Property measurements on spray formed Si-Al alloys

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 Nonferrous Metals,

Beijing General Research Institute for Non-ferrous Metals, Beijing 100088, China

Received 30 May 2006; accepted 21 December 2006

Abstract: A novel Si-Al alloy was prepared by spray forming process for electronic packaging. Property measurements on spray-formed Si-Al alloys after hot pressing were carried out. The results indicate that the alloys (Si-(30%-40%)Al) have advantageous physical and mechanical characteristics, including low coefficient of thermal expansion (6.9×10^-6-8.7×10^-6/K), high thermal conductivity (118-127 W/(m·K)), low density (2.421×10³-2.465×10³ kg/m³), high ultimate flexural strength (180-220 MPa) and Brinell hardness (162-261). The alloys are easy to machine to tight tolerances using standard machine tools and they can be electroplated with gold finishes and soldered with Sn-Pb alloy without any difficulty.

Key words: spray forming; Si-Al alloy; electronic packaging material; coefficient of thermal expansion

1 Introduction

Si-Al alloys (Si-(30%-50%)Al, mass fraction), prepared using the spray forming process, achieve homogeneous and isotropic properties. Their advantageous physical characteristics, such as low coefficient of thermal expansion (6.8×10^-6-11×10^-6/K), high thermal conductivity (120-149 W/(m·K)), and low density (2.42-2.51 g/cm³), make these alloys particularly attractive for packaging electronic circuitry[1-7]. Furthermore, the alloys are easy to machine to tight tolerance using standard machine tools and they can be electroplated with gold, silver or nickel finishes without difficulty. Being non-toxic, they do not require any special handling[8-10]. Therefore, exploitation and application of the alloy have extensive prospect. 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.[11-12]. Nowadays, these alloys have been produced in large scale and applied in RF packages, microwave packages, carrier plates for high frequency circuits and microwave amplifier modules etc. Si-Al alloys (Al-50%Si, Al-60%Si, Al-70%Si) have been prepared by spray forming process or powder metallurgy method in some institutes and universities in China[13-16].

In this study, Si-Al alloys were studied through measuring physical properties (coefficient of thermal expansion, thermal diffusivity and specific heat), mechanical properties (ultimate flexural strength and Brinell hardness), and electroplating and soldering tests. From thermal diffusivity and specific heat, it is possible to reliably calculate thermal conductivity of the alloys.

2 Experimental

The experimental materials were spray formed Si-Al alloys (Si-30%Al, Si-40%Al) after hot pressing. The preforms were prepared on spray forming equipment developed by Beijing General Research Institute for Non-ferrous Metals and Jinzhou Metallurgic Technology Institute (model SF720). The densification process was conducted on hot-pressed sintering furnace developed by Beijing General Research Institute for Non-ferrous Metals. Coefficients of thermal expansion (CTE) measurements were conducted on Formastor-Digital in Beijing Research Institute of Mechanical & Electrical Technology. Thermal diffusivity measurements were conducted in No.703 Institute in Beijing. Specific heat values were determined on Netzsch DSC204 in University of Science & Technology Beijing. Electro-plating tests were conducted in No.55 Institute in Nanjing. Soldering tests, the 3-point bending and Brinell hardness measurements were conducted in Beijing General Research Institute for Non-ferrous Metals. Microstructures of Si-Al alloys were observed on NEOPHOT-2 type Optical Microscope and CAMBRIDGE-2 type Scanning Electronic Microscope. The density of the alloys was tested by draining means.

3 Results and discussion

3.1 Microstructure of spray formed Si-30%Al alloy

It can be seen that the microstructure of as-deposited preforms is fine and homogenous. The primary silicon phases distributed 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 characteristics of spray forming process, as shown in Fig.1(a). Fig.1(b) shows metallograph 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 hot-pressed alloy. Moreover, the primary silicon and aluminium matrix realign. The primary silicon congregates in microzone under the stress, and aluminium matrix flows into the porosities and connects in harness.

Fig.1 Microstructures of spray formed Si-30%Al alloy: (a) As- deposited; (b) After hot pressing

3.2 Physical properties of spray formed Si-Al alloy

Coefficients of thermal expansion (CTE) measurements were made on samples of Si-30%Al and Si-40%Al alloys. Fig.2 shows CTE of Si-30%Al and Si-40%Al alloys. It can be seen that CTE of Si-40%Al alloy increases with increasing temperature before 400 ℃, however, CTE of Si-40%Al alloy at 500 ℃ is lower than that at 400 ℃. From Fig.2, it can also be seen that CTE of Si-30%Al alloy increases with increasing temperature before 300 ℃, however, CTE decreases with increasing temperature from 400 ℃ to 500 ℃. As seen in Fig.2, CTE of the alloys increases with increasing aluminium content at the same temperature.

