Microstructures variation of spray formed Si-30%Al alloy during densification process
WEI Yan-guang(魏衍广), XIONG Bai-qing(熊柏青), ZHANG Yong-an(张永安),
LIU Hong-wei(刘红伟), WANG Feng(王 锋), ZHU Bao-hong(朱宝宏)
State Key Laboratory for Fabrication and Processing of Non-ferrous Metals,
Beijing General Research Institute for Non-ferrous Metals, Beijing 100088, China
Received 28 July 2006; accepted 15 September 2006
Abstract:
Microstructure variation of spray-formed Si-30%Al alloy during densification process by hot pressing was studied. The results indicate that the microstructure of as-deposited preforms is fine and homogenous. The primary silicon phases distributing in aluminium matrix evenly are fine and irregular. Aluminium matrix is divided into two groups: supersaturated α-Al phase or α-Al phase and Al-Si pseudo-eutectic phase or Al-Si eutectic phase. During hot pressing, the primary silicon and the aluminium matrix realign as follows: the primary silicon fractures at a given compressive stress, the particles congregates in microzone with increasing stress, and the aluminium matrix flows and connects in harness. Al-Si pseudo-eutectic phase turns into Al-Si eutectic phase due to the diffusion of atoms during densification process.
Key words:
Si-30％Al alloy; spray forming; electronic packaging materials; microstructures variation; densification process;
1 Introduction
A new family of spray formed Si-Al alloy (Si-30%Al-Si-50%Al (mass fraction)) developed for electronic packaging application has excellent physical characteristics[1-6], which include low coefficient of thermal expansion (6.8×10-6-11×10-6 K-1) that matches demand of chip (Si or GaAs), high thermal conductivity (120-149 W/(m?K)) that conducts heat duly out, and low density (2.42-2.51 g/cm3) that satisfies lightmass demand of aerospace equipments and locomotive calculator (or communication device), furthermore, the alloys have adequate strength and rigidity and also can be machined and coated or plated by traditional technology. Therefore, the alloys have been exploited and applied extensively. A family of Si-Al alloy (Al-27%Si, Al-42%Si, Al-50%Si, Al-60%Si, Al-70%Si) has been developed for electronic applications by spray forming process in Osprey Metals Ltd.[7-8]. These alloys have been produced in large and applied in RF Packages & Microwave Packages, Carrier Plates for High Frequency Circuits and Microwave Amplifier Modules, etc, nowadays. In China, Si-50%Al alloy has been prepared by powder metallurgic method in Central South University[9]. Si-40%Al alloy or Si-30%Al has been prepared by spray forming process in University of Science and Technology Beijing, Beijing General Research Institute for Non-ferrous Metals or Institute of Metal Research Chinese Academy of Sciences[10-13].
However, there are few published literatures of microstructures variation of spray formed Si-30%Al alloy during densification process. In this paper, the microstructures of the preforms were studied. In our previous work, it was found that the relative density of the preforms was in the range of 95%-98%[14], so hot pressing was adopted in densification process of the alloy prior to using for electronic packaging. Research on microstructures variation of the alloys during hot pressing will help to optimize technologic parameters of densification process.
2 Experimental
The experimental material was spray formed Si-30%Al alloy. The preforms were prepared on spray forming equipment developed by Beijing General Research Institute for Non-ferrous Metals and Jinzhou Metallurgic Technology institute. In the experiment, a double deck nonrestrictive atomizing nozzle was adopted, and the atomization gas was pure nitrogen, the angle of normal of cross section of the delivery tube and parallel surface of substrate was 30?-40?, the eccentric distance was 20-30 mm, the distance of substrate and atomizer was 500-700 mm, while the range of atomization pressure was 0.45-0.6 MPa and the diameter range of the delivery tube was 3.8-4.5 mm.
The densification process was conducted on the hot-pressed sintering furnace developed by Beijing General Research Institute for Non-ferrous Metals. In the experiment, the alloys were pressed at 570 ℃ under 200, 250, 300 MPa for 4 h.
The microstructures of Si-30%Al alloy were observed on NEOPHOT-2 type optical microscope, CAMBRIDGE-2 type scanning electron microscopy (SEM) and JEM2000FX transmission electron micro- scopy(TEM). The differential scanning calorimeter(DSC) analysis of the alloy was conducted on NETZSCH STA 409C testing machine.
3 Results and discussion
3.1 Microstructure of as-deposited Si-30%Al alloy
Fig.1 shows the metallograph of spray-deposited Si-30%Al alloy. It can be seen that the microstructure of as-deposited preforms is fine and homogenous. The primary silicon phases distributing in aluminium matrix evenly are fine and irregular, whose size is in the range of 10-40 μm. There are porosities in the alloys because of the characteristics of spray forming process.
