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Thermal expansion of a novel hybrid SiC foam–SiC particles–Al composites
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 3404–3408 www.elsevier.com/locate/compscitech
Thermal expansion of a novel hybrid SiC foam–SiC particles–Al composites L.Z. Zhao a
a,* ,
M.J. Zhao a, X.M. Cao b, C. Tian b, W.P. Hu b, J.S. Zhang
b
School of Mechanical and Electronic Engineering, East China Jiaotong University, Shuanggang Road, Nanchang 330013, PR China b Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China Received 18 October 2006; received in revised form 30 January 2007; accepted 14 March 2007 Available online 27 March 2007
Abstract A new type of hybrid SiC foam–SiC particles–Al composites (VSiC = 53, 56.2 and 59.9%) to be used as an electronic packaging substrate material were fabricated by squeeze casting technique, and their thermal expansion behavior was evaluated. The coefficients of thermal expansion (CTEs) of the hybrid composites in the range of 20–100 °C were found to be between 6.6 and 7.7 ppm/°C. The measured CTEs are much lower than those of SiC particle-reinforced aluminum (SiCp–Al) composites with the same content of SiC because of the characteristic interpenetrating structure of the hybrid composites. A material of such a low CTE is ideal for electronic packaging because of the low thermal mismatch (and therefore, low thermal stresses) between the electronic component and the substrate. To achieve similar CTEs in SiCp–Al composites, the volume fraction of SiC would be much higher than that in the hybrid composites. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Metal-matrix composites; Hybrid reinforcement; Interpenetrating; Thermal expansion
1. Introduction Metal-matrix composites attract attention for packaging applications because of their integrated thermal and physical properties and superior mechanical strengths. Among them, particle-reinforced and interpenetrating aluminum matrix composites have shown promises as candidates for such applications [1–4]. Generally, the volume fraction of particles has to exceed 70% in order to make thermal expansion match between the substrate and chips when SiCp–Al composites are used as substrate [5]. However, such high content of SiC particles reduces the thermal conductivity of the composites, which makes the high heat dispersion requirements difficult to meet in high-performance electronic packaging. The interpenetrating composites reinforced by three-dimensional ceramic networks, such as SiC foams, have much lower thermal expansion than SiCp–Al
composites containing same content of SiC [6–8]. Nevertheless, foams with fine-sized pores and high SiC volume fractions are difficult to attain, and it is difficult to produce such composites to meet the requirements of low thermal expansion for electronic packaging. In this paper, a novel hybrid composite is proposed to overcome the shortcomings of 3D SiC foam-reinforced composites, by substituting the aluminum matrix in interpenetrating composites with SiCp–Al composites. They are called SiC foam–SiC–particles–Al hybrid composites. The hybrid composites not only have the desirable low CTEs, but also have relative low content of SiC reinforcement compared with SiCp–Al composites. The hybrid composites are fabricated by squeeze casting technology, and their thermal properties are investigated in order to apply them in electronic packaging. 2. Experimental procedure
*
Corresponding author. Tel.: +86 791 7126824; fax: +86 791 7046124. E-mail address: [email protected] (L.Z. Zhao).
