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Preparation of micron-sized flake copper powder for base-metal-electrode multi-layer ceramic capacitor
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1129–1133
journal homepage: www.elsevier.com/locate/jmatprotec
Preparation of micron-sized flake copper powder for base-metal-electrode multi-layer ceramic capacitor S.P. Wu a,∗ , R.Y. Gao b , L.H. Xu a a b
College of chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China College of Environmental Science and Engineering, South China University of Technology, Guangzhou 510641, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
The preparation of flake micron-sized copper powders with the chemical–mechanical
Received 29 June 2006
method was investigated. Reaction of [Cu(NH3 )4 ]2+ complex with hydrazine hydrate at 85 ◦ C
Received in revised form
produced monodispersed fine spherical copper powders, which were used as precursor
2 March 2008
to synthesize flake copper powders by the ball milling process. The flake copper powders
Accepted 16 March 2008
having an excellent dispersibility and a uniform size of 9 ± 2 m could be achieved. Thermogravimetry (TG), differential thermogravimetry (DTG) and differential thermal analysis (DTA) of the flake copper were investigated with thermal analyzer. The results showed that
Keywords:
the oxidizing temperature increased with a decreasing specific area. The flake copper pow-
Micron-sized copper powders
der particles were employed as functional conductive materials in copper thick film paste
Flake powders
for base-metal-electrode multi-layer ceramic capacitors (BME-MLCCs). Excellent connection
Chemical reduction
between internal and terminal electrode and even distribution of glass in copper thick film
Ball milling
can be observed by polarized light photograph. The dense thick films were also found by
BME-MLCCs
scanning electron microscopy (SEM) analysis, and the high densification of the fired films
Thick films
could be attributed to the “framework” effects. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Thick film technology was widely used for versatile application. The steady growth of thick film technology has been due to the continual growth of new microelectronic circuit needs and the ability to develop materials to accommodate them. Amongst the various conductive materials, copper has been established as an important choice because of high electrical conductivity, relatively higher melting point, excellent solderability, low electrochemical migration behavior and low materials cost (Wu et al., 2007; Wu, 2007a). The fired-on type base-metal-based pastes were successfully used in base-metal-electrode multi-layer ceramic capacitors (BME-MLCCs) as demonstrated (Im et al., 2006). In copper-based conductive paste, it is necessary to employ
∗
Corresponding author. E-mail address: [email protected] (S.P. Wu). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.03.010
flake copper powders as a supplement to spherical particles for optimizing rheological behavior of paste and improving electrical conductivity of thick film. Either spherical or flake particles, the parameters, such as particle size, shape and distribution, of copper particles are of utmost importance, which have direct bearing on the printing process and, in turn, the microstructure and electrical properties of the resulting films (Rane et al., 2003; Lin and Wang, 1996). For conductive paste application, it is required to use micron-sized nonagglomerated metallic powders (Deshpande et al., 2005; Wu, 2007b). Generally speaking, flake copper powders could be prepared with the two-stage method. Firstly, precursor copper particles were produced by electrolytic method (Xue et al., 2006), pyrolysis (Rosenband and Gany, 2004), atomized process (Karibyan et al., 1981) or chemical-reduction method (Wu,
1130
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1129–1133
2007a; Sinha and Sharma, 2002). Secondly, the spherical copper particles are processed by a high energy ball mill. Precursor particles are extruded by ball milling media, and the flake particles could be achieved. In the paper, we reported a chemical–mechanical method for the formation of the flake copper particles, which could meet the requirement of copper thick film paste. The behaviors and microstructures of BME-MLCCs made by assynthesized flake particles were also studied by scanning electron microscopy (SEM) and polarized light photograph.
by SA3100 (Beckman Coulter) with Brunauer–Emmett–Teller (BET) method. The BME-MLCCs were fixed with resin and processed with grinding and polishing machine, and then the connection of internal–terminal electrode and distribution of glass in termination were analyzed with the polarized light microscope (Olympus BX51M). Electrical conductivity of the thick films was measured by the four-point probe method. The bulk resistivity of copper thick film was calculated according to the following equation:
2.
Experimental
=
2.1.
