The Effect of Nano Fillers in Electrical and Mechanical Properties of Isotropic Conductive Adhesive

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1 The Effect of Nano Fillers in Electrical and Mechanical Properties of Isotropic Conductive Adhesive C.S. Chew*, R. Durairaj, J. Hwang, J Tan, M. S. Liang, C. N. Tan and K. P. Chen *Corresponding Author: Faculty of Engineering and Science, Department of Mechanical and Materials Engineering, University Tunku Abdul Rahman, Jalan Genting Kelang, Setapak, Kuala Lumpur, Malaysia. Tel: 006-(0) Fax: 006-(0) E -mail: chewcs@utar.edu.my Abstract In electronic packaging, most of the components on the circuit board are hold together using lead solders. Due to the toxicities of lead, a lot of non-toxic alternatives have been investigated to replace the lead containing solders. One of the alternatives is isotropic conductive adhesives (ICAs). Nano fillers are often incorporated into ICAs to improve its electrical conductivity and mechanical properties. In this study, epoxy resin (DGEBA), microcrystalline cellulose (MCC), silver powder and silver nano particles and carbon nanotubes (CNTs) were used to produce ICAs. A total of 0 different compositions were formulated for experimentation and testing. Tensile test was carried out by using INSTRON 5582 to measure the tensile strength, digital micro hardness tester machine was used for hardness tests and conductivity was measured using digital multimeter. The tensile strength, hardness and electrical conductivity of the produced ICAs were compared with lead-tin solder. It was found that the Young s Modulus and conductivity of the ICAs are lower whereas the hardness of the ICAs is higher than lead-tin solder.. Introduction Electronic industry is one of the fastest-growing industries worldwide, and most of the components on the circuit board are hold together using solder with high content of lead. Due to the toxicities of lead, a lot of non toxic alternative were researched to eventually replace lead containing solders. One of the alternatives is isotropic conductive adhesives (ICAs). ICAs are known as polymer solder for the reason being a composition of polymer resin and conductive fillers. The epoxy which can cure at a low temperature makes them a very speedy polymer binder. Nano fillers are often used with ICAs to improve its basic characteristic and properties such as electrical conductivity and mechanical properties like tensile strength, hardness.[, 2] There are many advantages in using ICAs as compared to lead. Some of the advantages are; fewer steps in processing which helps to reduce cleaning and fluxing of the components to achieve a low processing cost, fine pitch interconnection capability, lowering sintering temperature to reduce production cost and etc. Despite of the advantages, there are also drawbacks to the usage of ICAs. It has relatively unstable contact resistance and low conductivity, which limits the reliability of ICAs. This would challenge the practical application of ICAs to a very critical level. In addition, electrochemical corrosion on the contact interface created of the polymer matrixes by humid environment may cause different stress concentration caused by surrounding influence, internal hydrolysis of colloids and delamination between devices would weaken the bonding strength and increase electrical resistance, which could cause electronic device failure. Therefore, understanding the mechanical properties of ICAs of different composition could give better manipulation of surface properties In this study, the aim is to investigate the influence of different compositions such as the addition of carbon nanotubes and microcrystalline cellulose on the mechanical and electrical property of the formulated ICAs. 2. Methodology The epoxy resin chosen for this study was DGEBA using with Ag powder and MCC as the filler, Multi walled CNT and SNP as nano-fillers and EDA as the curing agent. Pure DGEBA and 99% EDA, MCC were purchased from Sigma-aldrich. Multi walled CNTs was purchased Ted Pella, Inc. Ag powder was purchased from. Nacalai Tesque, Inc. The chemical reagents were used without any further purification. Meanwhile, SNP was self-prepared using 2: concentration ratio of Sodium borohydride, NaBH₄ and Silver Nitrate, AgNO 3. In this study, 0 samples were prepared according to different formulation to study the effect of nano-fillers (SNPs and CNTs) composition, the effect of MCC addition and the effect of addition of different type of nano-filler to the dynamical mechanical properties of ICA. Each sample has a total mass of 4 g. The weight of materials needed was calculated according to the weight percent of the respective materials as shown in Table. The tensile testing sample preparation was prepared by using two copper bars and having the ICAs paste joining both the copper bar together as shown in Figure. Figure Tensile Testing Sample The effective length was set to be 60 mm where both ends clamped, and the pulling rate was set to be 2 mm/min. The mild steel rod was first cut to a suitable length and the rough edge filed using abrasive papers of different roughness to a smooth and flat surface. Next, the surface of the mild

