Electrical and Thermal Properties of Electrically Conductive Adhesives Using A Heat-resistant Epoxy Binder

Electrical and Thermal Properties of Electrically Conductive Adhesives Using A Heat-resistant Epoxy Binder Masahiro Inoue 1),3) and Johan Liu 1),2) (1) Department of Microtechnology and Nanoscience, Chalmers University of Technology Kemivägen 9, SE-412 96, Gothenburg, Sweden (2) Key Laboratory of Advanced Display and System Applications and SMIT Center, Shanghai University 149 Yan Chang Rd., Shanghai 200072, China (3) The Institute of Scientific and Industrial Research, Osaka University Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan Abstract Heat resistant conductive adhesives composed of a multi-functional epoxy matrix containing Ag flakes were developed in this work. The adhesives are potentially stable up to 200-250 C because the primary relaxation mechanism of the matrix resin occurs at ~ 250 C. However, the adhesives appeared to exhibit another relaxation mechanism at an intermediate temperature range (100-180 C) when a mono-epoxide was added to the mixture as the reactive diluent. Adhesives cured under appropriate curing conditions exhibited relatively low electrical resistivity and high thermal conductivity in the in-plane direction. Because the reactive diluent influences the electrical and thermal properties as well as the thermo-mechanical properties of the adhesives, the selection of the most appropriate reactive diluent will be the key to developing conductive adhesives that exhibit superior heat resistance. Introduction Recently, the thermal management of chip-based electronic devices is becoming a major bottleneck to attempt to for increase performance and integration density [1]. Power electronics applications are also being significantly limited by the inability to transfer heat across interfaces into heat-sinks [1]. Therefore, the development of high-performance thermal interface materials (TIMs) including thermal greases, gels, adhesives and phase-change materials is essential for the thermal management of microsystems such as these. Within the various types of TIMs that are available, the present work focuses on the heat-resistant conductive adhesives that are required for high-power die-attach applications. In order to develop heat-resistant conductive adhesives, there are a number of subjects that must be addressed, including the design of suitable materials for the matrix resin and for the fillers. Since the electrical and thermal properties of conductive adhesives are directly governed by loading and geometrical factors (such as shape and size distribution) which are determined by the fillers, the design of filler materials has been actively studied in order to improve these properties [2]. However, the chemistry of the matrix resin can also affect the electrical and thermal properties of conductive adhesives [3-5], and therefore the matrix resin should be designed carefully prior to investigations into filler materials. The present work mainly discusses material design for the matrix resin in order to simultaneously achieve high heatresistance and superior electrical and thermal properties. First of all, the adhesive pastes should be easy to handle during the assembly processes and should enable high performance after curing. Hence, the viscosity of the pastes needs to be tunable depending on the mounting process, such as pin transfer, dispensing and stencil printing. Although the viscosity can be controlled using solvents (non-reactive diluents), these are unfavorable for advanced microsystems because large voids can be formed inside the adhesive layer and/or on the interface due to the vaporization of solvents during the curing process. Therefore, the viscosity of the pastes should be controlled using reactive diluents. The thermo-mechanical properties of the adhesives are determined during the curing process, since a crosslinked polymeric structure is formed in the matrix resin. Furthermore, the electrical conductivity in the adhesives is also determined during the curing process [6] because the rheological percolation structure of the fillers [7] in the adhesive paste is transformed to an electrical percolation structure due to the formation of electrical contacts promoted by the curing shrinkage and subsequent cooling shrinkage of the matrix resin. Therefore, the curing reaction should be controlled using appropriate curing agents in order to obtain adhesives that exhibit superior properties under moderate curing conditions. In the present work, a multi-functional epoxy resin was examined as a potential matrix for heat-resistant conductive adhesives. The effect of reactive diluents on the viscosity, the curing reactivity of the pastes and on the thermo-mechanical, electrical and thermal properties of the cured adhesives was investigated in order to develop heat-resistant conductive adhesives. Experimental procedure A multi-functional epoxy resin was used as the main matrix component of the adhesives. A curing agent was employed to promote the curing reaction of the matrix resin. Ag flakes (average diameter: 3 μm) were mixed into the matrix at up to 85wt% loading. In order to control the viscosity of the adhesive pastes, a mono-epoxide was additionally used as the reactive diluent, as shown in Tab. 1. The viscosity of the adhesive pastes was evaluated 978-1-4244-2814-4/08/$25.00 2008 IEEE 1147

