficients of thermal expansion for compositions sintered from these composite powders were near theoretical predictions. REFERENCES 1. A.J. Stevens, Powder-Metallurgy Solutions to Electrical-Contact Problems, Powder Metall. Int., 1974, Vol. 17, pp. 331-346. 2. N.C. Kothari, Factors Affecting Tungsten-Copper and Tungsten-Silver Electrical Contact Materials, Powder Metall. Int., 1982, Vol. 14, pp. 139-143. 3. E. Kny, Properties and Uses of the Pseudobinary Alloys of Cu with Refractory Metals, Proc. 12th Int. Plansee Seminar, edited by H. Bildstein and H.M. Ortner, Metallwerk Plansee, Reutte, Austria, 1989, Vol. 4, pp. 763-772. 4. W. Neumann and E. Kny, Thermal Properties of Materials Used for Heat Sink Applications, Proc. 12th Int. Plansee Seminar, edited by H. Bildstein and H.M. Ortner, Metallwerk Plansee, Reutte, Austria, 1989, Vol. 4, pp. 113-137. 5. S. Lee, M.-H. Hong, J.-W. Noh, E.-P. Kim, H.-S. Song, and W.-H Baek, Method of Forming Tungsten-Coated W-Cu Composite Powder, U.S. Patent No. 6,863,707 B2, Mar. 8, 2005. 6. A.D.Brailsford and K.G. Major, The Thermal Conductivity of Aggregates of Several Phases, Including Porous Materials, Brit.J.Appl.Phys., 1963, Vol. 15, pp. 313. 9-59