0266-3538/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.03.020
An Al–Si alloy (4032Al), used as the matrix whose chemical composition is listed in Table 1, has a relative
L.Z. Zhao et al. / Composites Science and Technology 67 (2007) 3404–3408 Table 1 Chemical composition of 4032Al (wt%) Si
Cu
Mg
Ni
Al
11.8
1.0
1.1
1.0
Balance
low CTE. The physical properties of SiC particles and 4032Al are presented in Table 2. The hybrid composites were fabricated by modifying an established procedure [11]. First, the SiC foams with a cell size of about 1 mm, shown in Fig. 1a, were produced by a solid-state sintering process through the polymer foam replication method [12]. The SiC foam is made out of cells and struts that consist of triangular shaped opening holes surrounded by strut walls as shown in Fig. 1b. Furthermore, there are numerous continuous tiny cavities in the strut walls (Fig. 1c), which makes the struts a three-dimensional reticular structure similar to that of the foam. This special structure was deliberately designed in the experiment in order to increase interfacial bonding between the matrix and struts. SiC foams with different volume fractions were obtained through tuning the strut diameters with the cell size kept constant. Second, SiC particles of 20 lm were dispersed in the SiC foam cells to form a preform by vibration. The volume fractions of SiC of the foams were 16.4, 22.2 and 28.8%, and the total SiC volume fractions in the
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composites were 53, 56.2 and 59.9%, respectively. Then, the preform and the mould were preheated to 800 °C and 300 °C, respectively. Meanwhile, 4032Al alloy was molten, degassed, and cleaned in a graphite crucible, and heated to 700–800 °C. The last stage was to place the preform in the mould, and squeeze the molten aluminum alloy into the preform with a pressure of 120 MPa to produce the composite. The microstructure of the composites was examined with an MEF4A optical microscope (OM) and an S360 scanning electron microscope. The CTEs were measured using a DIL 402EP dilatometer system from 25 to 500 °C with a heating rate of 5 °C/min. To eliminate system errors, the dilatometer was calibrated by measuring a standard alumina specimen under identical conditions. Thermal conductivity was measured by the laser flash method with the NETZSCH LFA447 thermal conductivity meter. The bulk density of the composites was measured based on Archimedes’ principle. 3. Results and discussion 3.1. Composite characterization It is found that the hybrid composites are composed of the gray SiC particles, the white aluminum alloy matrix
Table 2 Properties of SiC particles [9] and 4032Al [10] Material
Density (g/cm3)
CTE (ppm/°C)
Young’s modulus (GPa)
Shear modulus (GPa)
Bulk modulus (GPa)
Poisson’s ratio
SiC 4032Al
3.18 2.72a
4.7 20.8
450 69
192 29.7
225 77.5
0.17 0.33
a
Attained by experiment.
Fig. 1. Morphology of a typical SiC foam reinforcement: (a) a low magnification image; (b) cross section of a strut and (c) surface of a strut.
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L.Z. Zhao et al. / Composites Science and Technology 67 (2007) 3404–3408
Fig. 2. Morphology of a SiC foam–SiC particles–Al hybrid composite: (a) low magnification view and (b) a higher magnification view.
22
ROM Kerner's Model Turner's Model Experimental data SiCp/Al in reference [10]
20 18
CTE (ppm/˚C)
and the gray SiC foam struts, shown in Fig. 2. The SiC particles distribute uniformly in the cells of a SiC foam, but they are not found in the triangle holes of the struts. The SiC particles cannot be placed (by vibration) into the holes because the particles are larger than the cavities in the strut walls. Because the strut walls are reticular, the walls have an interpenetrating structure after aluminum infiltrating the cavities. Such an interpenetrating structure, which is called local interpenetrating structure, will boost the bonding of the SiC foam struts to aluminum in the triangle holes and in the cavities. In addition, a SiC foam network makes the overall hybrid composites an interpenetrating structure. The structure, which is called the overall interpenetrating structure, restricts the SiC particles and aluminum in the foam cells. The composites, possessing both the local interpenetrating structure and the overall interpenetrating structure, are called double interpenetrating structure composites.
16 14 12 10 8 6 4 0
20
40
60
80
100
Volume fraction of SiC (%)
Fig. 3. Comparison between theoretical predictions and experimental CTEs of SiC foam–SiC particles–4032Al composites.
3.2. Thermal expansion behavior The average CTEs of the hybrid composites measured between 25 and 100 °C shown in Fig. 3, are 7.7, 7.1 and 6.6 ppm/°C, for composites with total SiC volume fractions of 53.0, 56.2 and 59.9%, respectively. It is clear that the CTEs of the hybrid composites are much lower than those of conventional SiCp–Al composites with the same volume fraction of SiC, and the CTEs decrease with the volume fraction of SiC which is similar as observed in conventional SiCp–Al composites. The thermal expansion behavior of the hybrid composites depends on the intrinsic thermal expansion properties of the SiC, aluminum matrix, as well as the special double interpenetrating structure. In this structure, the restriction imposed by the SiC foams on the matrix plays a predominant role in reducing the thermal expansion of the hybrid composites. In contrast, SiC particles have little restriction on the aluminum matrix in conventional SiCp–Al composites. Furthermore, there is little interaction between the SiC particles in SiCp–Al composites, and they move with the aluminum matrix during thermal expansion. The cells in the hybrid composites do not allow large displacement of the matrix, and there is a large interaction among cells of SiC foams which restricts the thermal expansion of the matrix.
