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Microstructure, thermo-physical and mechanical properties of spray-deposited Si–30Al alloy for elec

Microstructure,thermo-physical and mechanical properties of spray-deposited Si –30Al alloy for electronic packaging application

Feng Wang ?,Baiqing Xiong,Yongan Zhang,Baohong Zhu,Hongwei Liu,Yanguang Wei

State Key Laboratory for Fabrication and Processing of Nonferrous Metals,General Research Institute for Nonferrous Metals Beijing,Beijing,100088,China

A R T I C L E D A T A

A B S T R A C T

Article history:

Received 24May 2007Received in revised form 5January 2008

Accepted 9January 2008In this study,Si –30Al alloy was synthesized by the spray atomization and deposition technique.The microstructure and properties of the alloy were studied using optical microscopy,scanning electron microscopy,coefficient of thermal expansion (CTE)and thermal conductivity (TC)measurements,and 3-point bending tests.It was found that the microstructure of the alloy after hot pressing is composed of a continuous network of globular primary Si and interpenetrating secondary Al-rich phase.The property measurements results indicate that the spray-deposited 70Si30Al alloy has advantageous physical and mechanical characteristics,including low coefficient of thermal expansion (6.8×10?6/K),high thermal conductivity (118W/mK),low density (2.42g cm ?3),high ultimate flexural strength (180MPa)and Brinell hardness (261).

Keywords:Spray-deposited Microstructure Properties Si –30Al alloy

1.Introduction

The thermal management materials are needed to dissipate heat associated with high performance microelectronic cir-cuits [1].The properties of traditional packaging can no longer satisfy the needs of rapid development of microelectronic technology.Challenges arise both with the materials selection and the component fabrication.Electronic packaging requires protecting and interconnecting integrated circuits and other component while,at the same time,fast removal of heat gen-erated during the operation of the components is necessary [2].In short,the ideal packaging materials should have a coef-ficient of thermal expansion (CTE)matching ceramic sub-strates (such as alumina,beryllia or aluminium nitride)or semiconductors (such as silicon or gallium arsenide)to avoid thermally induced stresses that often cause eventual device failure,high thermal conductivity (TC)of N 100W/mK to re-move excess heat [3].Additionally,good mechanical properties

and low density are desirable in applications that require maximal performance at a low weight [4].

Hypereutectic Al –Si alloy possess a combination of physi-cal properties that make them attractive for thermal manage-ment applications,including the required relatively high thermal conductivity,low density and a linear variation in CTE with temperature [5].Powder metallurgy techniques pro-vide a means of fabricating hypereutectic Al-high Si alloys with tailored thermal properties [6].However,limitations of the technique include relatively higher cost,complicated pro-cessing steps,longer production cycles or specific protecting atmosphere.In contrast to the powder metallurgy technique,spray-deposition process is believed to be an effective tech-nique because of the advantages of higher production rate,low inherent oxide contents and near-net products.In spray-deposition process,droplets are first atomized from a molten metal stream,quickly cooled by an inert gas,then deposited on a substrate,and finally built up to form a low-porosity

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?Corresponding author.Tel.:+861082241163;fax:+861082240096.E-mail address:wfbs@https://www.wendangku.net/doc/b616326778.html, (F.

Wang).

10.1016/j.matchar.2008.01.012

deposit with the required shape [7,8].Spray-deposition process has been developed for the fabrication of Al-high Si alloys with Si concentrations up to 70wt.%.

In this study,Si –30Al alloy was fabricated by the spray atomization and deposition technique.The objective of this study is to investigate the microstructural evolution,thermo-physical and mechanical properties of spray-deposited Si –30Al alloy.

2.Experimental Procedures

The nominal composition of the alloy was 70wt.%Si and 30wt.%Al.The spray-deposition experiments were conducted in a SF-200type environmental chamber (manufactured by General Research Institute for Nonferrous Metals,Beijing,China).During spray-deposition process,the molten metal was atomized by N 2at 1300°C,the distance of atomizing deposition was kept constant at 550mm.For the densification,the perform,after being cut into cylindrical billets was hot pressed for 4h at a temperature of 570°C with a pressure of 240MPa.After hot pressing,the materials were cooled in air.

The microstructures of the alloy were characterized using light optical microscope (LOM)and scanning electron micro-scopy (SEM).Coefficient of thermal expansion (CTE)measure-ments were conducted on Formastor-Digital tester with a heating rate of 5°C/min.Thermal conductivity measurements were performed on TCT 416(NETZSCH Corporation)at 50°C.The 3-point bending tests with 30mm span were carried out at room temperature on an Instron 5569universal electron

tension testing system.Brinell hardness measurements were performed on a HBV-30double-purpose tester with a 1mm ball indenter.

Results and Discussion

3.1.

Microstructure

Fig.1shows optical micrographs of as-cast and as-spray-deposited and hot pressed Si –30Al alloy for comparison.It can be seen that the optical microscopy microstructure of as-cast alloy is composed of the large block-like primary Si phases and coarse flake eutectic,in Fig.1(a).Some microcracks exist in Si phases,especially in coarse ones,and thus result in the crack growth or even new crack initiation under presence of the stress.Fig.1(b)shows the LOM microstructure of as-spray-deposited and hot pressed alloy,which is composed of a continuous network of globular primary Si and interpenetrat-ing secondary Al-rich https://www.wendangku.net/doc/b616326778.html,work of globular primary Si is surrounded by an Al-rich phases.The presence of the globular primary Si was attributed to the high cooling rate,associated with the rapid solidification processes.Fig.2(a)represents a scanning electron micrograph of spray-deposited and hot pressed Si –30Al alloy.Elemental map scan traces of Si and Al in the microstructure shown in Fig.2(b)and (c),respectively.It can be seen that spray-deposited alloy exhibited no Al –Si eutectic in inter-Si regions,and instead these regions con-sisted of a single Al-rich phase.The formation of the globular primary Si phase in the spray-deposited Si –30Al alloy

resulted

Fig.1–LOM micrographs of Si –30Al alloy (a)as-cast (b)

as-spray-deposited.