Synthesis of spherical copper powders
where R is the resistance measured with four-point technology, S is the area of cross-section of thick film estimated by SEM analysis and L is the length. The adhesion strength of electrode was determined with FDV-50 force apparatus (Wagner Instruments, USA). Resistance behavior of termination of BME-MLCCs to soldering was tested by immerging termination of MLCCs in solder at 260 ± 5 ◦ C for 5 s.
All chemicals of reagent grade quality were used without further purification. 1500 ml solution containing 400 g CuSO4·5H2 O, 200 g NH4 Cl and 900 ml NH3 ·H2 O was added during 60 min to a stirred hydrazine hydrate (N2 H4 ·H2 O) solution. The temperature was kept at 70 ◦ C. The solution was heated to 85 ◦ C for 1 h to reduce copper ion in the solution to metallic copper.
2.2.
Preparation of flake copper powders
Above-mentioned spherical copper powders, ZrO2 ball milling media and dispersing agent, i.e. sodium dodecyl sulfate, were mixed in a container fixed on a planet-type grinding mill rotating at a rate of 250 rpm for 8–12 h, so that the precursor particles were processed to flake copper particles. The flake copper powders were recovered from the solution, washed and dried under vacuum.
2.3.
(1)
3.
Results and discussion
3.1.
Synthesis of spherical and flake copper powders
In this work, spherical copper particles were obtained by reaction of [Cu(NH3 )4 ]2+ complex with hydrazine hydrate in ammonia chloride solution. In this process, the following chemical reaction occurred: Cu2+ + 4NH3 → [Cu(NH3 )4 2+
(2)
2[Cu(NH3 )4 ]2+ + N2 H4 → 2Cu + N2 + 4NH3 + 4NH4 +
(3)
Fabrication of thick films for BME-MLCC
The thick film pastes were formulated having the composition of as-prepared copper powders as the functional material, ZnO–SiO2 –B2 O3 glass frit as the permanent binder, a mixture of ethyl cellulose and solvent, i.e. ethylene glycol acetate, as a vehicle. The weight ratio of the composition was kept at 65:10:25. The pastes were rolled several times by a threeroll mill and coated on the termination of MLCC chip, whose specification was 0805Y5V, and dried, then, copper end terminations were fired at the peak temperature of 910 ◦ C for 10 min in a nitrogen atmosphere, with a controlled level of oxygen.
2.4.
RS L
Characterization
The behaviors of powder particles and microstructure of copper thick films were directly observed with the scanning electron microscopy on a XL30␦DX-4i (Philips). The crystal structure was characterized by X-ray diffraction (Philips). Thermogravimetry (TG), differential thermogravimetry (DTG) and differential thermal analysis (DTA) were carried out at scanning rates of 10 ◦ C/min in a flowing air atmosphere (Q600, SDT, TA Inc., USA). The purity of powder was determined with inductively coupled plasma spectrometer (ICPS) (PE Optima 3000). The specific areas of powder particles were measured
In aqueous ammonia media, ammonia reacts with Cu2+ according to Eq. (2) and [Cu(NH3 )4 ]2+ complex was reduced to obtain fine copper powder as seen in Eq. (3). It was obvious that Eq. (3) occurred in basic solution. If pH value was low, reaction could not occur smoothly. Ammonia chloride/aqueous ammonia buffer was employed to keep constant pH values at 8–8.5, in where the dispersability of powder particles increased. It was very important to choose appropriate dispersion agent for preparing fine metallic particles with chemicalreduction method. Dispersion agent could effectively control particle size and dispersability according to electrostatic repulsion effect, space hindrance effect and interfacial tension effect. We could distinctly find out that the micron-sized monodispersed non-agglomerated single spherical copper particles have been synthesized when sodium tartrate was employed as an inorganic dispersing agent. In an optimal condition, as-synthesized particles have an excellent dispersability and uniform single particle size of 3.5 ± 0.5 m by SEM analysis as given in Fig. 1a. By ball milling precursor spherical copper powders, the thin flake copper particles with a uniform size of 9 ± 2 m were formed as shown in Fig. 1b. The powder particles have a uniform size, less specific area and suitable ratio of diameter–thickness. By size distribution analysis, one could see that the powder particles have an excellent dispersabil-
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1129–1133
1131
Fig. 3 – Thermal analysis of flake copper particles.
