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In this study, novel fast-curing electrically conductive adhesives were prepared from a functional epoxy, a reactive diluent, a silane coupling agent, a curing agent, and micro silver flakes. Differential scanning calorimetry, Fourier transform infrared spectroscopy, four-probe method, shear test, impact test, scanning electron microscopy, and ener...
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... temperature and long time are required for the curing of ECAs. This problem should be solved well to improve the electronic packaging efficiency. In a previous study, a matrix resin containing a functional epoxy, a reactive diluent, a silane coupling agent, and a curing agent was determined, and also a ECAs formulation containing 24 % of matrix resin and 76 % electrically conductive fillers by weight [13–18]. They were cured at 150 C for 30 min or 1 h, and had a high decomposition temperature above 350 C and a high glass transition temperature at 180 C. In this study, the functional epoxy, reactive diluent, silane coupling agent, and the ECAs formulation mentioned above were still used. A new curing agent was introduced into the matrix resin. By increasing the content of curing agent, the curing of ECAs was to be improved. The physical, electrical, mechanical, and thermal properties were investigated by using the differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), four-probe method, shear test, impact test, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The ECAs were made up of a matrix resin and electrically conductive fillers. The matrix resin contained a functional epoxy ( N,N -diglycidyl-4-glycidyloxyaniline, Sigma-Aldrich, St. Louis, MO, USA) as the main resin, a reactive diluent (2-ethylhexyl glycidyl ether, Sigma-Aldrich, St. Louis, MO, USA), a silane coupling agent (3-glycidoxypropyltrimethoxysilane, Sigma-Aldrich, St. Louis, MO, USA), and a curing agent (2,4-dimethylimidazole, Sigma-Aldrich, St. Louis, MO, USA). The content of micro silver flakes was fixed at 76 % by weight. The content of curing agent varied from 1.2 % to 2.4 % , 4.8 % , and 7.2 % by weight, and the content of functional epoxy, reactive diluent, and silane coupling agent changed accordingly, as shown in Table 1. They were named as ECAs-A, ECAs-B, ECAs-C, and ECAs-D, respectively. To fabricate these ECAs, the matrix resin was prepared firstly: the functional epoxy, reactive diluent, silane coupling agent, and curing agent were mixed together by a 78-1 magnetic mixer (Lantian, Hangzhou, Zhejiang, China) until they became homogeneous. Then the micro silver flakes with a size of 3–10 m m and a thickness of about 0.5 m m (Fig. 1a, Fukuda Metal Foil & Powder, Kyoto, Japan) were incorporated into the matrix resin by a MIX500D SLOPE solder cream mixer (Smtech, Shenzhen, Guangdong, China) with a mixing speed of 3000–4000 rpm and a mixing time of 10–20 min. After the fabrication, the ECAs (Fig. 1b) were stored at À 20 C. When tested, they were taken out to room temperature, thawed for 30 min, and stirred strongly by hand for about 5–10 min to make sure the electrically conductive fillers were distributed more evenly. The curing of the samples was studied using a Perkin Elmer Pyris 1 differential scanning calorimeter (Perkin Elmer Inc., Eden Prairie, MN, USA) operated under an atmosphere of pure N 2 . The sample ( ca. 7 mg) was placed in a sealed aluminum sample pan. The curing scans were conducted from 50 C to 200 C at a rate of 10 C = min. FTIR spectra of the sample pellets were recorded using a Nicolet Avatar 370 Fourier transform infrared spectrometer (Thermo Nicolet, Tacoma, Washington, USA) and the conventional KBr disk method; 32 scans were collected at a spectral resolution of 1 cm À 1 ; the pellets used in this study were sufficiently thin to obey the Beer-Lambert law. To prepare the tested samples for bulk resistivity, liquid ECAs were manually printed flatly onto a glass substrate using a small scraper to form stripes of 40 (4.0 0.5) (0.08 0.015) mm. After being cured at 150 C for 15 min and cooled to room temperature in an oven, the stripes (Fig. 2a) were tested by four-probe method (Fig. 