using a corn-plate type viscometer at 30 C. The curing behavior of the pastes was examined using differential scanning calorimetry (DSC) under dynamic and isothermal conditions. The degree of conversion was estimated by using the results of DSC measurements. In order to confirm the electrical and thermomechanical properties of the cured adhesives, two kinds of specimens, i.e. printed and free-standing specimens, were prepared by the isothermal curing process. The adhesive pastes were printed on glass and PTFE substrates before the curing process. After curing, the adhesives that were formed on PTFE were peeled off the substrates in order to obtain free-standing specimens. The mechanical behavior of the cured adhesives was examined using a dynamic mechanical analyzer (DMA) in the temperature range between 30 and 320 C under a frequency of 1 Hz and a heating rate of 8.33 x 10-2 C s -1. The thermal decomposition behavior was analyzed by thermogravimetry (TG) at a heating rate of 0.333 C s -1. The electrical resistivities of the specimens (3 mm x 76 mm x 50 μm) cured on glass substrates were evaluated in the in-plane direction using the four-point probe method at ambient temperature. The thermal conductivities of the cured adhesives were measured at ambient temperature, both in the in-plane and vertical directions, by the laser flash method using free-standing specimens with dimensions of 25 mm x 25 mm x 250 μm. Tab.1 Adhesive pastes prepared in this work. Content of reactive diluent Adhesives in matrix resin A1 0 wt% A2 10 wt% A3 20 wt% A4 30 wt% Results and discussion (1) Viscosity control using the reactive diluent Figure 1 shows the viscosity of the adhesive pastes containing various compositions of the mono-epoxide diluent as a function of shear rate. The viscosity of pastes containing Ag fillers always decreases with increasing shear rate (shear thinning [7]). This kind of rheological behavior is often observed in resin systems containing filler particles. A rheological percolation structure of Ag flakes is considered to be formed in the adhesive pastes because the shear rate dependence of viscosity is attributed to the mechanical behavior of the structure formed by the fillers. The formation of a rheological percolation structure results in a significant increase in the viscosity of the pastes compared to the matrix resin, as shown in Fig. 1. In the case of the rheological percolation structure, the fillers interact with each other through the molecules of the matrix resin. Therefore, the viscosity of the adhesive pastes can be lowered when a diluent that weakens the inter-filler interactions in the rheological percolation structure is mixed into the matrix. In the present work, a mono-epoxide was used as the reactive diluent. In fact, the viscosity of the pastes clearly decreased when using the diluent, as shown in Fig. 1. Because the viscosity of the pastes also decreases with increasing temperature, the viscosity is tunable as a parameter of diluent composition and temperature, depending on mounting processes. Viscosity, η (Pa s) 10 3 10 2 10 1 10 0 10-1 A1 (0% diluent) A2 (10% diluent) A3 (20% diluent) A4 (30% diluent) Matrix of A1 10 0 10 1. Shear rate, γ (s -1 ) Fig.1 Shear rate dependence of viscosity (at 30 C) for adhesive pastes containing various compositions of a mono-epoxide diluent. (2) Curing behavior of the adhesive pastes Figure 2 shows the results of a dynamic DSC scan of the adhesive pastes at a heating rate of 8.33 x 10-2 C s -1. An exothermic peak originating from the curing reaction of the matrix resin was detected in the temperature range between 130 and 160 C. Furthermore, the exothermic peak shifted slightly toward the high temperature side with increasing diluent composition. This peak shift suggests that the curing rate of the matrix resin is decreased by the effect of the reactive diluent molecules. Figures 3 (a) and (b) show the isothermal DSC curves for A1 and A4 at 130, 150 and 180 C. Because the curing reaction is a thermally activated process, the reaction rate is accelerated with increasing curing temperature. In fact, the curing reaction is slightly suppressed by the effect of the diluent. However, these adhesive pastes are clarified as being sufficiently cured (above 98 % conversion) by heating at these temperatures for 0.5 h. (3) Thermo-mechanical properties of the adhesives In order to examine the thermo-mechanical behavior of the adhesives, DMA measurements were performed after a curing process at 150 C for 0.5 h. Figures 4 (a) and (b) respectively show the storage modulus and loss tangent (tan δ) of the adhesives. The DMA results indicate that these specimens exhibit two different relaxation processes which result in softening at elevated temperatures. All of the specimens show a relaxation process above 200-250 C. This relaxation process is attributed to the primary relaxation of the main 1148

