Current Applied Physics 9 (2009) S72–S74

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Current Applied Physics

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Sinterability and conductivity of silver paste with Pb-free frit

D.S. Seo a, S.H. Park a,b, J.K. Lee a,b,*

a Department of Advanced Materials Engineering, Chosun University, 375, Seosuk-Dong, Dong-Gu, Gwangju 501-759, Republic of Koreab BK21 Education Center of Mould Technology for Advanced Materials and Parts, Chosun University, 375, Seosuk-Dong, Dong-Gu, Gwangju 501-759, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Available online 31 August 2008

PACS:81.07.�b

Keywords:Silver pasteNanoparticlesLow sintering temperaturePb-free frit

1567-1739/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.cap.2008.08.011

* Corresponding author. Fax: +82 (062) 230 7899.E-mail address: jklee@chosun.ac.kr (J.K. Lee).

Conductive silver paste for low sintering temperature was prepared by mixing two commercial silverpowders with different particle size of 0.8 lm and 1.6 lm, and nanoparticles of 20–50 nm. The silvernanoparticles were synthesized by a chemical reduction method using surfactant and 10 wt% of thenanoparticles was added as a low sintering temperature aid. About 3, 6 and 9 wt% of Pb-free frits wereadded to the mixed silver powder, respectively. Using the paste, thick films were prepared by ascreen-printing on an alumina substrate and the films were sintered at temperatures from 400 to550 �C. As increasing the sintering temperatures and glass frit contents, the thick films became denseand their sheet resistivity decreased corresponding to dense microstructure. The films had thickness of4–6 lm and sheet resistivity of approximately 5–10 mX/h.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Electrodes with a form of thick film have been used extensivelyas conductors of hybrid IC, internal and external electrodes in elec-tronic ceramics [1]. The thick film electrodes consist of conductivemetal (Au, Ag and Cu), and binder such as glass and low meltingpoint oxides [2]. Of particular, silver powders are used in electron-ics due to unique properties such as high electrical and thermalconductivity [3]. Silver paste made of silver particles and PbO-based glass composites has been used for thick film conductorsof electronic devices due to their excellent conductivity [4]. How-ever, in order to avoid harmful effects of PbO-based glass and re-duce production cost, intensive researches on preparation of Pb-free silver paste for low temperature sintering have been carriedout [5]. Nano-sized particles were known to sinter at lower tem-peratures due to their high surface energy. Therefore, Pb-free silverpaste adding nano-sized particles can help the desired material tosinter at lower temperatures.

In this work, we aimed to prepare Pb-free silver paste with goodconductivity at low sintering temperature. The Pb-free silver pastewith glass content was investigated to improve densification inconductive thick film and to understand the effects of glass contenton microstructures of the films. Besides, the silver nanoparticleswere used as a sintering aid for low sintering temperature.

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2. Experimental

The silver nanoparticles were synthesized by chemical reduc-tion method. About 0.05 M silver nitrate (AgNO3) was reduced by0.5 M hydrazine monohydrate (N2H4) in presence of 0.1 M tri-so-dium citrate as a surfactant. The silver nanoparticles obtained wereused as a low sintering aid. To prepare silver paste, the amount ofsilver particles was fixed at 70 wt% and commercial silver particleswith 1.6 lm and 0.8 lm in size were mixed with ratio of 40:20(1.6:0.8 lm in wt%). Thereafter, the powders were mixed with10 wt% of silver nanoparticles. The particles were mixed with 3–9 wt% of Pb-free frit and 21–27 wt% of commercial vehicles,respectively. The pastes, composed of 40:20:10/3/27, 40:20:10/6/24 and 40:20:10/9/21 wt% were prepared by using a paste mixer.Hereafter, the pastes of 40:20:10/3/27, 40:20:10/6/24 and40:20:10/9/21 wt% were denoted as N10G3Paste, N10G6Pasteand N10G9Paste, respectively. Thick films from the pastes wereprepared by screen-printing technique on an alumina substrateusing ST#400 (10 lm) screen mask. After screen-printing, the thickfilms were dried at 120 �C for 20 min to remove a volatile organicsolvent. Then, the films were fired at 300–550 �C for 15 min.Microstructures of the films were observed by FE-SEM. The sheetresistivity of the fired films was measured using four-pointtechnique.

3. Results and discussion

Fig. 1 shows XRD pattern and FE-SEM micrograph of thesynthetic silver nanoparticles. It was well crystallized and showed

Fig. 3. FE-SEM micrographs of silver thick films prepared from N10G3Pastesintered at (a) 400 �C, (b) 450 �C, (c) 500 �C and (d) 550 �C.