Fig.1 Microstructure of as-deposited Si-30%Al alloy
Fig.2 shows the scanning electron microscope photograph of as-deposited Si-30%Al alloy. Table 1 shows the composition of micro area in the alloy. From the preliminary analysis of SEM photograph and the composition at different positions, symbol position a represents supersaturated solid solution of the primary silicon phase, and symbol positions b, c and d represent supersaturated α(Al) phase or phase α(Al) because the silicon content of phase α(Al) is ranging between 1.43% and 5.77%, and the symbols of position d and position f represent Al-Si pseudo-eutectic phase or Al-Si eutectic phase because the silicon content of the eutectic phase is ranging between 12.57% and 19.55%. However, the two phases cannot be distinguished in SEM photograph because they are too small.
Fig.2 SEM image of as-deposited Si-30%Al alloy
Table 1 Composition of six positions in Fig.2 (mass fraction, %)
Fig.3 shows the TEM image of as-deposited Si-30%Al alloy. Table 2 shows the composition of micro area in the alloy. From the analysis of SEM photograph and the composition at different positions, symbol position a represents Al-Si eutectic phase, symbol position b represents α-Al phase, and symbol position c represents the primary silicon. As seen in Fig.3(b), Al-Si eutectic phase showing sandwich can be observed distinctly.
Table 2 Composition of three positions in Fig.2 (mass fraction, %)
Fig.4 shows the DSC thermogram of as-deposited Si-30%Al alloy. As seen in Fig.4, there is an endothermal reaction ranging from 566.4 to 581.7 ℃ during heating process, and then the remolten metal starts to emerge at 566.4 ℃ in as-deposited Si-30%Al alloy. Because the melting point of Al-Si eutectic phase is 576.5℃ according to the phase equilibrium diagram of Al-Si alloy, it shows that the remolten metal in spray formed Si-30%Al alloy is Al-Si pseudo-eutectic phase. The melting temperature of the remolten phase is lower than that of Al-Si eutectic phase because Al-Si pseudo- eutectic phase is metastable phase that has higher distortion energy, which combines the energy owing to high temperature making the material remolten.
Fig.3 TEM image of as-deposited Si-30％Al alloy
Fig.4 DSC curve of spray formed Si-30%Al alloy
3.2 Microstructure of spray formed Si-30%Al alloy after hot pressing
Fig.5 shows the microstructures of spray formed Si-30%Al alloy after hot pressing which is different from that of as-deposited preforms. There aren’t porosities in the alloy after hot pressing. Moreover, the primary silicon and the aluminium matrix realign as follows: the primary silicon fractures at a given stress, the particles congregate in microzone with increasing stress, and the aluminium matrix flows into the porosities and connects in harness.
Fig.5 Variation of microstructures of Si-30%Al alloy during hot pressing (570 ℃, 4 h): (a) 200 MPa; (b) 250 MPa; (c) 300 MPa
The primary silicon rotates and slips under the stress, however, the particles fracture because many big particles around the primary silicon restrict its movement. The fractured particles are rejoined by the aluminium matrix which flows into the crack under the stress, as seen in Fig.5(a). Deformation of the alloy depends on movements of the particles and the aluminium matrix: the cracks of the primary silicon provide new routes for fluxion of the aluminium matrix, which also conduces to the movements of particles.
The primary silicon congregates in microzone with increasing stress, as shown in Fig.5(b). The aluminium matrix connects in harness with increasing stress, and its shape relies on the figure of the primary silicon. The aluminium matrix surrounds the primary silicon, which represents typical reticulations in some part areas, as seen in Fig.5(c).
Fig.6 shows the DSC thermogram of spray formed Si-30%Al alloy after hot pressing. As seen in Fig.6, there is an endothermal reaction ranging from 575.4 ℃ to 585 ℃ during heating process, so the remolten metal emerging at 575.4 ℃ is Al-Si eutectic phase. Because of the diffusion of silicon atoms during hot pressing, Al-Si pseudo-eutectic phase that is high-energy nonequilibrium phase turns into Al-Si eutectic phase that is in low-energy state. Second phase comes into being due to the diffusion of silicon atoms, and the congregation of which represents abnormal clubbed precipitates, as seen in Fig.7.
Fig.6 DSC curve of spray formed Si-30%Al alloy after hot pressing
Fig.7 TEM image of Si-30%Al alloy after hot pressing
4 Conclusions
1) The microstructures of spray-formed Si-30%Al alloys are fine and homogenous. The primary silicon phases distributing in the aluminium matrix evenly are fine and irregular. The aluminium matrix includes supersaturated α-Al phase or α-Al phase and Al-Si pseudo-eutectic phase or Al-Si eutectic phase.
2) The microstructures of spray formed Si-30%Al alloy after hot pressing are also fine and homogenous. The primary silicon fractures at a given compressive stress, the particles congregate in microzone with increasing stress, and the aluminium matrix flows and connects in harness. Al-Si pseudo-eutectic phase turns into Al-Si eutectic phase due to the diffusion of silicon atoms.
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(Edited by LI Xiang-qun)
Foundation item: Project(G20000672) supported by the National Basic Research Program of China
Corresponding author: WEI Yan-guang; Tel: +86-10-82241206; E-mail: weiyg99@163.com