2b) using a Hewlett & Packard HP-34401A (HP, Palo Alto, CA, USA) to measure the voltage ( U ) and a Agilent E3631A Triple Output DC Power Supply (HP, Palo Alto, CA, USA) was used to measure the supply current ( I ). Five sets of samples were tested for each value. The tested samples for shear strength were prepared according to Fig. 3. A metal coating pad (8 Â 8 mm) on a FR-4 printed circuit board (PCB) substrate (50 Â 8 Â 2 mm) was used to determine the location of ECAs which provides the shear strength and also to mimic the practical application. After being cured at 150 C for 15 min and cooled to room temperature in a oven, the samples were tested using an Instron 5548 Microtester (Instron, Boston, Massachusetts, USA) at a tensile rate of 5 mm = min. The thickness of cured ECAs was 0.1 mm. There were 25 sets of samples tested for each value. To prepare the tested samples for impact strength, liquid ECAs were manually printed in a rectangle duct-like mold of 80 10 4 mm formed by polytetrafluoroethene tape attached onto a glass substrate by the help of a small scraper. After being cured at 150 C for 15 min and cooled to room temperature in a oven, the de-molded ECAs rods (Fig. 4) were tested using a ZBC1400-2 J pendulum impact testing machine (MTS, Minneapolis, MN, USA). The span length was 60 mm, the impact energy of the pendulum was 2 J and the impact speed was 2.9 m = s. Five sets of samples were tested for each value and there was no notch in these tested samples. The humid-thermal aging was carried out in a MCU Constant Temperature and Humidity Experimental Box (Minch, Taipei, Taiwan). The tested samples were prepared according to Fig. 5. After cured at 150 C for 15 min and cooled to room temperature in a oven, the tested samples conducted the humid-thermal aging under a constant relative humidity level of 85 % at 85 C for 500 h. The surface morphology of the samples was observed using a JSM-6700 F scanning electron microscopy (JEOL, Kyoto, Japan). The cross- section EDS analysis of the interconnection from FR-4 PCB and ECAs was scanned using an INCA Energy X-ray energy dispersive spectroscopy (Oxford, High Wycombe, UK). The water absorption was obtained in the humid-thermal aging. The tested samples (2 Â 2 Â 0.1 mm) were weighed every certain time. Five sets of samples were tested for each value. Figure 6a shows the curing of these fast-curing ECAs. As the curing agent increased from 1.2 % (ECAs-A) to 2.4 % (ECAs-B), 4.8 % (ECAs-C), and 7.2 % (ECAs-D), the curing temperatures decreased accordingly. They were about 130 C for ECAs-A, 115 C for ECAs-B, 108 C and 96 C for ECAs-C, and 96 C for ECAs-D, respectively, at the curing peak temperatures. It can be seen that the curing temperature dropped more than 30 C by increasing the content of curing agent in ECAs to accelerate the curing rate. This will contribute to the realization of fast-curing at low temperatures for ECAs. To determine the curing conditions, the ECAs-A was scanned by DSC at 120 C and 150 C, respectively. As Fig. 6b shows, ECAs-A only had sharp curing peaks around 15 min. No peak was found out as the time exceeded this time. This showed the ECAs-A could be cured at 120 C and 150 C for 15 min by increasing the curing agent content. In addition, during the curing reaction with functional epoxy, the steric effect of 2,4-dimethylimidazole was smaller than that of 1-cyanoethyl-2-ethyl-4-methylimidazole which was used as the curing agent in our previous work. Therefore, the curing conditions were improved greatly compared to that at 150 C for 30 min to 1 h in previous study [13–18]. Moreover, the peak at 150 C was sharper than that at 120 C showing that the ECAs-A had a faster curing ability at 150 C. Therefore, the curing conditions at 150 C for 15 min were fixed and used in this study. The ECAs-A was chosen to further study the related electrical, mechanical, and thermal properties in the following. During curing, there were some complex, continuous chemical reactions between the curing agent (2,4-dimethylimidazole) and functional epoxy ( N,N diglycidyl-4-glycidyloxyaniline). Figure 7 shows the curing mechanism. The imidazole curing agents usually have a secondary