Fig.2 Coefficient of thermal expansion of spray formed Si-Al alloys after hot pressing

Thermal conductivities of the alloys were computed over the measured temperature range from the specific heat capacity and thermal diffusivity measurements. Specific heat capacity (c_p) measurements were made on samples of Si-30%Al and Si-40%Al alloys over a temperature range of 50-500 ℃, as shown in Fig.3. It can be seen that c_p values of Si-Al alloys increase with increasing temperature and aluminium content.

Fig.3 Heat capacity of spray formed Si-Al alloys after hot pressing

Thermal diffusivity measurements were made on samples of Si-30%Al and Si-40%Al alloys over the same temperature range (50-500 ℃), as shown in Fig.4. It can be seen that thermal diffusivity values of Si-Al alloys decrease with increasing temperature and silicon content.

Fig.4 Thermal diffusivity of spray formed Si-Al alloys after hot pressing

From the specific heat capacity and thermal diffusivity measurements, the thermal conductivity values of the alloys were computed, as shown in Fig.5. The derived thermal conductivities similarly drop with increasing temperature and silicon content.

Fig.5 Thermal conductivity of spray formed Si-Al alloys after hot pressing

Drainage is adopted in density measurements of spray formed Si-Al alloys after hot pressing. Three specimens were measured for each alloy, and the average values are listed in Table 1. It can be seen that the average density of Si-30%Al alloy is 2.421×10³ kg/m³ and that of Si-40%Al alloy is 2.465×10³ kg/m³. The values of this parameter measured are attractive for light mass of the devices.

Table 1 Density of spray formed Si-Al alloys after hot pressing

3.3 Mechanical properties of spray formed Si-Al alloy

Table 2 lists the measurements of the ultimate flexural strength of Si-Al alloys after hot pressing. It is shown that the average value of the ultimate flexural strength of Si-30%Al alloy is 180 MPa and that of Si-40%Al alloy is 220 MPa. For the alloys that could be reliably measured, the ultimate flexural strength decreases with increasing silicon content because the alloy becomes more brittle.

Table 2 Ultimate flexural strength of spray formed Si-Al alloys after hot pressing

Table 3 lists the measurements of Brinell hardness of spray formed Si-Al alloys after hot pressing. It can be seen that the average value of Brinell hardness of Si-30%Al alloy is 261 and that of Si-40%Al alloy is 162. The hardness increases with increasing silicon content because the primary silicon is an unyielding phase.

Table 3 Brinell hardness of spray formed Si-Al alloys after hot pressing

3.4 Electroplating and soldering tests

SEM photograph of densified Si-Al alloy after gold plating is shown in Fig.6. It is shown that appearance of uniform film is perfect. When the alloy was held at 450 ℃ for 120 s in atmosphere according to GJBl420A, the film didn’t change color and bubble. When the alloy was pressed in vacuum according to GJB548-1041, ratio of leaking is less or equal to 5×10^-3 Pa·cm³/s.

Fig.6 SEM photograph of densified Si-Al alloy after gold plating

Interphase of the film is shown in Fig.7. It can be seen that the film combines the primary silicon or aluminium matrix well. The thickness of the nick layer ranges between 2 and 10 μm. Nick layer is coated with gold whose thickness is in the range of 2-4 μm. For diffusion of atoms, diffusion layer between nick layer and the primary silicon or aluminium matrix is about 2 μm, and the layer between nick layer and gold layer is in the range of 1-3 μm. It is shown that the alloy could be plated easily.

Fig.7 SEM photograph and element distributions of plating of Si-Al alloy

After electroplating tests, the alloy was soldered with Sn-Pb alloy. There isn’t porosities and flaws in the weld joint, as seen in Fig.8. Diffusion layer is in the range of 6-9 μm according to interdiffusion of stannum or plumbum atoms with gold atoms, which makes the phases integrate firmly. It is shown that the alloy can be infiltrated and soldered easily.