2 steel rod was cleansed by dipping into 2% hydrochloric acid, Millipore water, ethanol consequently. The mild steel rod was dipped into each substance for 5 seconds and was dried thoroughly before dipping into another solution. This process tend to remove the impurities that were attached on the surface of the mild steel rod to achieve a better bonding between the ICAs paste and the mild steel rod surface. was cleansed by dipping into 2% hydrochloric acid, Millipore water, ethanol consequently. The mild steel rod was dipped into each substance for 5 seconds and was dried thoroughly before dipping into another solution. Table List of Sample with Different Compositions Sample DGEBA Matrix (wt%) MCC Silver (Ag) Powder S S S S S S S S S S Filler (wt%) 49 SNPs / CNTs (CNTs) 2 (CNTs) 0 0 Curing Agent (wt%) Ethylenediamine (EDA) Figure 3 Conductivity Test Specimen. The ICAs paste was then filled on the insulated section between the copper plate bridging them together. After that, it was cured in the oven for one hour at 225 C. Once it was fully cured, a digital multimeter was used to measure the resistance across the two copper plates. Before measuring, the resistance of each of copper plates was measured to be 0 ohm. This shows that resistance on the copper plate was negligible. The multimeter s test point was put on the two separate copper plates to measure the resistance across the ICAs paste, and the result obtained were collected and tabulated. The specimen used in the conductivity test was used further for the hardness test. The specimen was put under the digital microhardness tester machine to measure the hardness of each specimen with 0.098N force applied. The data obtained is then tabulated. 3. Results and discussion The ICAs paste was then applied to join two separate mild steel rods as shown in Figure and labeled accordingly. Then, the samples are being cured in the oven at 225 C for hour. Throughout the whole curing process, no external forces were applied on the specimen. Figure 2 Tensile Test Specimen after Test Run Tensile Test was done by using INSTRON The machine was stop once sample broke into 2 as shown in Figure 2, and the data obtained was saved. The width, thickness and length of the specimen were 3.3 mm,.3 mm and 6 cm, respectively. For conductivity test, printed circuit board (PCB) was used for the test. The PCB was first cut into the required dimension by using PCB cutter. Then, PCB was then attached back together with an insulated section in between as shown in Figure 3. The whole specimen was then cleansed Throughout the experiment, a control specimen known as sample is included in the result to have an insight of the comparison of the ICAs and the traditional lead-tin solder. The Young s Modulus is calculated by using Equation.The data is then tabulated in Figure 4. Where, E = Young s Modulus (MPa) σ = Stress (MPa) ε = Strain (%) E = σ/ε ()

3 Table 2 Stress and Strain of ICAs obtained from Tensile Test and relative Young's Modulus Sample STRESS (MPa) STRAIN (%) YOUNG S MODULUS (MPa) (con trol) Figure 4 Young's Modulus of ICAs From Figure 4, it is observed that mechanical strength increases with addition of CNTs compared those using SNP and MCC. From the slope of Young s Modulus of sample 7 and 8, it can be seen that the threshold of the addition of CNTs is near. From sample to sample 6, there is a steady trend except for sample 4. According to [2], by replacing the metal fillers with CNTs, high percentage of the mechanical strength can retain. They also say the MWCNTs threshold is less than 3%. From the graph that is plotted, it can be seen that increment from sample 7 to sample 8 was not much. This shows that the amount of CNTs used for sample 8 is close to the MWCNTs threshold. According to [3], they observed an increase in 25% for tensile strength and 36-42% for elastic modulus when they incorporate CNT into their polystyrene/cnt sample. With the addition of nanomaterials reinforcement, the stress transfer mechanism is improved thus the mechanical strength of the composite with the increasing surface area of the filler particles. In the study of [4], they mentioned that the dispersion of CNTs is not that significant in effecting tensile strength. Furthermore, it also said that a properly dispersed CNTs are better in load transferring as compared to that of agglomerated CNTs and cracks, which would initiate propagation easier than those with poor dispersion. This could be the possible reason why sample 8 has lower strength as compared to sample 7. A huge increase in strength of sample 4 was analysed by comparing the compositions. The only difference in sample 4 is the usage of 55 wt% of silver powder and 4 wt% of SNPs, while others remain the same. The possible reason for the increase of Young s Modulus could be achievement optimized composition. As discussed by [5], a correct composition may result to extraordinary optimized result in properties when dealing with ICAs. By comparing the composition of sample 9 with sample, the only difference is the replacement of 5 wt% of DGEBA with 5 wt% MCC. The Young s Modulus drastically increases after the addition of 5 wt% MCC. This is because MCC is fiber in nature, and it helps to link the silver powder and SNPs by agglomeration, and this helps to create the link of network, which eventually increases the mechanical strength of the ICAs. This is also reported by Fu [6] that the interface adhesion helps to increase the strength of the composite during loading by improving the load transfer between the fillers and polymer matrix. In sample 0, the replacement of 0 wt% of MCC with DGEBA turns out to weaken the mechanical strength so severe that it is incomparable with sample 9 in terms of mechanical strength. The possible reasoning behind this radical change can be due to the excessive amount of MCC. The metal fillers are there for electrical conductivity purpose, and the polymer matrix is for mechanical properties. By reducing the amount of DGEBA, it would definitely result in lower mechanical strength but this is compensated by the MCC addition to the composition but the excessive amount of MCC added will not help in increasing the mechanical strength instead it has a negative effect on the mechanical strength. The Young s Modulus of sample 0 is closest to sample 2. When comparing the result of ICAs with the control sample, it is observed that the composed ICAs were much weaker than normal conventional tin-lead solder. This may result to drawbacks for the composed ICAs for any practical application. More invesitgations are needed in order to replace the solders to ICAs. The electrical resistance is measured by the digital multimeter and tabulated in Table 4.2. Calibration is done to eliminate zero errors. The reading is allowed to stabilize before the final reading is taken. The resistivity is then converted to conductivity by using Equation 2.