component of the matrix resin. Hence, this matrix resin potentially provides superior thermo-mechanical stability up to 200-250 C. motion of fragments of the reactive diluent molecules in the polymeric structure of the matrix resin. Because the reactive diluent molecules become components of the polymeric structure, its mechanical properties should be sensitively affected by the diluent molecules. Fig.2 Dynamic DSC curves for adhesive pastes at a heating rate of 8.33 x 10-2 C s -1. Fig.4 (a) Storage modulus and (b) loss tangent of adhesives cured at 150 C for 0.5 h. Fig.3 Isothermal DSC curves for (a) A1 and (b) A4 at 130, 150 and 180 C. However, another relaxation process (peak in tan δ) appears at an intermediate temperature range (100-180 C) as well as an apparent decrease in storage modulus when a reactive diluent was mixed into the matrix. This relaxation is considered to be related to the molecular Fig.5 TG curves measured at 0.333 C s -1 for adhesives cured at 150 C for 0.5 h. Figure 5 shows the TG curves ( measured at 0.333 C s -1 ) of the adhesives which were cured at 150 C for 0.5 h. A decrease in weight due to the thermal decomposition of the matrix resin is promoted in the temperature range 1149

between 300 and 450 C. In addition, the addition of the reactive diluent seems to affect the temperature at which the on-set of decomposition of the matrix resin occurs. The on-set temperature is slightly decreased by the addition of the reactive diluent. As shown in this section, the inclusion of a reactive diluent can reduce the thermo-mechanical stability of the adhesives. Therefore, the selection of the reactive diluent will be a key factor for the development of conductive adhesives that have superior heat resistance. (4) Electrical resistivity of the adhesives Figure 6 shows the electrical resistivity of adhesives cured at 130-180 C (measured by the four-point probe method) as a function of the composition of the reactive diluent. The electrical resistivity of the adhesives tends to decrease with increasing curing temperature. The electrical resistivity of the adhesives apparently decreases with increasing content of reactive diluent. Although the role of the reactive diluent in decreasing the electrical resistivity is still unclear within the limit of the present work, a decrease in the viscosity of the matrix resin may be advantageous for improving the dispersion states of the fillers in the matrix resin during the mixing process. Adhesives A3 and A4 exhibited relatively low electrical resistivity (~2 x 10-5 Ωcm) in the in-plane direction compared to the conventional conductive adhesives when they were cured above 150 C. Electrical resistivity, ρ (Ωcm) 10-3 10-4 10-5 130 ºC 150 ºC 180 ºC 0 5 10 15 20 25 30 35 Content of reactive diluent (wt%) Fig.6 Electrical resistivity of adhesives cured at 130-180 C for 0.5 h as a function of the composition of the reactive diluent. (5) Thermal conductivity of the adhesives Figures 7 (a) and (b) show the thermal conductivities of the free-standing specimens (A1 and A3) in the inplane and vertical directions at ambient temperature, as measured by the laser flash method. One of the present authors (MI) pointed out that the contributions from conducting electrons should have been taken into account when we analyzed the thermal conductivities of advanced conductive adhesives in previous papers [5,8]. According to a prediction using the Wiedemann-Franz Law (W-F law), the contributions of conducting electrons become the predominant factor in determining thermal conductivity for conductive adhesives that exhibit low electrical resistivity below 10-4 Ωcm. In fact, the thermal conductivities in the in-plane direction of the present adhesives tend to increase with decreasing electrical resistivities. Here, the values of electrical resistivity shown in Fig. 6 are used to briefly predict the thermal conductivity in the in-plane direction. By using the W-F scheme, specimens that exhibit low electrical resistivity (e.g. 2-4 x 10-5 Ωcm) are predicted to exhibit high thermal conductivities in the range of 20-30 Wm -1 K -1 in the in-plane direction. The thermal conductivity in the in-plane direction for these specimens, as measured using the laser flash method, was roughly consistent with the prediction, as shown in Figs. 7 (a) and (b). By contrast, the specimens exhibit relatively low thermal conductivity in the vertical direction (2 6 Wm - 1 K -1 ) when compared with the values in the in-plane direction. When considering the causes of this anisotropy in thermal conductivity, two factors, including the effects of the ratio of filler size/specimen thickness (matrix-rich layer theory) and of the alignment (orientation) of flakes [3,9], must be taken into account. Thermal conductivity, λ (Wm -1 K -1 ) 35 30 25 20 15 A3 (in-plane) A1(in-plane) 10 A3(vertical) 5 A1(vertical) 0 120 130 140 150 160 170 180 190 Curing temperature, T (ºC) Fig.7 Thermal conductivities in the in-plane and vertical directions of adhesives A1 and A3 at ambient temperature, as measured by the laser flash method. In the surface regions of the adhesive specimens, layers that have a low filler concentration (matrix-rich layer) are always formed during the curing process. The conditions at which the effect of the matrix-rich layers became obvious are unclear. Some researchers have reported that the effect of the matrix-rich layers is obvious when the ratio of filler size/specimen thickness (d/l) is >0.1 [9]. Although the values of d/l is <0.05 in the present specimens, the effect of the matrix-rich layer still needs to be examined further. In addition, fillers with high aspect ratio often align along the in-plane direction in composite materials 1150