The hybrid composites have much lower volume fractions of SiC than conventional SiCp–Al composites with the same CTEs. For instance, when the CTE is about 7.7 ppm/°C, the volume fraction of SiC is 53% in the case of the hybrid composites, while it is larger than 70% in the SiCp–Al composites, as revealed in Fig. 3. The low value of SiC volume fraction attributed to the double interpenetrating structure of the hybrid composites, leads to the possible increase in the thermal conductivity of the composites. To further understand the thermal expansion behavior of the hybrid composites, several existing theoretical models of composites are compared. When the interfaces are free to slide and the constituent phases are free to flow, the CTEs of the composites can be expressed by the ruleof-mixture [8]: ac ¼ am V m þ ar V r ;
ð1Þ
where a is the CTE, V is the volume fraction, and subscripts c, m, r refer to the composite, matrix and reinforcement, respectively. For a composite with perfect interfacial bonding between particles and matrix, Kerner’s model is suitable for predicting the CTEs of composites. It assumes a sphere of one phase (reinforcement) enclosed by a uniform layer
L.Z. Zhao et al. / Composites Science and Technology 67 (2007) 3404–3408
of a second (matrix) phase, and the CTE of such a composite can be expressed as [10]: ac ¼ V r ar þ V m am þ V r V m ðar am Þ
Kr Km ; V m K m þ V r K r þ ½3 K r K m =ð4Gm Þ
ð2Þ
where, G and K are the shear modulus and bulk modulus, respectively. In Turner’s model, each component of a composite undergoes a homogeneous strain throughout the composite. The CTE can be calculated by the following formula [9]: Pn ai V i K i a ¼ Pi n : ð3Þ i V iK i A comparison between theoretical calculations and experimental results for the composites produced in this work is shown in Fig. 3. The experimentally measured CTEs of the SiC foam–SiC particle–Al composites are lower than the predictions of all the existing models and those measured on the SiCp–Al composites, which can be attributed to the restriction imposed by the SiC foam on the matrix. The experimental CTE values of the hybrid composites are lower than those of particles reinforced composites. We consider the role of phase morphology in determining the overall CTEs of the hybrid composites. For fixed content of the constituent phases, the CTEs of composites varies significantly depending on that the brittle ceramic phase is continuous. This is because the ductile metal phase has a higher coefficient of thermal expansion and thus, is the more compliant phase with temperature changes. When the brittle ceramic phase, less compliant phase is the continuous, deformation of metal phase is constrained by the surrounding, less compliant phase [1,13,14]. Thereby, as the temperature is raised, the restriction of SiC foam reinforcement on the expansion behavior of aluminum matrix in the hybrid composite is much stronger than that of discrete particles on matrix in particles reinforced composites, the CTEs of the hybrid composites is lower compared to particles reinforced composites. Besides, the morphology of phases in composites affects the residual thermal stresses in metal matrix, and the stresses influence the CTEs of composites. Multiphase materials, where the constituents have large thermal expansion mismatch such as metals and ceramics, exhibit significant residual stresses when cooled from a higher processing temperature. This results in large residual stresses within the materials: tensile in the metal phase, compressive in the ceramic. As the composite sample cools, these residual stresses increase with decreasing temperature. When the ductile metal phase is confined within the continuous network SiC phase of the hybrid composite, due to increased mechanical constraint hindering yielding, tensile stresses in the aluminum phase constrained by SiC foam are much greater than those evolved in the ductile phase when the