Fig. 1 – SEM photographs of spherical (a) and flake (b) copper particles.
ity and normal size distribution. With the help of dispersing agent, the soft agglomeration between particles was destroyed by strong mechanical power from ball milling, resulting in appearing flake monodispersed particles. The ICPS analysis shows that purity of flake copper particle is more than 99.95%.
3.2.
XRD of flake copper particles
Fig. 2 showed the XRD pattern of the flake copper particles. From the pattern, it was obvious that the diffractogram exhib-
Fig. 2 – XRD spectrum of flake copper powders.
Fig. 4 – Influence of specific area on oxidizing temperature.
ited the characteristic peaks of crystalline metallic copper ˚ = 2.089, 1.809, 1.279, corresponding to (1 1 1), (2 0 0), (fcc). d (A) (2 2 0) plane, respectively, which is very close to that given by JCPDS file no. 4-836. The intensity of peaks was higher than those of its precursor spherical particles.
3.3.
Thermal analysis of flake copper powders
The thermal behaviors of metallic flake copper prepared by the above-method were analyzed by TG/DTG/DTA in an air flow. Fig. 3 showed the TG/DTG/DTA curves with a heating rate of 10 ◦ C/min. According to TG curve, the increase in weight began at 250 ◦ C, which was content with the results by DTG/DTA analysis. There was a distinct exothermal peak attributed to formation of Cu2 O at 262 ◦ C in DTA curve. The oxidation of copper became very rapid at 383.4 ◦ C corresponding to 15% weight gain by TG curve, which leveled off above 550 ◦ C, at which the CuO formed. Influence of specific area of flake copper powders on oxidizing temperature was investigated as given in Fig. 4. The oxidizing temperatures decreased proportionally as an increasing specific area. For the oxidation of copper at firing in air, the chemical adsorption of oxygen atoms on the copper particles was one of the critical factors because the physical adsorption would be easily desorbed at a so high temperature
1132
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1129–1133
Fig. 5 – Polarized light photo of cross-section of MLCC. Fig. 6 – SEM photograph of copper thick films. (more than 200 ◦ C). According to the monolayer adsorption theory (Langmuir equation), the adsorptive capacity of oxygen atoms was the function of specific area of particles, so the amount of oxygen atoms adsorbed on the surface of copper particles decreased as a decreasing specific area, resulting to the higher oxidizing temperature for forming Cu2 O. However, for copper particles, the lower specific area meant the lower flaky degree, which was unfavorable for particle’s homogeneous dispersion in organic vehicle and deteriorated the stability of pastes, despite its higher oxidation-resistance power. The suitable specific area of flake copper particles would be chosen based on the overall consideration.
3.4.
Copper end termination of BME-MLCCs
The flake copper particles could effectively manage the rheologic properties of thick film paste due to their platy shape. The coordination of flake copper particles with spherical copper particles effectively was a very important question for making conductive termination pastes. The cross-section of MLCCs packaged with pastes comprising as-synthesized flake copper particles was discussed by polarized light microscope, and the photo was given in Fig. 5, in which the connection between internal and terminal electrode, distribution of glass and interfacial layer were exhibited. As seen from this photograph, the distribution of glass was uniform in copper thick films. The connection of internal–terminal electrode was excellent. The gray transitional region occurred in the interface between electrode and chip due to the interfacial reaction. The proper attachment of the glass to the dielectric bodies, whose basic material is BaTiO3 , produced high adhesion strength. No separation of electrode appeared when the applied force was 1.8 kgf mm−2 . The average adhesion strength of electrode was 2.4 kgf mm−2 . The SEM photograph of copper thick films was given in Fig. 6. The densification of the fired copper thick film was high. It could be attributed to the so-called “framework” effects. In the printed thick films, the larger flake copper particles could be considered to be the “bricks”, and formed a similarly stable “framework”. On the other hand, the smaller spherical copper particles could be jammed into the “gaps” between “frameworks”. The compact structures in thick films should be formed, resulting in a high densification of the fired films.