Fig.8 SEM photograph and element distributions of solderable coating of Si-Al alloy

4 Conclusions

1) Spay formed Si-Al (Si-(30%-40%)Al) alloys exhibit attractive properties, i.e. low coefficient of thermal expansion (6.9×10^-6-8.7×10^-6/K), low density (2.421×10³-2.465×10³ kg/m³), high thermal conducti- vity (118-127 W/(m·K)), high ultimate flexural strength (180-220 MPa) and high Brinell hardness (162-261).

2) Spay formed Si-Al alloys could be electroplated with gold easily according to standard technology. During soldering tests, it can be seen that the alloy could be soldered without any difficulty.

[1] SANGHA S P S, JACOBSOM D M, ARUN JUNAI A, BOTTER E. Novel aluminium-silicon alloys for electronics packaging [J]. Engineering Science and Education Journal, 1997, 11: 195-201.

[2] ZHANG Ji-shan. Spray formed new type of light weighted Si-Al electronic packaging materials with low thermal expansion and high heat conducting [J]. Materials Review, 2002, 16(1): 1-4. (in Chinese)

[3] ZHANG Chen, SHEN Neng-yu. Present state and development of electronic packaging materials [J]. Industrial Forum of Advanced Materials, 2003, 112(3): 5-11. (in Chinese)

[4] JACOBSON D M, SANGHA S P S. Future trends in materials for lightweight microwave packaging [J]. Microelectron Intl, 1998, 15(3): 17-21.

[5] JACOBSON D M. Spray-forming silicon-aluminum [J]. Advanced Materials and Process, 2000, 3: 36-39.

[6] WEI Yan-guang, XIONG Bai-qing, ZHANG Yong-an, LIU Hong-wei, WANG Feng, ZHU Bao-hong. Microstructure variation of spray formed 70Si30Al alloy during heating procedure [J]. The Chinese Journal of Nonferrous Metals, 2006, 16(4): 681-687. (in Chinese)

[7] TUMMALA R R, RYMASZEWSKI E J. Electronic Packaging Committee of Electronics’ Academy of China, Electronic Packaging Series Compiler’S Committee (Tr.). Microelectronics Packaging Handbook (2nd Edi) [M]. Beijing: Electronics Industry Press, 2001.

[8] CHEN Mei-ying. Study of Spray-Formed Novel and Lightweight Silicon-Aluminum Alloys Used for Electronics Panckaging [D]. Beijing: University of Science and Technology Beijing, 2003.

[9] ZHANG Di. Study of Spray-Formed Si-Al Alloys Used for Electronics Panckaging [D]. Beijing: University of Science and Technology Beijing, 2005.

[10] YANG Bin, CHEN Mei-ying, YAO Jun-ping, ZHANG Ji-shan. Fundamental study of new Si-Al electronic packaging materials [J]. Journal of Nanchang Institute of Aeronautical Technology (Natural Sciences), 2004, 18(1): 1-4. (in Chinese)

[11] JACOBSON D M, SANGHA S P S. Novel low expansion packages for electronics [J]. The GEC Journal of Technology, 1997, 14(1): 48-52.

[12] JACOBSON D M. Lightweight electronic packaging technology based on spray formed Si-Al [J]. Powder Metallurgy, 2000, 43(3): 200-202.

[13] WEI Yan-guang, XIONG Bai-qing, ZHANG Yong-an, LIU Hong-wei, WANG Feng, ZHU Bao-hong. Effect of ratio of p/m on preforms of spray formed 70Si30Al alloy [J]. The Chinese Journal of Nonferrous Metals, 2005, 15(11): 1160-1164. (in Chinese)

[14] WANG Xiao-feng, ZHAO Jiu-zhou, TIAN Chong. Study of novel electronic packaging material 70%Si-Al prepared by the spray deposition [J]. Atca Metallurgica Sinica, 2005, 41(12): 1227-1279. (in Chinese)

[15] YANG Pei-yong, ZHENG Zi-qiao, CAI Yang, LI Shi-chen, FENG Xi. P/M process of Si-Al electronic packaging materials [J]. Chinese Journal of Rare Metals, 2004, 28(1): 160-165. (in Chinese)

[16] ZHANG Yong-an, LIU Hong-wei, ZHU Bao-hong, XIONG Bai-qing, SHI Li-kai, ZHANG Ji-shan, CHEN Mei-ying. Novel 60Si40Al alloy packaging material by spray forming process [J]. The Chinese Journal of Nonferrous Metals, 2004, 14(1): 23-27. (in Chinese)

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

(Edited by YANG Bing)