4 Where, σ = Conductivity (S/m) ρ = Resistance (Ohm) σ = /ρ (2) Table 3 Resistance and Conductivity of ICAs Sample Resistance (Ohm) Conductivity (S/M) (control) Figure 5 Conductivity of ICAs From the graph shown as Figure 5, it was observed that sample have a high conductivity as it contains high wt% of SNPs because silver powder is in micron size it is porous thus with the addition of nano sized SNPs, it could fill up the porosity thus helps to increase the conductivity. This reasoning is proven by the decreasing conductivity from sample to sample 6 as there is more and more porosity with less SNPs to fill the pores [5, 7]. The existence of porosity is due to the stacking behaviour of the silver powder. The silver powder is round in shape thus during stacking, voids are formed when adjacent silver powder attached to each other. Since matrix resin is nonconductive thus this would reduce the conductivity of the ICAs. As reported by [2], addition of CNTs into ICAs would improve the conductivity as it helps in conducting electricity but the result shows otherwise. Sample 7 and sample 8 show a very low conductivity and based on the review on [8], it is suggested that the MWCNTs in our sample is not dispersed properly and there is little to no bridge connection between the CNTs due to agglomeration thus the CNTs are being useless in ICAs structure. Sonification method can be used to have a better dispersion. They also said that by using injection method, the pathway for conduction of CNTs is ensured and this would help to increase conductivity. Since the CNTs in sample 7 and 8 are not being utilized, with the lack of SNPs to fill the voids made from aggregation of silver powder, it results in low conductivity. For sample 9 and sample 0, the conductivity increases and achieves higher conductivity as compared to sample 3, 4, 5, 6, 7 and 8. This phenomenon is due to the ability of the MCC to agglomerate the silver powder together with the SNPs to form a better bridging, which helps to increase the conductivity. MCC was proven to be conductive and may assist in conductivity network [9]. A decrease in conductivity in sample 0 could possibly be the bulkiness of the fiber content causing the some of the metal network to disconnect. With the drop in mechanical strength and conductivity from sample 9 to 0, it is observed that it is better to used 5%wt of MCC compare to 0%wt. At balance composition of polymer matrix and nano metal fillers, the mechanical strength and electrical conductivity can be enhanced without the need for any of the properties of ICA to suffer for the gain of another. In comparison of the ICAs conductivity of the control sample (sample ), sample achieve higher conductivity than the traditional lead-tin solder whereby its conductivity is on par with sample 2. Thus, this data shows that, in terms of conductivity, the ICAs can have a greater result which makes it one of the acceptable criteria to be used in the industry. The hardness of the ICAs was tested by using the digital microhardness tester and the only available type of hardness test by the machine is Vickers Hardness test. The data is then tabulated in Table 4. Table 4 Hardness of ICAs Sample Hardness(HV) (control) 42.9

