­­Fabrication and thermo-physical properties of TiB_2p/Cu composites for electronic packaging applications

CHEN Guo-qin(陈国钦)¹, XIU Zi-yang(修子扬)², MENG Song-he(孟松鹤)³,

WU Gao-hui(武高辉)¹, ZHU De-zhi(朱德志)¹

1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

2. Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology,

Harbin 150001, China;

3. Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, China

Received 10 June 2009; accepted 15 August 2009

Abstract: TiB_2p/Cu composites with high reinforcement content (φ_p=50%, 58% and 65%) for electronic packaging applications were fabricated by squeeze casting technology. The microstructures and thermo-physical properties of the TiB_2p/Cu composites were investigated. The results show that TiB₂ particles are homogeneous and distribute uniformly, and the TiB₂-Cu interfaces are clean and free-from interfacial reaction products and amorphous layers, the densifications of the TiB_2p/Cu composites are higher than 98.2%. The mean linear coefficients of thermal expansion at 20-100 ℃ for TiB_2p/Cu composites range from 8.3×10^-6 to 10.8×10^-6/K and decrease with increasing volume fraction of TiB₂. The experimental coefficients of thermal expansion agree well with the predicted values based on Turner’s model. The thermal conductivities of TiB_2p/Cu composites range from 167.3 to 215.4 W/(m?K), decreasing with increasing volume fraction TiB₂.

Key words: TiB_2p/Cu composites; densification; coefficient of thermal expansion; thermal conductivity

1 Introduction

The development of packaging technology has resulted in need for new materials with superior properties[1]. Firstly, packaging materials should have coefficient of thermal expansion(CTE) matching the ceramic substrates such as alumina, beryllia or aluminum nitride or semiconductors such as silicon or gallium arsenide to avoid thermally induced stresses that often cause eventual device failure. Additionally, since the reliability of semiconductors drops dramatically as temperature rises, higher thermal conductivity is demanded to remove excess heat and keep operating temperature low. Further, large mechanical properties and low density are desirable in applications that require maximal performance at low mass[2-3].

Particulate reinforced copper matrix composite combines the benefits of compatible and tailorable CTE, high thermal conductivity, lightmass, enhanced specific strength and stiffness[4-5]. So, it has been identified as an ideal candidate material for power module baseplates, printed wiring board cores, microprocessor lids and heat spreaders. Titanium diboride (TiB₂) is well known for its high thermal and electrical conductivities, good chemical stability and good thermal shock stability[6-7]. Thus, the addition of TiB₂ to copper matrix greatly decreases its coefficient of thermal expansion, while reducing the electrical and thermal conductivities much less than the addition of most other ceramic reinforcements[8-9]. Therefore, TiB₂ reinforced metal matrix composites have received a great attention recently. The previous researches about the TiB_2p/Cu composites were mostly focused on the thermal shock resistance properties and fabrication methods[10-12].

In the present study, high particle content TiB_2p/Cu composites with particle volume fraction of 50%, 58% and 65% were fabricated by the patent squeeze casting technology, the densification of which was higher than 98.2%, with their microstructures, thermo-physical properties were tested and analyzed.

2 Experimental

The reinforcements used in this work were titanium diboride, TiB₂, particles with nominal diameters of 2-3 μm, and the reinforcements volume fraction were 50%- 65%. The copper matrix was commercially available pure copper (w(Cu)≥99.7%). This pure copper was chosen for the purpose of high thermal conductivity and low cost. Table1 lists the typical parameters of TiB₂.

Table 1 Typical parameters of TiB₂ particle reinforcements

The TiB_2p/Cu composites were fabricated by squeeze casting technology. The TiB₂ preform was first fabricated and preheated. At the same time, copper alloy was melted, after which the molten copper was infiltrated into TiB₂ preform under the pressure and held for some time. And then, TiB_2p/Cu composite was solidified. A flow chart of the above process is shown in Fig.1. The composites were annealed in vacuum at 700 ℃ for 1.5 h and furnace cooled in order to release residual stress within the composites.

Fig.1 Flow chart of squeeze casting technology used for fabricating TiB_2p/Cu composites

An S-570 scanning electron microscope(SEM) was used to examine the microstructure of as-fabricated TiB_2p/Cu composites. The measured density was obtained using the Archimedes method and compared with the theoretical density to obtain various degree of densification. The CTE was measured on a DIL 402C (NETZSCH Corp.) with a heating rate of 5 ℃/min. The thermal conductivity was measured by the laser flash method with the NETZSCH LFA427 thermal constant measuring equipment. The testing sample was cylindrical, 12.7 mm in diameter and 3 mm in thickness.

3 Results and discussion

3.1 Densification

The effect of TiB₂ volume fraction on the densification for TiB_2p/Cu composites is shown in Fig.2. The densification of the TiB_2p/Cu composites decreases with increasing TiB_2p volume fraction under the same processing conditions. The densification of three composites is in the range of 98.2%-99.3%, which can completely meet the high dense requests for the electronic package materials.

Fig.2 Dependence of TiB₂ volume fraction on densification for TiB_2p/Cu composites

3.2 Microstructure observation

Fig.3 reveals the microstructure of as-cast TiB_2p/Cu composites. The TiB₂ particles are observed to be homogeneously distributed in the copper matrix. And the composites are free from common cast defects such as porosity and shrinking cavities because pressure was applied during the solidification of TiB_2p/Cu composite.