Fig. 4. FE-SEM micrographs of silver thick films prepared from N10G6Pastesintered at (a) 400 �C, (b) 450 �C, (c) 500 �C and (d) 550 �C.

Fig. 1. (a) XRD pattern and (b) FE-SEM micrograph of silver nanoparticles.

D.S. Seo et al. / Current Applied Physics 9 (2009) S72–S74 S73

typical silver peaks corresponding to face centered cubic (FCC)structure. FE-SEM micrograph shows that the silver nanoparticleswere spherical and relatively mono-dispersed with a size of 20–50 nm.

Fig. 2 shows FE-SEM micrographs of the films made ofN10G3Paste, N10G6Paste and N10G9Paste. Silver nanoparticleswere dispersed among commercial particles at 120 �C (Fig. 2a–c).When the thick films were sintered at 300 �C, contact between par-ticles was initiated. It seems that the nanoparticles may assist thefilm to be sintered at lower temperatures (Fig. 2d–f). However,many voids between particles were still observed meaning thatthe sintering temperature at 300 �C is not high enough for thickfilm conductors. Besides, glass frit (marked by arrows in Fig. 2d–f) was presented as an irregular form, which in turn was not stillsintered.

Fig. 3 shows morphologies of the thick films produced by usingthe N10G3Paste with sintering temperature. Fig. 3a and b shows aporous network of large silver grain due to lack of movement of li-quid glass frit at these sintering temperatures. Thus, a glass poolwas formed (as indicated by arrows) and did not contribute to den-sification of the film. The porous microstructure may be explainedby that development of microstructure occurred by grain boundarydiffusion such as solid phase sintering or shape accommodation;hence many voids were presented in thick film.

However, denser network of thick film was obtained at highersintering temperature (Fig. 3c and d), where liquid phase sinteringof glass melt. Sheet resistivity of the thick films was 10, 9, 8 and7 mX/h in order of increasing sintering temperatures (Fig. 6).

Fig. 4 presents the microstructures of the thick films made ofthe N10G6Paste. Fig. 4a and b showed similar tendency to the caseof N10G3Paste (Fig. 3a and b). As increasing the sintering temper-atures, grain growth took place and denser microstructure was ob-served. Glass pool (marked by arrows in Fig. 4a) was observed dueto lack of movement of liquid glass frit. At higher sintering temper-

Fig. 2. FE-SEM micrographs of thick films prepared from N10G3Paste ((a), (d)), N10G

atures (Fig. 4c and d), the degree of densification was enhancedwith less porosity compared with the N10G3Paste (Fig. 3c and d)due to increase of glass frit content. Therefore, the particles con-tacts were formed more rigid network than that of N10G3Paste.As a result, sheet resistivity of the thick films prepared fromN10G6Paste had lower values of 9, 8, 7 and 6 mX/h in order ofincreasing sintering temperatures than that of N10G3Paste (Fig. 6).

6Paste ((b), (e)) and N10G9Paste ((c), (f)) fired at 120 �C and 300 �C, respectively.

Fig. 5. FE-SEM micrographs of silver thick films prepared from N10G9Pastesintered at (a) 400 �C, (b) 450 �C, (c) 500 �C and (d) 550 �C.

Fig. 6. Sheet resistivity of thick films with sintering temperature.

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A further increase of glass frit content (Fig. 5), active repackingof thick film was accelerated by the larger amount of liquid phaseglass frit which assists fast rearrangement process and intercon-nection of metal particles. As a result, metal particles enlarge theircontact area and make a short current path corresponding to thelower resistivity. The N10G9Paste had the sheet resistivity of 11,8, 6 and 5 mX/h in order of increasing sintering temperatures(Fig. 6). The lowest value of sheet resistivity was 5 mX/h at a sin-tering temperature of 550 �C.

4. Conclusion

Silver pastes were prepared by adding Pb-free frit and nanopar-ticles as a sintering aid. The glass frit was useful to develop micro-structure of the film by liquid phase sintering. As content of glassfrit changing (3–9 wt%), sheet resistivity was changed correspond-ing to densification of thick film. The films prepared byN10G9Paste with 9 wt% of glass frit had better conductive networkand the lowest sheet resistivity of 5 mX/h. Furthermore, the nano-particles with 10 wt% assisted the film to be sintered at lower tem-peratures for conductive network of thick film. Accordingly, thedenser interconnection contributed to excellent conductivity ofthe sintered thick films. The results indicate that silver paste withsuitable contents of nanoparticles and glass frit can be a good sub-stance for electronic devices.

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