5 conductivity test; there is a positive result for samples with MCC added to the composite. Since hardness and tensile strength is correlated to each other, it is expected that an increase in hardness would also mean an increase in tensile strength. 4. Conclusions Figure 6 Hardness Graph of ICAs From Figure 6, it is observed sample 9 has the highest hardness among all the ICAs. The sample 4 would have a more positive result as compared to the other samples from sample to sample 6 and right after sample 4, it would always be a sudden plunge at sample 5. This clearly shows when dealing with SNP the optimized composition of the composed ICAs is sample 4. In this test, all samples have higher hardness (HV) as compared to the traditional lead-tin solder. From [0] the addition of CNTs in polymer matrix would help to increase the hardness because the CNTs would be able to increase the stacking and overlapping between the material basis and the additives thus this reduces the movement of the polymer molecules which ultimately increases the resistance to cutting and stretching of the material, consequently, the material becomes more resistant to plastic deformation. From the data obtained, the samples with the addition of MWCNTs show higher hardness as compared to sample, 2, 3, 5 and sample 6 but it should be expected an increase of about 40% should be observed with the addition of the MWCNTs. This was also reported by [] where the increment of hardness is directly proportional to the addition of CNTs in composition but from their data, it seems to be that the mechanical threshold for the addition of CNTs is higher than 0 wt% as compared to that of in conductivity where the threshold is about 3 wt% thus it could be that more CNTs are needed to show a more prominent result for hardness test. However, without proper stacking and linkage, the ICA s mechanical property was not enhanced. The agreement is supported by [] findings where their material shows promising mechanical strength increment with addition of CNTs with well dispersed composite. This problem can be compromised by going through long duration of sonification. There is a hike in hardness of the ICA at sample 9. This is the sample where MCC is added to the composite to increase the network linkage between the fillers. It is expected that the hardness of the material will increase as with more network linkage, it is harder for the material to deform and from previous data from tensile test and In conclusion, the tensile strength and conductivity of the ICAs are lower than lead tin solder whereas hardness the hardness is higher. Although not all property of ICAs show better than conventional solder, but the conductivity of sample and the achievement in hardness show that ICAs is possible to replace solder with certain implementations. The MWCNTs filler helps in all properties such as mechanical and electrical properties when added in polymer matrix composite. MWCNTs exhibit many desired characteristics such as having a high tensile strength, Young s Modulus and high conductivity. When linkage is created to bridge the fillers, all these properties will be improved. The sample preparation needs to be revised as ultrasonic bath is needed to disperse the MWCNTs in the sample evenly to be able to bridge the fillers properly in the composite. More wt% of MWCNTs should be added to increase the mechanical property too. The addition of microcrystalline cellulose helps to bridge the polymer matrix and the fillers together, and this helps to agglomerate the composite together to improve the mechanical and electrical property as the composite will be more closely packed with fewer micro to nano voids in the composite. A more precise threshold value should be researched in order to further increase the mechanical strength of the ICAs. Acknowledgments The authors acknowledge the supports of UTAR Research Fund (UTARRF) - Project Number: IPSR/RMC/UTARRF/203-C2/C04 References. Lu, D., K.T. and C.P. Wong, Conductivity mechanisms of isotropic conductive adhesives (ICAs). IEEE transactions on electronics packaging manufacturing, : p. pp Li, Y.M., C.P. Wong, and K.-s. Moon, Nano-conductive Adhesives for Nano-electronics Interconnection. Nano- Bio-electrironic, Photonic, and MEMS Packaging, 200. : p. p Qian, D., et al., Load transfer and deformation mechanisms in carbon nanotube polystyrene composites. Appl. Phys. Lett. Carbon, : p. pp Song, Y.S. and J.R. Youn, Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites. Carbon, (7): p. pp

6 5. Cui, H.-W., et al., Electrical and mechanical properties of electrically conductive adhesives from epoxy, microsilver flakes, and nano-hexagonal boron nitride particles after humid and thermal aging. International Journal of Adhesion and Adhesives, : p. pp Fu, S.-Y., et al., Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate-polymer composites. Composites Part B, : p. pp Li, J., Electrical and Mechanical Characterization of MWNT Filled Conductive Adhesive for Electronics Packaging. 2008, University of Kentucky Doctoral Dissertations: Kentucky, USA. 8. Abbasi, S., P.J. Carreau, and A. Derdouri, Flow induced orientation of multiwalled carbon nanotubes in polycarbonate nanocomposites: Rheology, conductivity and mechanical properties. Polymer, (4): p. pp Zhukov, A.N., et al., Conductivity and Electrokinetic Potential of Microcrystaline Cellulose Particles in Aqueous HCL and NaOH Solutions. Colloid Journal, (3): p. pp Husaen, S.I., Mechanical properties of carbon nanotube reinforced Epoxy Resin composites. J. Badhdad for Sci., 20. 9(2): p. pp Kim, K.T., et al., Microstructures and tensile behavior of carbon nanotube reinforced Cu Matrix nanocomposites. Mater. Sci. Eng. A, : p

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