amine nitrogen atom at position 1 and a tertiary amine nitrogen atom at position 3. It is considered generally that the nitrogen atom at position 3 on the imidazole ring makes the epoxy group ring open firstly, as shown in Fig. 7A(a) and B(a). Next, two different curing reactions will appear. One is that when the nitrogen atom at position 1 has a hydrogen atom (Fig. 7A), the hydrogen proton transfers [Fig. 7A(b)], and then the nitrogen atom at position 1 opens the epoxy group ring and reacts with the epoxy [Fig. 7A(c)]. The other one is that the nitrogen atom at position 1 has a substituent, not a hydrogen atom, which cannot react with epoxy. Only the nitrogen atom at position 3 can open the epoxy group ring to have a chemical reaction with epoxy [Fig. 7B(a)]. The oxygen anions obtained from the above two cases catalyze the epoxy continuously to open the epoxy group ring and polymerize further [Figure 7A(d) and B(b)]. By this continuous opening ring reaction, the epoxy develops the network structures to give it good physical, mechanical, and thermal properties. In this study, the chemical reactions between the curing agent (2,4-dimethylimidazole) and functional epoxy ( N,N -diglycidyl-4-glycidyloxyaniline) followed the former curing mechanism, as shown in Fig. 7A. Figure 8 shows the bulk resistivity of ECAs-A during humid-thermal aging. It can be seen that the bulk resistivity decreased sharply from 3.32 Â 10 À 3 X Á cm for un-aged ECAs-A to 4.77 Â 10 À 4 X Á cm for ECAs-A aged for 250 h. The decrement was more than 85 % . As Fig. 6 shows, ECAs-A was cured at 150 C for 15 min, and the related intermolecular reaction basically stopped. During the next reflow process or humid-thermal aging, ECAs-A coating or film was heated and kept this heating state for a certain time. The intermolecular reaction continued, and the density of ECAs-A coating or film increased. This process is called the post-curing. ECAs-A showed a post-curing effect at the ...
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Citations
... In summary, in previous studies with resistance measurement for ICA curing process, the resistance values are measured at a few (<20) curing times and/or temperatures [5,11,12,[14][15][16][17], and/or monitored with a slow sampling rate [12]. Some studies [3,[11][12][13][14] have missing resistance values because of the range limit of the technology applied. ...
Real time resistance monitoring technology is used to study the silver sintering process. Signals of joint resistance show events such as resistance increase to >10 GΩ, abrupt resistance drop from >10 GΩ to <1 kΩ, and gradual resistance drop to <1 mΩ. Based on cross-sectioning of samples at various stages of sintering and differential scanning calorimetry (DSC), we propose a correlation between resistance signal and solvent evaporation, capping agent degradation, and silver sintering. We identified distinct clusters of sintered silver of samples removed from the oven when the resistance drops to ∼2.94 Ω.
... The low percolation threshold is critical to maintain the mechanical properties of the polymer matrix and deliver high conductivity at low filler loading levels. Micro-sized silver flakes are known for their high conductivity because of their lower contact resistance [22,23]. Generally, a lubricant layer (e.g. ...
... The cured products should be able to bear short-term high temperatures around 350°C in hot-press leading, long-term high temperatures at 150-170°C, and above 200°C, in storage assessments [1][2][3]. Therefore, functional resins, e.g., epoxy [4][5][6], silicone resin [7,8], polyimide resin [9,10], phenolformaldehyde resin [11,12], polyurethane [13,14], and acrylic resin [15,16], and functional fillers, e.g., copper particles [17,18], micro silver flakes [19,20], nano silver rods [21,22], functionalized carbon nanotubes [23][24][25], silver-plated nano graphite sheets [26,27], nano silver wires [28,29], and nano hexagonal boron nitride particles [30][31][32], have been developed to improve the thermal conductivity and thermal resistance. Most of these ECAs are cured by heat in 30-60 min at 120-150°C or by ultraviolet rays in several to tens of minutes. ...
