ELECTRICALLY CONDUCTIVE ADHESIVES MODIFIED USING IONS AND NANOPARTICLES. David BUŠEK, Ivana PILARČÍKOVÁ, Pavel MACH

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1 ELECTRICALLY CONDUCTIVE ADHESIVES MODIFIED USING IONS AND NANOPARTICLES David BUŠEK, Ivana PILARČÍKOVÁ, Pavel MACH CTU-FEE - Technická 2, Praha 6, Czech Republic, busekd1@fel.cvut.cz, pilarcik@fel.cvut.cz, mach@fel.cvut.cz Abstract Ordinary electrically conductive adhesives (ECAs) cannot cope with solders due to their worse electrical and also mechanical properties. The general goal of this work is to improve electrical conductivity which is nowadays up to two orders worse when compared to lead or lead free solders. This work summarizes the results of experiments, where two basic adhesives AX20 and AX70MN are either additionally filled with silver nanoparticles, carbon nanotubes or AgNO 3, or modified using nitric acid (HNO 3 ), chloride acid (HCl), potassium cyanide (KCN) or treated with elevated temperature (200 C) for the duration of 20 minutes (annealed). Keywords: ECA, modifications, CNT, nanosilver 1. MOTIVATION Though soldering is and probably will be the major joining technique for at least another decade, increasing number of applications require conditions that cannot be met with solders. Among applications, where solder is not suitable belong high pitch applications because lead and lead free solders typically fail when scaled down to less than 100 micron pitch due to poor fatigue resistance. [2] Electrically conductive adhesives (ECA s) already have many advantages over solders. When ECA is used, then high soldering temperature can be avoided and organic substrates can be used. When adhesives are used, no fluxes are necessary and as already mentioned, fine pitch (<0.1 mm) can be realized. Our motivation behind this work are current disadvantages of electrically conductive adhesives. ECA s are having lower electrical conductivity, lower thermal conductivity, worse long time stability and worse rework. Even if the samples were mounted using fully automatic pick and place machine, the resulting resistance was not uniform, not even for the same processed batch. As the adhesive is based on organic epoxy, the deviations in parameters are hardly avoidable. Despite these disadvantages, the electrically conductive adhesives are promising material for the future. The results of the real experiments, when additional filler or chemicals were added to the adhesive mix are presented in this paper. 2. DESCRIPTION OF USED FILLERS AND MODIFICATIONS Silver nanopowder (nag) Multi-walled Carbon NanoTubes (CNT) Acids (HCl, HNO 3, KCN) & AgNO 3 Annealing, thinning and 50% Ag filling

2 2.1 Silver nanopowder (nag) Silver nanopowder (nag), <150 nm particle size, 99% trace metals basis. CAS Number: It is presumed, that after thorough mixing, the nanoparticles could fill-in the gaps between micro sized particles within the ECA and create additional conducting bridges and chains that would then lower the resistance of the adhesive (and consequently the measured resistance of the joint). Fig. 1 TEM picture of nag (particle <150 nm) [4] 2.2 Multi-walled Carbon NanoTubes (CNT) Multiwall carbon nanotube, MWNT CAS Number: A nanotube is in fact a single sheet of graphite, rolled up into a tube. The dimensions are in nanoscale. The electronic properties of the resulting nanotube depend on the direction in which the sheet was rolled up. Some nanotubes are metals with high electrical conductivity (the electrical current that could be passed through a multi-wall nanotube corresponds to Fig. 2 Single wall nanotube [4] (left) and multi-walled CNT agglomerates within the adhesive (right) a current density of 107 A/cm 2 [2]), while others are semiconductors with relatively large band gaps. The diameter of a multi-wall nanotube is tens of nanometers; for a single-wall nanotube it is one or two nanometers. 2.3 Acids (HCl, HNO 3, KCN) & AgNO 3 Experiments where nitric acid or chloride acid were added to a silver filled adhesive were undertaken, unfortunately, the adhesive pulverized right after the acid was added so the mixture became unusable. The nitric acid was expected to interact with silver particles, to change the surface of the particles and create additional ions and thus improve conductivity. Yet another way was to add conductive ions directly in the form of AgNO 3. The results is not quite clear, while AX20 was not measurably influenced, AX70MN exhibited approximately 12% resistance lowering. 2.4 Annealing, thinning and 50% Ag filling When the modification using powder silver was conducted, then as a mean to disperse nanoparticles and homogenize the mixture better, the powder was first mixed to a thinner and only then it was mixed to the original adhesive. The manufacturer suggests to dilute the adhesive as low as possible, in order to confirm that thinning itself does not influence the whole process more than negligible; a control measurement with thinner only was therefore carried out. MEASURING SETUP In this experiment, two different epoxy based adhesives that were chosen according to our previous research, in several modifications were used. One tested sample consists of seven 0R0 serially interconnected resistors. Experimentally obtained resistance of the 0R0 component was 15 mω, therefore the four-point probe method had to be used for the measurement.