The high densification could effectively avoid the leakage of electroplating solution, which may produce the failure of BMEMLCCs. The degree of densification was also important in determining the electrical resistivity of the fired thick film. Norton (1991) reported that the increased film density reduced the electrical resistivity as the degree of contact between particles was increased. Bulk resistivity of as-prepared thick films was 5.5 × 10−5 cm. 95% of terminal electrode covered by new solder showed that the thick films had excellent solderability behavior. Resistance behavior to soldering was very good. The results showed that no micro-crack appeared in MLCC at such a high impact temperature of 260 ◦ C, and 80% of terminal electrode was covered by new solder.
4.
Conclusions
In conclusion, spherical copper particles with a size of 3.5 ± 0.5 m were used as precursor to prepare flake copper powders by ball milling process. Flake copper particles with a uniform size of 9 ± 2 m were formed. The oxidizing temperature of flake copper powders increased as a decreasing specific area. The fired-on type films prepared by conductive pastes comprising as-synthesized flake particles have a high densification because of “framework” effects. The copper end termination of BME-MLCCs has an excellent electrical and mechanical behavior.
references
Deshpande, V., Kshirsagar, A., Rane, S., Seth, T., Phatak, G.J., Mulik, U.P., Amalnerkar, D.P., 2005. Properties of lead-free conductive thick films of co-precipitated silver–palladium powders. Mater. Chem. Phys. 93 (2–3), 320–324. Im, D.H., Hyun, S.H., Park, S.Y., Lee, B.Y., Kim, Y., 2006. Preparation of Ni paste using binary powder mixture for thick film electrodes. Mater. Chem. Phys. 96 (2–3), 228–233. Karibyan, A.N., Medvedovskii, A.B., Belyakov, V.A., et al., 1981. Atomized copper powders. Sov. Powder Metall. Met. Ceram. 20 (5), 316–319. Lin, J.C., Wang, C.Y., 1996. Effect of surface properties of silver powder on the sintering of its thick-film conductor. Mater. Chem. Phys. 45 (3), 253–261.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1129–1133
Norton, M.G., 1991. Glass–ceramic interactions and thick-film metallization of aluminum nitride. J. Mater. Sci. 26 (9), 2322–2328. Rane, S.B., Seth, T., Phatak, G.J., Amalnerkar, D.P., Das, B.K., 2003. Influence of surfactants treatment on silver powder and its thick films. Mater. Lett. 57 (20), 3096–3100. Rosenband, V., Gany, A., 2004. Preparation of nickel and copper submicrometer particles by pyrolysis of their formats. J. Mater. Process. Technol. 153–154, 1058–1061. Sinha, A., Sharma, B.P., 2002. Preparation of copper powder by glycerol process. Mater. Res. Bull. 37 (3), 407–416.
1133
Wu, S.P., Jiao, L., Ni, J., Zeng, Z.O., Liu, S., 2007. Preparation of ultra fine copper–nickel bimetallic powders for conductive thick film. Intermetallics 15 (10), 1316–1321. Wu, S.P., 2007a. Preparation of ultra-fine copper powder and its lead-free conductive thick film. Mater. Lett. 61 (16),3526–3530. Wu, S.P., 2007b. Preparation of fine copper powder using ascorbic acid as reducing agent and its application in MLCC. Mater. Lett. 61 (4–5), 1125–1129. Xue, J.Q., Wu, Q., Wang, Z.Q., Yi, S.F., 2006. Function of additives in electrolytic preparation of copper powder. Hydrometallurgy 82 (3–4), 154–156.