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ceramic SiC phase is discrete [1,9,13,15,16]. When the stress in metal matrix is increasing to the critical value, the pores are formed in the metal, and the critical stress is called cavitation stress. So the cavitations in the hybrid composites are much more than those in particles reinforced composites. When the temperature is raised, part of aluminum matrix will expand into the pores, thus limiting the increase of its bulk volume [9,15,17], therefore, the hybrid composites exhibit lower CTEs when compared to particles reinforced composites. On the other hand, the presence of microvoids at the ceramic metal interface is another factor which reduces the values of overall CTEs of the hybrid composites [8,18,19]. The double interpenetrating structure of the hybrid composites not only makes composites have sufficient surface area, which ensures that the SiC foam reinforcement has effective restriction on the thermal expansion of the aluminum matrix [17], but also cause the hybrid composites to preexist voids at concave reinforcement surface regions, because the molten matrix does not wet the reinforcement, and hence cannot fill very narrow pores. The closure of the microvoids at the interface is also responsible for reducing the CTEs of the hybrid composites. Both constrained deformation of aluminum matrix and the microvoids induced by the special double interpenetrating structure of SiC foam reinforcement are the factors why the hybrid composites have much lower CTEs compared to particles reinforced composites. From the applications of electronic packaging materials [17], Kovar and Invar alloy are often used in electronic packaging for the advantages of low CTE, but their thermal conductivity is much lower, and their densities are rather high when compared to the hybrid composites achieved in this work (Table 3). Alumina used quite often in electronic packaging substrate, has the advantages of low density and low CTE, but its thermal conductivity is rather low, which cannot meet the demand of high-power electronic component. The traditional packaging materials, such as Kovar, Invar and alumina are not the ideal candidate substrate for high packaging density; by contrast, the hybrid SiC–Al composites in this work are the perfect packaging materials for the thermal conductivity of the hybrid SiC–Al composite is eight times of Kovar or Invar and three times that of alumina. From the thermal physical properties mentioned above, it is found that the achieved hybrid composites reinforced by silicon carbide foam and particles exhibit outstanding packaging properties, thereTable 3 Physical properties of electronic packaging materials [17] Materials
Density (g/cm3)
CTE (ppm/°C)
TC (W/m °C)
Kovar Invar Alunina 59.9% SiC–Al compositesa
8.36 8.04 3.96 2.96
6 0.4 7.6 6.6
17 11 39 135
a
Attained by experiment.
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L.Z. Zhao et al. / Composites Science and Technology 67 (2007) 3404–3408
fore, the hybrid composites will be used instead of traditional packaging materials. 4. Conclusions In this work, a novel hybrid composite was fabricated by squeeze casting technology. The CTEs of the hybrid composites are much lower than those of SiCp–Al composites due to the double interpenetrating structure of the hybrid composites. The CTEs of the hybrid composites increase from 6.6 to 7.7 ppm/°C when the volume fraction of SiC goes down from 59.9% to 53%. In order to achieve the same CTEs, the volume fractions of SiC in the hybrid composites are much lower than those in conventional SiCp–Al composites. The low SiC volume fraction leads to the possible rise in thermal conductivity of the composites. Therefore, the hybrid composites are prospective materials as high-density electronic packaging substrates. Acknowledgement The authors thank Professor H.Y. Zhang (Department of MIME, The University of Toledo, USA) for revising this manuscript. References [1] Shen YL, Needleman A, Suresh S. Coefficients of thermal expansion of metal-matrix composites for electronic packaging. Metall Mater Trans A 1994;25:839–50. [2] Kumar AH, Tummala RR. The present and future of multilayer ceramic multichip modules in electronic packaging. JOM 1992;7:10–4. [3] Zweben C. Advances in composite materials for thermal management in electronic packaging. JOM 1998;6:47–51. [4] Premkumar MK, Hunt WH, Sawtell RR. Aluminum composite materials for multichip modules. JOM 1992;7:24–8.