Fig.2–SEM micrographs of spray-deposited Si –30Al alloy (a)secondary electron micrograph (b)elemental map of Si (c)elemental map of Al.

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mainly from the relativity high cooling rates during spray-deposition,which has the intrinsic ability to alter the mor-phological stability of the Si phase.Furthermore,impact deformation and fracture experienced by the partially solidi-fied droplets during spray-deposition may also effectively break the Si phase that formed prior to the deposition stage.The presence of such Si phase lead to the increase of the number density of nuclei in the top layer of the spray-deposited material,and effectively inhibited the formation of eutectic Al –Si during deposition.

3.2.Thermo-physical and Mechanical Properties

Table 1summarizes some key property data for spray-deposited Si –30Al alloy and other electronic packaging mate-rials.From the applications of electronic packaging materials,it is found that the Kovar and Invar have been the preferred packaging materials because of its welding capacity and its CTE that is compatible with those of semiconductors.How-ever,these materials suffer from poor thermal conductivity and high density.Cu and Al are often used as baseplates of power modules for their good thermal conductivities,but their CTEs are much higher than desired for attachment of sub-strates.Alumina is also used quite often in electronic pack-aging applications,although it has the advantages of low density and CTE,its thermal conductivity is rather low.Al-68%SiC composite combines the benefits of compatible and tai-lorable CTE,high thermal conductivity and low density,it is a pity that the composite with high SiC content is not easi-ly machinable.Beryllia composites,with perfect thermo-physical properties,low density and high modulus,are restricted by their toxicity.

By contrast,The density of spray-deposited 70Si30Al alloy achieved in this study is only one-third that of Kovar,Invar and Cu,and lower than Al –68%SiC,BeO and Al,but its TC is nine times that of Kovar or Invar and three times that of alumina.From Table 1,it can be seen that the values of the Brinell hardness and Flexural strength were 261and 180MPa,respectively.It is beneficial for the packaging materials to have a reasonable stiffness to provide adequate mechanical support for mechanically sensitive components and substrates.Obviously,spray-deposited 70Si30Al alloy exhibits lower den-sity,CTE and higher TC,will have a good potential in electron packaging applications.

4.Conclusions

Based on the results of this study on 70Si30Al alloy fabricated using spray-deposition technique,the following conclusions can be drawn:

(1)The microstructure of spray-deposited 70Si30Al alloy

after hot pressing is composed of a continuous network of globular primary Si and interpenetrating secondary Al-rich phase.

(2)The thermo-physical characteristics of spray-deposited

70Si30Al alloy combine low coefficient of thermal expansion,high thermal conductivity and low density.Additionally,the alloy has a reasonable stiffness and adequate flexural strength.This makes them ideal can-didate materials for electronic packaging and thermal management.

R E F E R E N C E S

[1]Jacobson David M.Spray-formed silicon –aluminum alloy.

Adv Mater Process 2000;3:36–9.

[2]German Randall M,Hens Karl F,Johnson John L.Powder

metallurgy processing of thermal management materials for microelectronic applications.Int J Powder Metall 1994;30(2):205–15.

[3]Hogg SC,Lambourne A,Ogilvy A,Grant PS.Microstructural

characterization of spray formed Si –30Al for thermal management applications.Scr Mater 2006;55:111–4.

[4]Gaohui Wu,Qiang Zhang,Chen Guoqin,Jiang Longtao,Xiu

Ziyang.Properties of high reinforcement-content aluminum matrix composite for electronic packages.J Mater Sci 2003;14:9–12.

[5]Jacobson D.M,Sangha S.P.S.Novel low expansion packages for

electronics.GEC J Technol 1997;14(1):48–52.

[6]Zweben C.Advances in composites materials for thermal

management in electronic packaging.JOM 1998;50(6):47–50.[7]Zhou J,Duszczyk J,Korevaar M.As-spray-deposited structure

of an Al –20Si –5Fe Osprey preform and its development during subsequent processing.J Mater Sci 1991;26:5275–91.[8]Wu Y,Cassada W.A,Lavernia E.J.Microstructure and

mechanical properties spray-deposited Al –17Si –4.5Cu –0.6Mg wrought alloy.Metall Mater Trans 1995;26A:1235–42.

Table 1–Thermo-physical and mechanical properties of spray-deposited Si –30Al alloy and other electronic packaging materials Materials

Density,g cm ?3

CTE,×10?6°C ?1

TC,W/mK

Flexural strength,MPa Brinell hardness

Kovar 8.36617Invar 8.040.411Cu

8.917.8398Alumina 3.967.639Al –68%SiC 3.0 6.9150BeO 2.9 6.8240Al

2.72

3.6230SD 70Si30Al

2.42

6.8

118

180261

CTE:coefficient of thermal expansion;TC:thermal conductivity;SD:spray-deposited.

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