Materials Transactions, Vol. 49, No. 3 (2008) pp. 685 to 687 #2008 The Japan Institute of Metals RAPID PUBLICATION
Production and Properties of a Spray Formed 70%Si-Al Alloy for Electronic Packaging Applications
Kun Yu*, Chao Li, Richu Wang and Jun Yang
School of Materials Science and Engineering, Central South University, Changsha 410083, P. R. China
The high silicon content Si-Al alloy is a typical heat dissipation material that used in the electrical packaging field. A spray forming process is used to produce a 70%Si-Al alloy specimen as a heat dissipation material with a diameter of 76.2 mm (3 inch) and a thickness of 6 mm. Then the spray formed Si-Al alloy specimens are hot pressed at 570 C with different pressure ranged from 200 MPa to 700 MPa to increase their density. The physical properties of the experimental alloy specimen are measured. And the microstructures are observed by using optical microscopy and scanning electronic microscopy. The results show that Spray forming is suitable to produce a 70%Si-Al alloy. The size of primary Si phase in the spray formed 70%Si-Al alloy is refined only 20 30 mm. The relative density of 70%Si-Al alloy after spray forming is about 90%. With a following hot pressure of 700 MPa, the relative density value can obtain 98%. The typical physical properties such as the thermal conductivity, coefficient of thermal expansion and electrical conductivity of spray formed 70%Si-Al alloy are acceptable as a heat dissipation material for many electronic packaging applications. [doi:10.2320/matertrans.MRP2007630]
(Received December 10, 2007; Accepted January 15, 2008; Published February 25, 2008) Keywords: 70% silicon aluminum alloy, spray forming, thermal properties
1. Introduction 2. Experimental Method
Modern electronics demands fast and efficient heat The polycrystalline pure Silicon (99.9%) and pure Alumi- removal during the operation of the components.1) Such heat num (99.9%) were induction melted at about 1500 C, then dissipation is achieved by using heat sink and heat spreader the alloy melt was poured into a pre-heated graphite tundish packaging materials to prevent the failure of semiconductor with a diameter 4 mm. The melt was atomized by a delivery devices. Ideal materials for the thermal dissipation need a nozzle with N2 gas. The spray melt was deposited on an iron coefficient of thermal expansion (CTE) match to Si or GaAs plate and build up a billet with a diameter about 100 with a value of 7 9 10 6/ C. And it need a high thermal 110 mm in the spray chamber. The specimens with a diameter conductivity of >100 W/(mK) and a low density of <3 g/ 76.2 mm (3 inch) and a thickness of 6 mm were cut from the cm 3. High Si-Al alloy occupies all these physical properties solid billet. Then the specimens were hot pressed in a pre- for the thermal management.1,2) Especially the 70%Si-Al heated mould with different pressure 200 MPa, 400 MPa and alloy obtains a CTE about 7 10 6/ C 1 that matches the 700 MPa respectively at 570 C to reduce the porosity. 1) Al2O3 and GaAs semiconductor devices. Furthermore, the The microstructure of the spray forming specimens Si-Al alloy has a high specific stiffness and the platability compared with that of the gravity cast Si-Al alloy by using with Au or Cu. So it becomes an advanced electronic optical microscopy and a KYKY 2800 scanning electronic packaging material.2) There are different methods to product microscopy (SEM). The density of Si-Al alloy specimens are a high Si-Al alloy such as conventionally gravity casting, measured by using Archimedes method. The thermal semi-solid processing, and powder metallurgy technology.3) diffusion ratio was test by laser flash method in a JR-3 laser But the primary Si is too large to be used in the high Si-Al tester, then the thermal conductivity of alloy specimen was alloy through the gravity casting method.4) And the relative calculated by K ¼ C, where is thermal diffusion density of alloy is too low (only 70% 80%) for a powder ratio, is the density, and C is the thermal capacity. And the metallurgy Si-Al production. Therefore, a spray forming CTE was measured in a differential thermal analysis technology is used to manufacture the high Si-Al alloys by apparatus from 25 C to 400 C with a heating speed of Osprey Metals Ltd recently.2,6,7) This technology provides a 150 C/h. useful method to produce a high Si-Al alloy with a Si concentration up to 70%. And it can provide a suitable 3. Results and Discussion microstructure for such material. Therefore, in this paper, the production technology of a spray forming 70%Si-Al alloy is The specimens with the diameter of 76.2 (3 inch) are also researched on a self-designed spray deposition instru- shown in Fig. 1. ment. The microstructure and some physical properties of Figure 2(a) shows the optical micrographs of the spray such 70%Si-Al alloy are studied to evaluate the technology formed 70%Si-Al alloy and a comparison of an as-cast and to produce a test sample with a diameter of 76.2 mm 70%Si-Al alloy. The microstructure of spray formed 70%Si- (3 inch) and a thickness of 6 mm. Al alloy consisted of a continuous network of primary Si and a secondary Al phase interpenetrated. The spray formed Si-Al alloy exhibits no Al-Si lamellar eutectic in the alloy and only consisted of a single Al-rich phase. In contrast, *Corresponding author, E-mail: [email protected] in gravity as-cast 70%Si-Al alloy, a very large flake shape 686 K. Yu, C. Li, R. Wang and J. Yang
100
98
96
94
Spray formed + hot pressed density,% relative 92 Spray formed 70%Si-Al 70%Si-Al 90
Fig. 1 Specimens of spray formed and hot press 70%Si-Al alloy. 0 100 200 300 400 500 600 700 pressure,MPa
Fig. 3 Relation between the relative densities with the pressure.