[5] Zweben C. Metal-matrix composites for electronic packaging. JOM 1992;7:15–23. [6] Clarke DR. Interpenetrating phase composites. J Am Ceramic Soc 1992;4:739–59. [7] Daehn GS, Starck B, Xu L, Elfishawy KF, Ringnalda J, Fraser HL. Elastic and plastic behavior of a co-continuous alumina/aluminum composite. Acta Mater 1996;1:249–61. [8] Skirl S, Hoffman M, Bowman K. Thermal expansion behavior and macrostrain of Al2O3/Al composites with interpenetrating networks. Acta Mater 1998;7:2493–9. [9] Elomari S, Skibo MD, Sundarrajan A, Richards H. Thermal expansion behavior of particulate metal-matrix composites. Compos Sci Technol 1998;58:369–76. [10] Zhang Q, Wu GH, Chen GQ, et al. The thermal expansion and mechanical properties of high reinforcement content SiCp/Al composites fabricated by squeeze casting technology. Compos: Part A 2003;34:1023–7. [11] Xing HW, Cao XM, Hu WP, Zhao LZ, Zhang JS. Interfacial reactions in 3D-SiC network reinforced Cu-matrix composites prepared by squeeze casting. Mater Lett 2005;12:1563–6. [12] Schwartzwalder K, Somers H, Somers AV. Method of making porous ceramic articles. US3090094, 1963-05-21. [13] Shen YL. Thermal expansion of metal-ceramic composites: a threedimensional analysis. Mater Sci Eng 1998;252 A(2):269–75. [14] Vaidya RU, Chawla KK. Thermal expansion of metal-matrix composites. Compos Sci Technol 1994;50:13–22. [15] Elomari S, Boukhili R, Marchi CS, Mortensen A, Lloyd DJ. Thermal expansion responses of pressure infiltrated SiC/Al metal-matrix composites. J Mater Sci 1997;32(8):2131–40. ¨ del J. Thermal residual strains and [16] Hoffman M, Skirl S, Pompe W, RO stresses in Al2O3/Al composites with interpenetrating networks. Acta mater 1999;47(2):565–77. [17] Chien CW, Lee SL, Lin JC, Jahn MT. Effect of Sip size and volume fraction on properties of Al/Sip composites. Mater Lett 2002;52: 334–41. [18] Balch DK, Fitzgerald TJ, Michaud VJ, Mortensen A, Shen YL, Suresh S. Thermal expansion of metals reinforced with ceramic particles and microcellular foams. Metall Mater Trans 1996;27A(11): 3700–17. [19] Lee HS, Hong SH. Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites. Mater Sci Technol 2003;19(8):1057–64.
Thermal expansion of a novel hybrid SiC foam–SiC particles–Al composites L.Z. Zhao a
a,* ,
M.J. Zhao a, X.M. Cao b, C. Tian b, W.P. Hu b, J.S. Zhang
b
School of Mechanical and Electronic Engineering, East China Jiaotong University, Shuanggang Road, Nanchang 330013, PR China b Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China Received 18 October 2006; received in revised form 30 January 2007; accepted 14 March 2007 Available online 27 March 2007
Abstract A new type of hybrid SiC foam–SiC particles–Al composites (VSiC = 53, 56.2 and 59.9%) to be used as an electronic packaging substrate material were fabricated by squeeze casting technique, and their thermal expansion behavior was evaluated. The coefficients of thermal expansion (CTEs) of the hybrid composites in the range of 20–100 °C were found to be between 6.6 and 7.7 ppm/°C. The measured CTEs are much lower than those of SiC particle-reinforced aluminum (SiCp–Al) composites with the same content of SiC because of the characteristic interpenetrating structure of the hybrid composites. A material of such a low CTE is ideal for electronic packaging because of the low thermal mismatch (and therefore, low thermal stresses) between the electronic component and the substrate. To achieve similar CTEs in SiCp–Al composites, the volume fraction of SiC would be much higher than that in the hybrid composites. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Metal-matrix composites; Hybrid reinforcement; Interpenetrating; Thermal expansion
1. Introduction Metal-matrix composites attract attention for packaging applications because of their integrated thermal and physical properties and superior mechanical strengths. Among them, particle-reinforced and interpenetrating aluminum matrix composites have shown promises as candidates for such applications [1–4]. Generally, the volume fraction of particles has to exceed 70% in order to make thermal expansion match between the substrate and chips when SiCp–Al composites are used as substrate [5]. However, such high content of SiC particles reduces the thermal conductivity of the composites, which makes the high heat dispersion requirements difficult to meet in high-performance electronic packaging. The interpenetrating composites reinforced by three-dimensional ceramic networks, such as SiC foams, have much lower thermal expansion than SiCp–Al
composites containing same content of SiC [6–8]. Nevertheless, foams with fine-sized pores and high SiC volume fractions are difficult to attain, and it is difficult to produce such composites to meet the requirements of low thermal expansion for electronic packaging. In this paper, a novel hybrid composite is proposed to overcome the shortcomings of 3D SiC foam-reinforced composites, by substituting the aluminum matrix in interpenetrating composites with SiCp–Al composites. They are called SiC foam–SiC–particles–Al hybrid composites. The hybrid composites not only have the desirable low CTEs, but also have relative low content of SiC reinforcement compared with SiCp–Al composites. The hybrid composites are fabricated by squeeze casting technology, and their thermal properties are investigated in order to apply them in electronic packaging. 2. Experimental procedure
*
Corresponding author. Tel.: +86 791 7126824; fax: +86 791 7046124. E-mail address: [email protected] (L.Z. Zhao).