In this study, the electrically conductive adhesives were fabricated using vinyl ester resin and micro silver flakes, and then a high-intensity pulsed light was introduced to cure the adhesives under an ambient atmosphere at room temperature. The thermal degradation kinetics of photonically cured products was studied using the Friedman method and deduced by assuming a variable activation energy (E). The value of E spanned from 110 to 233 kJ mol−1, which first increased, then decreased, and finally increased again as the thermal degradation proceeded. The kinetic equation of thermal degradation was obtained as dα/dt = e26.62(1 − α)2.22
α
1.85e(−E(α)/RT) with E(α) = 961.73α
3 − 1453.9α
2 + 669.79α + 79.859, α ∈ (0, 1), where α was the fractional extent conversion at a given time (or given temperature). The overall order of reaction was 4.07 and >1, demonstrating that the thermal degradation was complex. With the Friedman method, a comprehensive and in-depth understanding of the thermal degradation kinetics of photonically cured electrically conductive adhesives has been achieved.
... Electrically conductive fillers are combined together by the bonding of matrix resin to form electrically conductive channels and achieve electrically conductive connections [1,2]. The matrix resin, consisting of main resin (e.g., epoxy [3,4], silicone resin [5,6], polyimide resin [7,8], phenol-formaldehyde resin [9,10], polyurethane [11,12], acrylic resin [13,14], etc.), reactive diluent [15][16][17][18], curing agent [15][16][17][18], crosslinker [19,20], coupling agent [15][16][17][18], preservative [21,22], toughening agent [23,24], thixotropic agent [25], and photo initiator [26,27], can be cured under appropriate conditions to connect electronic devices. Meanwhile, ECAs are always made into pastes or adhesives, which can achieve high line resolutions, and meet the rapid developments on the miniaturization of electronic components and the high-density and high integration of printed circuit boards. ...
... Electrically conductive fillers are combined together by the bonding of matrix resin to form electrically conductive channels and achieve electrically conductive connections [1,2]. The matrix resin, consisting of main resin (e.g., epoxy [3,4], silicone resin [5,6], polyimide resin [7,8], phenol-formaldehyde resin [9,10], polyurethane [11,12], acrylic resin [13,14], etc.), reactive diluent [15][16][17][18], curing agent [15][16][17][18], crosslinker [19,20], coupling agent [15][16][17][18], preservative [21,22], toughening agent [23,24], thixotropic agent [25], and photo initiator [26,27], can be cured under appropriate conditions to connect electronic devices. Meanwhile, ECAs are always made into pastes or adhesives, which can achieve high line resolutions, and meet the rapid developments on the miniaturization of electronic components and the high-density and high integration of printed circuit boards. ...
... Electrically conductive fillers are combined together by the bonding of matrix resin to form electrically conductive channels and achieve electrically conductive connections [1,2]. The matrix resin, consisting of main resin (e.g., epoxy [3,4], silicone resin [5,6], polyimide resin [7,8], phenol-formaldehyde resin [9,10], polyurethane [11,12], acrylic resin [13,14], etc.), reactive diluent [15][16][17][18], curing agent [15][16][17][18], crosslinker [19,20], coupling agent [15][16][17][18], preservative [21,22], toughening agent [23,24], thixotropic agent [25], and photo initiator [26,27], can be cured under appropriate conditions to connect electronic devices. Meanwhile, ECAs are always made into pastes or adhesives, which can achieve high line resolutions, and meet the rapid developments on the miniaturization of electronic components and the high-density and high integration of printed circuit boards. ...
In this paper, we fabricated electrically conductive adhesives using vinyl ester resin and micro silver flakes, and then cured the adhesives by heat without any catalysts or initiators. The curing temperature was above 200 °C, and the curing time about 30 min. Under these heat curing conditions, the double bonds in the adhesives reached a high conversion (α) around 98.88 % calculated from the Fourier transform infrared spectroscopy analysis. The curing kinetics of heat curing products was studied using Ozawa method and deduced by assuming a constant activation energy (E). The curing kinetic equation was obtained as dα/dt = e17.70(1 − α)1.19
α
0.41e(−94.32)/RT) with E = 94.32 kJ mol−1. The heat curing followed the shrinking core model from the resin-particle system. The data calculated from the kinetic equation agreed well with the experimental data, showing that the Ozawa method could evaluate the curing kinetics effectively. Furthermore, a comprehensive and in-depth understanding of the curing kinetics of heat curing electrically conductive adhesives has been achieved with this Ozawa method.
