Tuning the electrical resistivity of conductive silver paste prepared by blending multi-morphologies and micro-nanometers silver powder

As an important interconnection material in electronics, conductive silver paste has attracted much research interest in chip packaging and printed circuit board due to its predominant properties like high conductivity and flexible interconnection. In this paper, the silver nanoparticles, the silver sphere particles, and flake silver powders are fabricated by various methods. Different proportions of silver powder are selected to prepare micro-silver paste and micro-nano (mn) silver paste (fabricated by silver nanoparticles:sphere particles:flake silver powder = 1:1:1). Compared to traditional silver paste with high silver content (containing 80 wt% silver), paste in this work contains only 66.67 wt% silver. When the ratio of sphere particle to flake silver powder is 1:1, the electrical resistivity of micro-silver paste cured at 200 °C for 45 min in air is about (3.31 ± 0.73) × 10−5 Ω cm, the resistance of silver paste rises a little bit after being folded by ten times. The addition of nano-silver particles can reduce the resistivity in lower temperature curing. The folding endurance of mn silver paste is comparable to that of micro-silver paste with sphere:flake = 1:1.


Introduction
Recently, with the rapid development of micro-chip packaging and flexible printed circuit board, traditional interconnection materials in electronics such as solder alloy are unable to meet the rising requirements in IC packaging and solar cell. Silver paste has attracted extensive attention as a key conductive material for fabricating electronic components for its low processing temperature, high bonding strength (170 MPa), prominent thermal conductivity, stable electrical properties, and suitability for screen printing [1][2][3][4][5]. Silver paste is composed of conductive filler, binder phase, solvent, and other auxiliary agents. According to the different binder phases, the silver paste can be divided into polymer type silver conductive paste and sintering type silver conductive paste [6]. People used to add Ag nanoparticles (Ag NPs) in silver paste to suppress the sintering temperature due to small size effect and surface effect [7]. In addition, the conductive silver paste can realize flexible interconnection [8]. Because of those excellent performance mentioned above, conductive silver paste is widely used in solar cells, flexible printed circuit boards, radio frequency identification system (RFID), electromagnetic shielding, membrane switch, and other devices [9][10][11][12][13].
At present, the bottleneck of silver paste is the approach to decrease the resistivity and reduce the cost. The conductive mechanism of metal-filled conductive paste is the result of the percolation theory, tunneling theory, and field-emission theory [14][15][16]. The resistance of paste includes three parts, that is, where R i is the intrinsic resistance in the packaging, R c is the contact resistance between particles, and R t is the tunneling resistance. Point contact is formed between spherical powder fillers, while surface contact and line contact are formed between flake powder fillers [17]. Adding appropriate spherical silver nanoparticles into the flake silver matrix can maximize the contact area and fill the gap between the conductive phase particles. In this way, the number of conductive network is increasing, leading to the rising of electrical conductivity. C. Liu [18] synthesized the silver paste by using 2 lm Ag micron-flakes and 20 nm Ag NPs, which has low resistivity of 3.673 9 10 -5 X cm. Jung [19] found that the mixed silver paste composed of Ag NPs and Ag micron-flake had good adhesion and flexibility, and when the content of silver flake is 50%, the performance of silver paste reaches the peak. In addition, the choice of organic carrier also affects the conductivity of the silver paste. The residues of organic carrier are inevitable in the actual sintering process. Accordingly, the effective resistivity of Ag paste after curing is controlled not only by the resistivity of the silver powder but also by the resistivity of the organic carrier [20]. Therefore, the dielectric constant should be considered when selecting organic carrier. At the same time, the volatilization temperature of organic carrier will affect the removal of organic.
There are two strategies to reduce the cost of conductive silver paste. The first method is reducing the filling ratio of silver powder. The high price of silver powder determines the high cost of silver paste [21]. However, reducing the mass fraction of silver powder may lead to a decrease in electrical conductivity. C. F. Li [22] prepared silver paste with high conductivity by mixing Ag flakes and liquid poly (dimethylsiloxane) (PDMS). The lowest resistivity is about 8.7 9 10 -5 X cm when the loading capacity of silver flakes reached 91%, but the resistivity is only 3.7 9 10 -4 X cm when the mass fraction of Ag flakes is reduced to 80%. Another way is to add more Ag micron-flakes in the mixed paste [23]. In industrial, the Ag flakes are generally prepared by ball milling method, which is simple to operate and low-cost. The third method is using low-cost organic carriers with simple composition, such as epoxy resin, ethyl cellulose, ethanol, ethylene glycol, polyvinyl alcohol, hexanol, and other organic solvents that can be volatilized at low temperature [24].
In this work, we have synthesized the nano-silver particle and flake silver powder in two different ways and purchased commercial silver sphere particle (average diameter: 1 lm). We fabricated micro-silver paste and mn silver paste containing 66.67 wt% silver by blending multi-morphologies and micronanometers silver powder successfully. Rheological properties of silver paste were tested. The effects of curing temperature and curing time on the conductivity, pencil hardness, and adhesion of silver paste were studied. The objective is to establish a correlation between the resistivity and the proportion of multi-morphologic silver powder. We also aim to explore a new way to fabricate high conductivity, low-cost silver paste. The Ag NPs (average diameter: 50 nm) are synthesized by chemical reduction procedure: first, 10 g AgNO 3 was dissolved in 50 ml EG as precursor solution, and 150 ml EG (reducing agent), 100 ml PEG (reducing agent), and 20 g PVP (protective agent) were mixed as reducing solution. Then the precursor solution was poured into the reducing solution and stirred at 160°C for 30 min. Finally, the precipitation was centrifuged and washed several times with ethanol and dried at room temperature. The micro-silver flakes used in this work were fabricated by high energy ball milling.