0266-3538/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.03.020
An Al–Si alloy (4032Al), used as the matrix whose chemical composition is listed in Table 1, has a relative
L.Z. Zhao et al. / Composites Science and Technology 67 (2007) 3404–3408 Table 1 Chemical composition of 4032Al (wt%) Si
Cu
Mg
Ni
Al
11.8
1.0
1.1
1.0
Balance
low CTE. The physical properties of SiC particles and 4032Al are presented in Table 2. The hybrid composites were fabricated by modifying an established procedure [11]. First, the SiC foams with a cell size of about 1 mm, shown in Fig. 1a, were produced by a solid-state sintering process through the polymer foam replication method [12]. The SiC foam is made out of cells and struts that consist of triangular shaped opening holes surrounded by strut walls as shown in Fig. 1b. Furthermore, there are numerous continuous tiny cavities in the strut walls (Fig. 1c), which makes the struts a three-dimensional reticular structure similar to that of the foam. This special structure was deliberately designed in the experiment in order to increase interfacial bonding between the matrix and struts. SiC foams with different volume fractions were obtained through tuning the strut diameters with the cell size kept constant. Second, SiC particles of 20 lm were dispersed in the SiC foam cells to form a preform by vibration. The volume fractions of SiC of the foams were 16.4, 22.2 and 28.8%, and the total SiC volume fractions in the
3405
composites were 53, 56.2 and 59.9%, respectively. Then, the preform and the mould were preheated to 800 °C and 300 °C, respectively. Meanwhile, 4032Al alloy was molten, degassed, and cleaned in a graphite crucible, and heated to 700–800 °C. The last stage was to place the preform in the mould, and squeeze the molten aluminum alloy into the preform with a pressure of 120 MPa to produce the composite. The microstructure of the composites was examined with an MEF4A optical microscope (OM) and an S360 scanning electron microscope. The CTEs were measured using a DIL 402EP dilatometer system from 25 to 500 °C with a heating rate of 5 °C/min. To eliminate system errors, the dilatometer was calibrated by measuring a standard alumina specimen under identical conditions. Thermal conductivity was measured by the laser flash method with the NETZSCH LFA447 thermal conductivity meter. The bulk density of the composites was measured based on Archimedes’ principle. 3. Results and discussion 3.1. Composite characterization It is found that the hybrid composites are composed of the gray SiC particles, the white aluminum alloy matrix
Table 2 Properties of SiC particles [9] and 4032Al [10] Material
Density (g/cm3)
CTE (ppm/°C)
Young’s modulus (GPa)
Shear modulus (GPa)
Bulk modulus (GPa)
Poisson’s ratio
SiC 4032Al
3.18 2.72a
4.7 20.8
450 69
192 29.7
225 77.5
0.17 0.33
a
Attained by experiment.
Fig. 1. Morphology of a typical SiC foam reinforcement: (a) a low magnification image; (b) cross section of a strut and (c) surface of a strut.
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L.Z. Zhao et al. / Composites Science and Technology 67 (2007) 3404–3408
Fig. 2. Morphology of a SiC foam–SiC particles–Al hybrid composite: (a) low magnification view and (b) a higher magnification view.
22
ROM Kerner's Model Turner's Model Experimental data SiCp/Al in reference [10]
20 18
CTE (ppm/˚C)
and the gray SiC foam struts, shown in Fig. 2. The SiC particles distribute uniformly in the cells of a SiC foam, but they are not found in the triangle holes of the struts. The SiC particles cannot be placed (by vibration) into the holes because the particles are larger than the cavities in the strut walls. Because the strut walls are reticular, the walls have an interpenetrating structure after aluminum infiltrating the cavities. Such an interpenetrating structure, which is called local interpenetrating structure, will boost the bonding of the SiC foam struts to aluminum in the triangle holes and in the cavities. In addition, a SiC foam network makes the overall hybrid composites an interpenetrating structure. The structure, which is called the overall interpenetrating structure, restricts the SiC particles and aluminum in the foam cells. The composites, possessing both the local interpenetrating structure and the overall interpenetrating structure, are called double interpenetrating structure composites.