(a) (b)
Al
Si Si
100µm 100µm
Fig. 2 Morphology of spray formed and as-cast 70%Si-Al alloy (a) Spray formed 70%Si-Al alloy (b) ordinary as-cast 70%Si-Al alloy. primary Si phase and coarse acicular eutectic phase are higher pressure, the density can be increased higher. But the shown in Fig. 2(b). This difference is because of the rapidly Si-Al alloy is too fragile to bear a much high pressure. The solidification speed of Si-Al melt during the spray deposited specimens will be brittle easily under a pressure of over process. The coarsening of both the primary Si and the 700 MPa in the experiment. And a 98% relative density can eutectic phase were prohibited by the high cooling rate be accepted by the electronic packaging applications. There- during spray forming (the cooling rate of spray forming is fore, the adaptive pressure is about 700 MPa. The micro- about 1000 times faster than that of the ordinary gravity structure of spray formed alloy and hot press alloy are shown casting). So the alloy obtain a refine microstructure with a in Fig. 4. There are some black holes in the spray formed Si- primary Si only about 20 30 mm. But the size of primary Si Al alloy Fig. 4(a). But these holes decrease obviously after of ordinary as-cast 70%Si-Al is lager than 0.5 mm. Such hot pressing in the Fig. 4(b). And the Si phase does not coarse refined phase of spray formed 70%Si-Al alloy provide many at the hot press temperature. good physical properties for using as an electronic packaging The physical properties of 70%Si-Al alloy as shown in materials but ordinary as-cast Si-Al alloy can not be used due Table 1. They are acceptable for a heat dissipation material. to its coarse phase that prohibit the heat disappoint.1,8) Especially, the value of thermal conductivity and the CTE are The density variation of spray formed Si-Al alloy with a good to be used as heat dissipation materials for the following hot pressure force is shown in Fig. 3. It can be seen semiconductor devices. Therefore, the spray forming is an that the alloy obtains a relatively low density (about 90%) useful method to produce a heat removal material for after spray deposition without a following hot press. But it electronic packaging applications. can obtain an acceptable high density (about 98%) after hot press at 570 C under a pressure 700 MPa in a preheated mold. 4. Conclusion The pressure temperature is set at 570 C because the Al-Si eutectic temperature is 577 C and a liquid phase will emerge (1) Spray forming is a useful method to produce a 70%Si- over such temperature. Al alloy that used as a heat dissipation material. The It is obviously that the density of spray formed alloy can be size of Si phase in the spray formed 70%Si-Al alloy is improved by hot press and the porosity decrease obviously only 20 30 mm. under a high pressure. It can be anticipated with an even (2) The relative density of spray formed 70%Si-Al alloy is Production and Properties of a Spray Formed 70%Si-Al Alloy for Electronic Packaging Applications 687
(a) (b)
Fig. 4 Microstructure of spray formed and hot press 70%Si-Al alloy (a) spray formed 70%Si-Al alloy (b) spray formed and hot press at 570 C with 700 MPa.
Table 1 Typical physical property of alloy specimens.
thermal coefficient of coefficient of Electrical Density conductivity thermal thermal conductivity g cm 3 W/(m K) expansion (25 C) K 1 expansion (400 C) K 1 cm 2.38 102 6:9 10 6 9:5 10 6 6:7 10 5
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