Sample preparation
First, 1 g PVP was dissolved in 20 ml 1-hexanol as organic carrier. Then, silver powder and organic carrier with a weight ratio of 2:1 were pre-mixed and rolled at least four times. Meanwhile, the four samples were prepared by the same method, marked #1, #2, #3, and #4, respectively, as shown in Table 1. Finally, those silver pastes were screen printed on a 0.075 mm thick polyimide (PI) substrate to form a 60 mm 9 0.6 mm line, as shown in Fig. 1 and then those patterns were cured in muffle furnace at different temperature and time in air.

Measurements and characterization
The rheological property of silver paste is performed using a rotary rheometer (AR2000EX, USA), equipped with a steel plate of 40 mm. The specific surface area of silver powder is determined by automatic specific surface analyzer (Monosorb, USA). The morphology of silver powder and microstructure of silver paste after curing were observed with a fieldemission scanning electron microscopy (SEM, FEI Quanta FEG 250). X-ray diffraction (XRD) test was performed using fully automatic X-ray diffractometer (D/max 2550 VB). DC low resistance tester was executed to measure the resistance of silver paste after curing.

Characterization of silver powder
The size and morphology of silver powder used in this work are shown in Fig. 2. It can be seen that the average particle size of spherical silver powder is 1 lm. The morphology of flake silver powder is irregular and the average size is 4 lm. The average size of nano-silver powder is 50 nm. The specific surface area of three kinds of silver powder is measured, as shown in Table 2. It can be seen that the specific surface area of flake silver powder is 1.85 m 2 g -1 , three times larger than that of spherical silver powder, which is beneficial to improve the electrical conductivity.
In order to identify the crystallinity of silver powder, XRD pattern is performed, as shown in Fig. 3. It can be seen that the peak is exactly consistent with the diffraction of Ag. It indicates that the nano-silver powder separated by centrifugation has high crystallinity and few impurities.

The steady-state rheology property of silver paste
In order to measure the rheology behavior of silver paste, the steady-state rheology test is compared and analyzed, as shown in Fig. 4. It is easy to tell that the viscosity of silver paste decreases with increasing shear rate, gradually. The initial viscosity decreases from 153.1 Pa s in #3 sample (sphere:flake = 1:2) to 38.14 Pa s in #2 sample (sphere:flake = 2:1), gradually. This is mainly due to the poor fluidity of flake silver powder compared with spherical silver powder. When the silver paste flows, the spherical particles are rolling, and give a small friction, leading to good liquidity. On the other hand, the friction between the flake particles is large, which will present a poor liquidity. Therefore, the more flake silver powder is, the worse the liquidity is. Both too much and too few flake silver powder can affect the fluidity of paste and then affect the printing performance of paste. Considering the printing performance of these pastes, the viscosity of silver paste prepared by sphere:flake = 1:1 is moderate.