(including conductive adhesives) when the specimen thickness becomes thinner. Such an alignment of fillers could be the cause of the anisotropy that has been noted in the thermal conductivity of composite materials. The thermal conductivity in the vertical direction will be one of the most important properties for interconnect materials for heat dissipation in microsystems such as dieattach applications. Therefore, the design of composite materials using the concept of the bimodal or tri-modal distribution of fillers [2, 10-11] is essential to improve thermal conductivity in the vertical direction. In the next step, the size distribution of the fillers should be optimized, as well as the chemistry of the matrix resin, in order to obtain high thermal conductivity in the vertical direction. Conclusions In order to develop conductive adhesives that have superior heat resistance, a variety of adhesives were fabricated using a heat resistant epoxy matrix and a mono-epoxide reactive diluent. The thermo-mechanical, electrical and thermal properties of the products were evaluated to check their performance as heat resistant adhesives. The main results obtained in this work are summarized below: (1) The viscosity of the adhesive pastes is tunable by using various parameters of diluent composition and temperature. (2) The adhesives were sufficiently cured at 130-180 C when an appropriate curing agent was selected. The molecules of the reactive diluent can affect to the rate of the curing reaction of the matrix resin. (3) A adhesive without reactive diluents exhibit high softening temperatures (~250 C). By contrast, the adhesives showed a relaxation behavior originating from the molecular motion of the diluent fragments in the temperature range of 100-180 C when a monoepoxide diluent was used. (4) The electrical resistivity of the adhesives is influenced by both the curing temperature and the diluent composition. The adhesives fabricated in this work exhibited relatively low electrical resistivities (~2 x 10-5 Ω cm) in the in-plane direction. (5) Thermal conductivity increases with decreasing electrical resistivity of the specimens. Conductive adhesives containing Ag flakes exhibit significant anisotropy in thermal conductivity. The adhesives exhibit higher thermal conductivity in the in-plane direction than in the vertical direction. References 1. Mahajan R., Nair R., Wakharkar V., Swan J., Tang J., Vandentop, Emerging Directions For Packaging Technologies, Intel Technolgy Journal, Vol. 6, No. 2 (2002), pp.62-75. 2. Jiang H., Moon K.-Y., Li Y., Wong C. P., Ultra High Conductive of Istropic Conductive Adhesives, Proc. 2006 ECTC, 2006, pp.485-490. 3. Campbell R. C., Smith S. E., Dietz R. L., Measurements of Adhesive Bondline Effective Thermal Conductivity And Thermal Resistance Using The Laser Flash Method, Proc. 15 th SEMI- THERM Symp., 1999, pp.83-97. 4. Inoue M., Suganuma K., J. Electron. Mater., The Dependence on Thermal History of The Electrical Properties of An Epoxy-based Isotropic Conductive Adhesive, J. Electron. Mater., Vol. 36, No. 6 (2007) pp. 669-675. 5. Inoue M. Muta H., Yamanaka S., Suganuma K., Temperature Dependence of Electrical And Thermal Conductivities of An Epoxy-based Isotropic Conductive Adhesive, J. Electron. Mater., vol. 37, No. 4 (2008) pp.462-468. 6. Inoue M., Suganuma K., Effect of Curing Conditions on The Electrical Properties of Isotropic Conductive Adhesives Composed of An Epoxybased Binder, Soldering and Surface Mount Technology, Vol. 18, No. 2 (2006) pp.40-46. 7. Du F., Zhou W., Brand S., Fischer J. E., Winey K. I., Nanotube Networks in Polymer Nanocomposites: Rheology and Electrical Conductivity, Macromolecules, Vol. 37 (2004), pp. 9048-9055. 8. Inoue M., Muta H., Yamanaka S., Suganuma K., J. Electron. Mater., in contribution 9. Agari Y., Shimada M., Ueda A., Measurement of Effective Thermal Diffusivity of a Polyethylene Composite Filled with Copper Particles by Laser Flash Method, Netsu Bussei, Vol. 9, No.1 (1995), pp.17-23. 10. Fu Y., Liu J., Willander M., Conduction Modelling of A Conductive Adhesive with Bimodal Distribution of Conductive Element, Adhesion and Adhesives, Vol.19 (1999), pp.281-286. 11. Ishida H., Rimdusit S., Very High Thermal Conductivity Obtained By Boron Nitride-Filled Polybenzoxazine, Thermochimica Acta, Vol. 320 (1998), pp. 177-186. Acknowledgments The present work was financially supported by the Seventh Framework Programme of the European Union (Project name: Nano Packaging Technology for Interconnection and Heat Dissipation). 1151

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