16 14 12 10 8 6 4 0
20
40
60
80
100
Volume fraction of SiC (%)
Fig. 3. Comparison between theoretical predictions and experimental CTEs of SiC foam–SiC particles–4032Al composites.
3.2. Thermal expansion behavior The average CTEs of the hybrid composites measured between 25 and 100 °C shown in Fig. 3, are 7.7, 7.1 and 6.6 ppm/°C, for composites with total SiC volume fractions of 53.0, 56.2 and 59.9%, respectively. It is clear that the CTEs of the hybrid composites are much lower than those of conventional SiCp–Al composites with the same volume fraction of SiC, and the CTEs decrease with the volume fraction of SiC which is similar as observed in conventional SiCp–Al composites. The thermal expansion behavior of the hybrid composites depends on the intrinsic thermal expansion properties of the SiC, aluminum matrix, as well as the special double interpenetrating structure. In this structure, the restriction imposed by the SiC foams on the matrix plays a predominant role in reducing the thermal expansion of the hybrid composites. In contrast, SiC particles have little restriction on the aluminum matrix in conventional SiCp–Al composites. Furthermore, there is little interaction between the SiC particles in SiCp–Al composites, and they move with the aluminum matrix during thermal expansion. The cells in the hybrid composites do not allow large displacement of the matrix, and there is a large interaction among cells of SiC foams which restricts the thermal expansion of the matrix.
The hybrid composites have much lower volume fractions of SiC than conventional SiCp–Al composites with the same CTEs. For instance, when the CTE is about 7.7 ppm/°C, the volume fraction of SiC is 53% in the case of the hybrid composites, while it is larger than 70% in the SiCp–Al composites, as revealed in Fig. 3. The low value of SiC volume fraction attributed to the double interpenetrating structure of the hybrid composites, leads to the possible increase in the thermal conductivity of the composites. To further understand the thermal expansion behavior of the hybrid composites, several existing theoretical models of composites are compared. When the interfaces are free to slide and the constituent phases are free to flow, the CTEs of the composites can be expressed by the ruleof-mixture [8]: ac ¼ am V m þ ar V r ;
ð1Þ
where a is the CTE, V is the volume fraction, and subscripts c, m, r refer to the composite, matrix and reinforcement, respectively. For a composite with perfect interfacial bonding between particles and matrix, Kerner’s model is suitable for predicting the CTEs of composites. It assumes a sphere of one phase (reinforcement) enclosed by a uniform layer
L.Z. Zhao et al. / Composites Science and Technology 67 (2007) 3404–3408
of a second (matrix) phase, and the CTE of such a composite can be expressed as [10]: ac ¼ V r ar þ V m am þ V r V m ðar am Þ
Kr Km ; V m K m þ V r K r þ ½3 K r K m =ð4Gm Þ
ð2Þ
where, G and K are the shear modulus and bulk modulus, respectively. In Turner’s model, each component of a composite undergoes a homogeneous strain throughout the composite. The CTE can be calculated by the following formula [9]: Pn ai V i K i a ¼ Pi n : ð3Þ i V iK i A comparison between theoretical calculations and experimental results for the composites produced in this work is shown in Fig. 3. The experimentally measured CTEs of the SiC foam–SiC particle–Al composites are lower than the predictions of all the existing models and those measured on the SiCp–Al composites, which can be attributed to the restriction imposed by the SiC foam on the matrix. The experimental CTE values of the hybrid composites are lower than those of particles reinforced composites. We consider the role of phase morphology in determining the overall CTEs of the hybrid composites. For fixed content of the constituent phases, the CTEs of composites varies significantly depending on that the brittle ceramic phase is continuous. This is because the ductile metal phase has a higher coefficient of thermal expansion and thus, is the more compliant phase with temperature changes. When the brittle ceramic phase, less compliant phase is the continuous, deformation of metal phase is constrained by the surrounding, less compliant phase [1,13,14]. Thereby, as the temperature is raised, the restriction of SiC foam reinforcement on the expansion behavior of aluminum matrix in the hybrid composite is much stronger than that of discrete particles on matrix in particles reinforced composites, the CTEs of the hybrid composites is lower compared to particles reinforced composites. Besides, the morphology of phases in composites affects the residual thermal stresses in metal matrix, and the stresses influence the CTEs of composites. Multiphase materials, where the constituents have large thermal expansion mismatch such as metals and ceramics, exhibit significant residual stresses when cooled from a higher processing temperature. This results in large residual stresses within the materials: tensile in the metal phase, compressive in the ceramic. As the composite sample cools, these residual stresses increase with decreasing temperature. When the ductile metal phase is confined within the continuous network SiC phase of the hybrid composite, due to increased mechanical constraint hindering yielding, tensile stresses in the aluminum phase constrained by SiC foam are much greater than those evolved in the ductile phase when the