3 The nonlinearity measurement is more sensitive, though more complicated when compared to resistance measurement and was therefore used only for selected ECAs, specifically for adhesives modified using CNT (carbon nanotubes) The difference between different concertrations of CNT is presented. (see Table 2.). C1 and K1 respresent a different batch of carbon nanotubes. MEASURED VALUES AND RESULTS Table 1 Resistance measurement and comparison table Adhesive modification Resistance (m) Percentual deviation from original adhesive Standard deviation of raw measured data AX20 (50%Ag) AX AX20+thinner N/A AX20+thinner (annealed) N/A AX20(50%Ag)+nAg AX20+nAg AX20(50%Ag)+CNT AX20+KCN AX20+AgNO AX70MN AX70MN (annealed) N/A AX70MN+nAg AX70MN+AgNO Table 2 Nonlinearity Resistance measurement and comparison table Marking Adhesive modification Mixing method Nonlinearity (dbm) A AX20 Rotary A+C1 AX20+ CNT Ultrasound+Rotary A+2xC1 AX20+ 2 x CNT Ultrasound+Rotary A AX20 Spatula A +K1 AX20+ CNT Spatula B AX70MN Spatula B +K1 AX0MN+K1 Spatula The following graph (Fig. 3.) shows an resistance overview of used samples, their average resistance is presented.

4 dbm R (m) , Brno, Czech Republic, EU Resistance comparison of different adhesives and their modifications adhesive type AX20 (50%Ag) AX20 AX20+thinner AX20+thinner (annealed) AX20(50%Ag)+nAg AX20+nAg AX20(50%Ag)+CNT AX20+KCN AX20+AgNO3 AX70MN AX70MN (annealed) AX70MN+nAg AX70MN+AgNO3 Fig. 3 Comparison graph It can be seen that the percolation threshold for the adhesives is lower than generally indicated 55%-70% wt. Though the 50% filling ratio increased the resistance significantly (for 39%), the final resistance of one joint of ((117m-15m)/2) = 51 m may still usable for low current applications. Annealing of AX20 adhesive lowers the resistance for 11%, but it is probable, that the adhesive after such modification becomes more fragile. Positive influence of nanoparticle sized filler (nag) can be noted. The addition of nag to 50wt% filled adhesive improved the adhesive for 7% when compared to 50wt% filled adhesive alone Nonlinearity comparison of different adhesives and modifications A A+C1 A+2C1 A A +K1 B B +K1 A A+C1 A+2C1 A A +K1 B B +K1 Fig. 4 Nonlinearity comparison graph for CNT modifications; different amounts and different mixing methods; A,B two types of epoxy adhesives Positive influence of AgNO 3 was observed (12% decrease, see Table 1.). Due to dbm being measured in negative values, a higher column represents better modification (better electrical properties). A very good soldered contact exhibits a nonlinearity value of -125 dbm or higher, unmodified conductive adhesive has around -100 dbm and we consider anything around -80 or less as very bad. A comprehensive summary and description of the samples is shown in Table 2. EXPERIMENTAL DIFFICULTIES Figure below (CNT agglomerate distribution after spatula mixing) shows that dispersion of the carbon is not good and it is obvious that the carbon nanotubes create agglomerates. Carbon agglomerates are dark areas. A Detail of the dark area was also obtained and the nanotubes can be recognised (Fig. 6.). Though ultrasound should have dispersed the nanotubes better, optical crossection shows nearly the same

5 distribution. High viscosity of the adhesive+cnt mixture may be the cause. Double amount of nanotubes weakened the coherence and mechanical (and also electrical) properties were influenced down to an unusable level. Fig. 5 Non-homogenous dispersion of the carbon. Fig. 6 Detail of the carbon nanotubes agglomerate. CONCLUSIONS Small addition of carbon nanotubes (0.3% weight) improves mechanical toughness [3], [5], but can also positively influence the electrical nonlinearity as our research suggests. A too high (double) amount of nanotubes on the contrary significantly lowers electrical quality (see Fig. 4.). Ultrasound mixing does not create significantly different results from spatula mixing, when nonlinearity comes into account. The dispersion of the nanotubes in the matrix is after both ways of mixing is heavily non-homogenous (see crosssection in Fig. 5.). The modification having most positive influence on resistance of the adhesive is the addition of silver nanoparticles, when the resistance lowered for 12%. ACKNOWLEDGEMENTS This research was supported by grant: Czech Republic - MSM No Diagnostic of Materials. LITERATURE [1.] Bušek, David, Mach, Pavel, Electrical connection network within an electrically conductive adhesive In: ISSE th International Spring Seminar on Electronics Technology [CD-ROM]. Budapest university of Technology and Economics, 2008, pp ISBN [2.] Matthias Heimann, Jorn Lemm, Klaus-Jurgen Wolter, "Experimental Investigations of Carbon Nanotubes / Epoxy Composites for Electronic Applications", XXXI International Conference of IMAPS, Poland, pp , 200. [3.] LIANG Shu-quan, JIA Chun-yan, TANG Yan, ZHANG Yong, ZHONG Jie, PAN An-qiang, Mechanical and electrical properties of carbon nanotube reinforced epoxide resin composites In Trans. Nonferrous Met. Soc. China 14, pp (2007) [4.] Matthias Heimann, Jorn Lemm, Klaus-Jurgen Wolter, "Experimental Investigations of Carbon Nanotubes / Epoxy Composites for Electronic Applications", XXXI International Conference of IMAPS, Poland, pp , 200. [5.] Sabyasachi Ganguli, Heshmat Aglan, Derrick Dean; Microstructural Origin of Strength and Toughness of Epoxy Nanocomposites in Journal of Elastomers and Plastics, Vol. 37, No. 1, (2005) [6.] Bušek, D. - Mach, P. Influence of Carbon Nanotubes Added to a Commercial Adhesive In: 32nd ISSE 2009 Proceedings [CD- ROM]. Brno: VUT v Brně, FEI, 2009, ISBN [7.] Wikipedia, the free encyclopedia: (as of )