Curing morphology
In order to study the cause of resistivity change, the morphologies of silver paste cured at multiple temperatures are observed by scanning electron microscopy, as shown in Fig. 5. It can be seen that spherical silver particles are evenly distributed in the pores between flake silver powder. Sintering neck is generating and porosity is increasing with temperature rising, which also explains why the resistivity drops as the temperature rising. Figure 6 shows the SEM image of micro-silver paste with spherical:flake = 1:1 cured at different   time. It can be seen that the number of sintering neck is gradually increasing as the extension of curing time. The porosity of silver paste first increased from 16 to 24% as curing time from 15 to 60 min and then no longer increase. This is because the organic volatilization in the silver paste can produce a lot of pores, when curing for 60 min, the organic has almost evaporated, and the value of pore is at its maximum. Porosity of silver paste does not change after 60 min. Figure 7 shows the SEM image of mn silver paste cured at multiple temperatures. It can be seen that nano-silver particles dispersed well at low temperature (\ 150°C) due to low surface activity and ultrasonic vibration. But when the temperature exceeds 150°C, the surface activity of silver nanoparticles increased and they begin to aggregate and grow together. Figure 8 shows the electrical resistivity of silver paste after curing under different conditions. The resistivity of micro-silver paste cured at various temperatures is shown in Fig. 8a. Rising the temperature leads to a decrease in the average electrical resistivity and tends to be stable gradually. Electrical resistivity of silver paste with sphere:flake = 1:1 is significantly lower than that of other two samples cured at 175°C, the resistivity reaches 5.61 9 10 -5 X cm after curing at 200°C and then falling slowly when the temperature exceeds 200°C. The resistivity of silver paste is similar with that reported by Yuhuan Meng et al [25]. Figure 8b shows the electrical resistivity of micro-silver paste cured at different time. Extending the curing time from 15 to 45 min results in a decrease in the average electrical resistivity at first, then the resistivity increases slightly at 60 min and descends after 60 min again. Figure 8c shows the electrical resistivity of mn silver paste. It can be seen that the addition of nano-silver particle reduced the resistivity of micro-silver paste. The resistivity of mn silver paste cured at 100°C reached to 3.4 9 10 -4 X cm, which is lower than that of micro-silver paste cured at 150°C. However, the resistivity of mn silver paste cured above 150°C is higher than that of micro-silver paste cured at the same temperature. Figure 8d shows the resistivity of mn silver paste cured at different time. Extending the curing time is beneficial to the reduction of electrical resistivity.

Electrical resistivity
The present results indicate that the variation of resistivity of micro-silver paste is as follows: as the temperature rising, the sintering neck generated between particles, gradually. The connection between particles changes from physical contact to chemical contact, as shown in Fig. 5, leading to the obvious decrease of resistivity of silver paste.
The silver paste line first shrinks 45 min ago, and the distance between particles is dominant. When the contact area increases, resistivity of the silver paste decreases. At the same time, organic volatilization in the silver paste can produce more and more pores. Porosity of silver paste reaches the maximum when curing time reaches 60 min, the resistivity of silver paste is controlled by porosity and increases slightly. When curing time is exceeded 60 min, sintering neck is dominant, and the increase in the number of sintering neck leads to the decrease in resistivity.
The addition of nano-silver particles can reduce the resistivity at low temperature curing, but the effect is not obvious at high temperature. It is mainly because the nanoparticles could disperse well at low temperature due to low surface activity and ultrasonic vibration. While the temperature is above 150°C, the surface activity of silver nanoparticles increased and the particles agglomerated together. The agglomeration of silver nanoparticles results in a decrease of contact area and an increase of resistivity.

Pencil hardness
Pencil hardness of silver paste cured at multiple temperatures is shown in Table 3. Rising the temperature results in an increase in the pencil hardness of the silver paste. This is due to the emergence of sintering neck, which makes the connections between particles tighter. The more the number of spherical silver powder is, the worse the pencil hardness of silver paste after curing at the same temperature is, because the real contact area of spherical silver powder is smaller than that of flake powder.
Adhesion level of silver paste cured at different temperature is shown in Table 4. The adhesion level of silver paste after curing decreases with temperature rising. Because as the temperature rises, the amount of organic carrier decreases, and the adhesion level falls. The change of adhesion level of different silver paste cured at the same temperature is consistent with the hardness of pencil. Figure 9 shows the variation of electrical resistance of silver paste with folding times. It can be seen that the resistivity of the silver paste with sphere:flake = 1:1 and sphere:flake:nano = 1:1:1 does not increase sharply until it is folded by ten times, so the folding endurance of two kinds of silver paste is good.

Conclusion
In summary, micro-silver paste and mn silver paste were prepared by blending multi-morphologies and micro-nanometers silver powders. The effects of silver powder proportion, curing time, and curing temperature on the conductivity, pencil hardness, and adhesion of the silver paste were studied. The specific conclusions are as follows: 1. Nano-silver particles are synthesized by one-step aqueous-phase reduction reaction. Flake silver powder is fabricated by high energy ball milling. Micro-silver paste and mn silver paste containing 66.67 wt% silver are prepared by blending the proportion of these silver powders, which saves cost compared to traditional silver paste with silver content of 80 wt%. 2. The resistivity of silver paste decreases with the increase of temperature, gradually. The resistivity of micro-silver paste with sphere:flake = 1:1 cured at 200°C for 45 min in air is about (3.31 ± 0.73) 9 10 -5 X cm. The resistivity of silver paste drops at first before 45 min, then slightly increases at 60 min, and finally decreases.
3. The addition of nano-silver particles can reduce the resistivity in low temperature. The prepared two kinds of conductive silver paste exhibited low resistivity and excellent folding endurance. The resistance of silver paste with sphere:flake:nano = 1:1:1 does not change abruptly after folding by ten times. Fig. 8 Variation of electrical resistivity of silver paste cured at multiple temperatures and time; a micro-silver paste and multiple temperatures; b micro-silver paste and multiple time; c mn silver paste and multiple temperatures; d mn silver paste and multiple time