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ceramic SiC phase is discrete [1,9,13,15,16]. When the stress in metal matrix is increasing to the critical value, the pores are formed in the metal, and the critical stress is called cavitation stress. So the cavitations in the hybrid composites are much more than those in particles reinforced composites. When the temperature is raised, part of aluminum matrix will expand into the pores, thus limiting the increase of its bulk volume [9,15,17], therefore, the hybrid composites exhibit lower CTEs when compared to particles reinforced composites. On the other hand, the presence of microvoids at the ceramic metal interface is another factor which reduces the values of overall CTEs of the hybrid composites [8,18,19]. The double interpenetrating structure of the hybrid composites not only makes composites have sufficient surface area, which ensures that the SiC foam reinforcement has effective restriction on the thermal expansion of the aluminum matrix [17], but also cause the hybrid composites to preexist voids at concave reinforcement surface regions, because the molten matrix does not wet the reinforcement, and hence cannot fill very narrow pores. The closure of the microvoids at the interface is also responsible for reducing the CTEs of the hybrid composites. Both constrained deformation of aluminum matrix and the microvoids induced by the special double interpenetrating structure of SiC foam reinforcement are the factors why the hybrid composites have much lower CTEs compared to particles reinforced composites. From the applications of electronic packaging materials [17], Kovar and Invar alloy are often used in electronic packaging for the advantages of low CTE, but their thermal conductivity is much lower, and their densities are rather high when compared to the hybrid composites achieved in this work (Table 3). Alumina used quite often in electronic packaging substrate, has the advantages of low density and low CTE, but its thermal conductivity is rather low, which cannot meet the demand of high-power electronic component. The traditional packaging materials, such as Kovar, Invar and alumina are not the ideal candidate substrate for high packaging density; by contrast, the hybrid SiC–Al composites in this work are the perfect packaging materials for the thermal conductivity of the hybrid SiC–Al composite is eight times of Kovar or Invar and three times that of alumina. From the thermal physical properties mentioned above, it is found that the achieved hybrid composites reinforced by silicon carbide foam and particles exhibit outstanding packaging properties, thereTable 3 Physical properties of electronic packaging materials [17] Materials
Density (g/cm3)
CTE (ppm/°C)
TC (W/m °C)
Kovar Invar Alunina 59.9% SiC–Al compositesa
8.36 8.04 3.96 2.96
6 0.4 7.6 6.6
17 11 39 135
a
Attained by experiment.
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fore, the hybrid composites will be used instead of traditional packaging materials. 4. Conclusions In this work, a novel hybrid composite was fabricated by squeeze casting technology. The CTEs of the hybrid composites are much lower than those of SiCp–Al composites due to the double interpenetrating structure of the hybrid composites. The CTEs of the hybrid composites increase from 6.6 to 7.7 ppm/°C when the volume fraction of SiC goes down from 59.9% to 53%. In order to achieve the same CTEs, the volume fractions of SiC in the hybrid composites are much lower than those in conventional SiCp–Al composites. The low SiC volume fraction leads to the possible rise in thermal conductivity of the composites. Therefore, the hybrid composites are prospective materials as high-density electronic packaging substrates. Acknowledgement The authors thank Professor H.Y. Zhang (Department of MIME, The University of Toledo, USA) for revising this manuscript. References [1] Shen YL, Needleman A, Suresh S. Coefficients of thermal expansion of metal-matrix composites for electronic packaging. Metall Mater Trans A 1994;25:839–50. [2] Kumar AH, Tummala RR. The present and future of multilayer ceramic multichip modules in electronic packaging. JOM 1992;7:10–4. [3] Zweben C. Advances in composite materials for thermal management in electronic packaging. JOM 1998;6:47–51. [4] Premkumar MK, Hunt WH, Sawtell RR. Aluminum composite materials for multichip modules. JOM 1992;7:24–8.
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