The interface and the existing of interface effect are the important factors which can affect the properties of the composite. Fig.4 illustrates the typical TEM micrographs of the interfaces in TiB_2p/Cu composites. A large a mount of observations indicate that the TiB₂-Cu interfaces are clean, smooth and free from interfacial reaction products and amorphous layers, and no TiB₂ particles dissolved are observed.

Fig.3 Microstructure of TiB_2p/Cu composite

Fig.4 TEM micrographs of TiB₂-Cu interfaces

3.3 Thermal expansion analysis

The measured CTEs of TiB_2p/Cu composites are  8.9×10^-6, 9.5×10^-6 and 10.4×10^-6 K^-1 for the composites with volume fraction of TiB₂ of 65%, 58% and 50%, respectively. The CTEs reduce with increasing volume fraction of TiB₂. In a TiB_2p/Cu composite, the thermal expansion behavior is influenced by the thermal expansion of copper matrix and the tightened restriction of TiB₂ particles. Several theoretical models were proposed to predict the CTE of particulate composite [13-14]. If the matrix modulus is much smaller than that of reinforcement, the CTE of a composite is expressed as rule-of-mixture(Rom):

α_c=α_mφ_m+α_pφ_p                                (1)

where α is the CTE, φ is the volume fraction; and subscripts c, m, p refer to the composite, matrix and particle, respectively.

Turner’s model considers the uniform hydrostatic stresses and gives the CTE of a composite as

                     (2)

where K is the bulk modulus.

Both the normal and shear stress are taken into account in Kerner’s model, and the CTE of a composite is expressed as

             (3)

where G is shear modulus.

Fig.5 shows the comparison between the above theoretical predictions and experimental data. It can be seen that the experimental data are in good agreement with the predicted values based on Turner’s model, but deviate from ROM and Kerner’s model. This may be attributed to the fact that the uniform hydrostatic stresses are included in Turner’s.

Fig.5 Comparison between theoretical predictions and experimental CTEs

3.4 Thermal conductivity

The measured thermal conductivities of TiB_2p/Cu composites are 215.4, 180.5 and 167.3 W/(m·K) for the 50%, 58% and 65% composites, respectively, which are enough to satisfy the high thermal conductivity requests for electronic package materials. The thermal conductivities of TiB_2p/Cu composites increased with increasing volume fraction of Cu. It is attributed to the thermal conductivity of TiB₂ which is lower so far than that of copper.

Although the thermal conductivity of PMMCs is mainly decided by the thermal conductivity and content of constituent components, it connects with the densification of materials, interface condition, the figures and distribution of particles[15-17]. There are two ways for heat transfer in TiB_2p/Cu composites: free electron in Cu matrix and phonon in TiB₂ particles. Both the movement would be scattered by interface. Therefore, heat conduction in TiB_2p/Cu composite depends on the Cu matrix, TiB₂ particles and their interface. Fortunately, the interface between Cu and TiB₂ particles are smooth without any reactant (see Fig.4), which is beneficial to heat transfer.

Accurate prediction of the composite properties is one important goal for researchers on materials, and two widely used models for prediction of PMMCs thermal conductivity are as the following[18-19]:

1) For Rom model

λ_c=λ_mφ_m+λ_pφ_p                                (3)

2) For Maxwell model

According to the conductance and the thermal conductivity property of biphase and multiphase, the expression of the thermal conductivity is deduced:

                     (4)

where λ is thermal conductivity; φ is volume fraction; x equals λ_m/λ_p; and subscripts c, p and m refer to composite, reinforcement particle and matrix, respectively.

Generally, the thermal conductivity of matrix Cu is 398 W/(m·K), the thermal conductivity of TiB₂ is 96 W/(m·K). The calculated thermal conductivity is obtained from the above models, and the comparison between predictions and experimental data is listed in Table 2.

Table 2 Predicted and experimental thermal conductivities of TiB_2p/Cu composite

The comparison of those with experimental data indicates that the calculated thermal conductivity of Maxwell models is close to that of TiB_2p/Cu composites. The achievement of higher thermal conduction is attributed the high dense composite fabricated by the patent squeeze casting technology, and the TiB₂-Cu interfaces are clean, smooth and free-from interfacial reaction products and amorphous layers.

4 Conclusions

1) The full densities of the TiB_2p/Cu composites with volume fractions of TiB₂ of 50%-65% were fabricated. The composites are full dense and porosity-free macroscopically. TEM observations indicate the TiB₂-Cu interfaces are clean, smooth and free from interfacial reaction products and amorphous layers.

2) The linear CTEs of TiB_2p/Cu composites range from 8.9×10^-6 to 10.4×10^-6/K, depending on the volume fraction of TiB₂. The experimental CTEs are in good agreement with the predicted values based on Turner’s model.

3) The thermal conductivities of TiB_2p/Cu composites at ambient temperature range from 167.3 to 215.4 W/(m?K) and decrease with increasing volume fraction of TiB₂, which agrees with the calculated values of Maxwell model.

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(Edited by LONG Huai-zhong)

Foundation item: Project(20080430895) supported by China Postdoctoral Science Foundation; Project(2008RFQXG045) supported by the Special Fund of Technological Innovation of Harbin, China

Corresponding author: CHEN Guo-qin; Tel: +86-451-86402372-5058; E-mail: chenguoqin@hit.edu.cn