... In our previous work [22][23][24][25][26][27], a matrix resin (containing a functional epoxy, a reactive diluent, a silane-coupling agent, and a curing agent) and a formulation of ECAs (25 wt% of matrix resin and 75 wt% of electrically conductive fillers) were obtained. These ECAs were cured at 150 1C for 30 min, had a high pyrolysis temperature above 350 1C and a high glass transition temperature around 180 1C. ...
In this study, we used 3-aminopropyltrimethoxysilane, hexanedioic acid, 3-mercaptopropyltrimethoxysilane, and thioglycolic acid to functionalize the surface of micro silver flakes through in-situ replacements. Hexanedioic acid had the best effect on the functionalization. When the content of hexanedioic acid was 0.5–1 wt% of micro silver flakes, it could fully replace the long chain fatty acid and form a very thin molecular film on the surface of micro silver flakes. The micro silver flakes functionalized by hexanedioic acid played a significant role in the regards of improving the properties of electrically conductive adhesives, and letting the adhesives have low viscosity, e.g., 28,000 cPs, low bulk resistivity, e.g., 2.4×10−4 Ω cm, and high shear strength, e.g, 10.4 MPa.
... These ECAs were cured completely at 150 1C for 30 min, had a high pyrolysis temperature above 350 1C and a high glass transition temperature at 180 1C [25][26][27][28]. In addition, some high performance uni-modal, bimodal, tri-modal, flexible, and fast curing ECAs were also developed from the matrix resin, micro-silver flakes, micro-silver spheres, nanosilver spheres, and carbon nanotubes [29][30][31][32][33][34]. In this study, the matrix resin and the formulation described above were still used. ...
In this study, we incorporated micro-silver flakes and nano-hexagonal boron nitride (BN) particles into a matrix resin to prepare electrically conductive adhesives (ECAs). The humid and thermal aging results under a constant relative humidity level of 85% at 85 °C revealed that the aged ECAs containing 3 wt% of nano-hexagonal BN particles had high reliability. The contact resistance was low and the shear strength high. Nano-hexagonal BN particles have a good effect on the reliability of ECAs that can be used to improve the properties of ECAs.
A combination of metal and non-metal filler in an epoxy resin, called a hybrid electrically conductive adhesive (ECA), is an important development in the field of highly conductive electronic interconnect materials. Carbon nanotubes (CNTs) are well known for contributing strength and stiffness to ECA and silver (Ag) has been widely used as a conductive metal. This study presents the characterization of a hybrid silver micro-flake (AgMF) with multiwalled carbon nanotubes (MWCNT) in an epoxy matrix ECA, in terms of electrical and mechanical properties. The weight percentage of AgMF used was varied, from 1 wt.% to 10 wt.%, whereas the weight percentage of MWCNT filler loading was maintained at 5 wt.%. The properties of the hybrid ECA were characterized using a four-point probe and a universal testing machine for lap shear tests. It was found that the filler hybridization lowered the performance of the ECA in terms of both electrical and mechanical properties, as compared with non-hybrid MWCNT-filled ECA. This may be attributed to the weak interaction between micro- and nano-filler particle sizes and agglomeration of the MWCNT in the epoxy matrix in the hybrid ECA system. The hybrid ECA electrical conductivity was successfully enhanced when a low ratio of AgMF and MWCNT was considered. In addition, failure analysis confirmed that the hybrid ECA with less AgMF filler loading exhibits improved adhesion strength, suggesting a superior epoxy-to-substrate bonding interface.
Silver electrical conductive adhesive (ECA) was prepared by silane capping and ball milling treatment to apply in through-hole filling for PCB (Printed Circuit Board) isolated layer connection. The characteristics of ECA were investigated by scanning electron microscopy, X-ray diffraction, fourier transform infrared spectroscopy and thermal gravimetric analysis. The results showed that as-contained silver flakes preferred to uniformly distribute in ECA adhesive even after 30 days reserve and the bulk resistivity of cured ECA reached 1.8 × 10−4 Ω cm. In addition of high thermal reliability, this prepared ECA demonstrated an excellent performances to apply in through-hole filling for PCB interconnection.