journal homepage: www.elsevier.com/locate/jmatprotec
Preparation of micron-sized flake copper powder for base-metal-electrode multi-layer ceramic capacitor S.P. Wu a,∗ , R.Y. Gao b , L.H. Xu a a b
College of chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China College of Environmental Science and Engineering, South China University of Technology, Guangzhou 510641, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
The preparation of flake micron-sized copper powders with the chemical–mechanical
Received 29 June 2006
method was investigated. Reaction of [Cu(NH3 )4 ]2+ complex with hydrazine hydrate at 85 ◦ C
Received in revised form
produced monodispersed fine spherical copper powders, which were used as precursor
2 March 2008
to synthesize flake copper powders by the ball milling process. The flake copper powders
Accepted 16 March 2008
having an excellent dispersibility and a uniform size of 9 ± 2 m could be achieved. Thermogravimetry (TG), differential thermogravimetry (DTG) and differential thermal analysis (DTA) of the flake copper were investigated with thermal analyzer. The results showed that
Keywords:
the oxidizing temperature increased with a decreasing specific area. The flake copper pow-
Micron-sized copper powders
der particles were employed as functional conductive materials in copper thick film paste
Flake powders
for base-metal-electrode multi-layer ceramic capacitors (BME-MLCCs). Excellent connection
Chemical reduction
between internal and terminal electrode and even distribution of glass in copper thick film
Ball milling
can be observed by polarized light photograph. The dense thick films were also found by
BME-MLCCs
scanning electron microscopy (SEM) analysis, and the high densification of the fired films
Thick films
could be attributed to the “framework” effects. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Thick film technology was widely used for versatile application. The steady growth of thick film technology has been due to the continual growth of new microelectronic circuit needs and the ability to develop materials to accommodate them. Amongst the various conductive materials, copper has been established as an important choice because of high electrical conductivity, relatively higher melting point, excellent solderability, low electrochemical migration behavior and low materials cost (Wu et al., 2007; Wu, 2007a). The fired-on type base-metal-based pastes were successfully used in base-metal-electrode multi-layer ceramic capacitors (BME-MLCCs) as demonstrated (Im et al., 2006). In copper-based conductive paste, it is necessary to employ
∗
Corresponding author. E-mail address: [email protected] (S.P. Wu). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.03.010
flake copper powders as a supplement to spherical particles for optimizing rheological behavior of paste and improving electrical conductivity of thick film. Either spherical or flake particles, the parameters, such as particle size, shape and distribution, of copper particles are of utmost importance, which have direct bearing on the printing process and, in turn, the microstructure and electrical properties of the resulting films (Rane et al., 2003; Lin and Wang, 1996). For conductive paste application, it is required to use micron-sized nonagglomerated metallic powders (Deshpande et al., 2005; Wu, 2007b). Generally speaking, flake copper powders could be prepared with the two-stage method. Firstly, precursor copper particles were produced by electrolytic method (Xue et al., 2006), pyrolysis (Rosenband and Gany, 2004), atomized process (Karibyan et al., 1981) or chemical-reduction method (Wu,
1130
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1129–1133
2007a; Sinha and Sharma, 2002). Secondly, the spherical copper particles are processed by a high energy ball mill. Precursor particles are extruded by ball milling media, and the flake particles could be achieved. In the paper, we reported a chemical–mechanical method for the formation of the flake copper particles, which could meet the requirement of copper thick film paste. The behaviors and microstructures of BME-MLCCs made by assynthesized flake particles were also studied by scanning electron microscopy (SEM) and polarized light photograph.
by SA3100 (Beckman Coulter) with Brunauer–Emmett–Teller (BET) method. The BME-MLCCs were fixed with resin and processed with grinding and polishing machine, and then the connection of internal–terminal electrode and distribution of glass in termination were analyzed with the polarized light microscope (Olympus BX51M). Electrical conductivity of the thick films was measured by the four-point probe method. The bulk resistivity of copper thick film was calculated according to the following equation:
2.
Experimental
=
2.1.
Synthesis of spherical copper powders
where R is the resistance measured with four-point technology, S is the area of cross-section of thick film estimated by SEM analysis and L is the length. The adhesion strength of electrode was determined with FDV-50 force apparatus (Wagner Instruments, USA). Resistance behavior of termination of BME-MLCCs to soldering was tested by immerging termination of MLCCs in solder at 260 ± 5 ◦ C for 5 s.
All chemicals of reagent grade quality were used without further purification. 1500 ml solution containing 400 g CuSO4·5H2 O, 200 g NH4 Cl and 900 ml NH3 ·H2 O was added during 60 min to a stirred hydrazine hydrate (N2 H4 ·H2 O) solution. The temperature was kept at 70 ◦ C. The solution was heated to 85 ◦ C for 1 h to reduce copper ion in the solution to metallic copper.
2.2.
Preparation of flake copper powders
Above-mentioned spherical copper powders, ZrO2 ball milling media and dispersing agent, i.e. sodium dodecyl sulfate, were mixed in a container fixed on a planet-type grinding mill rotating at a rate of 250 rpm for 8–12 h, so that the precursor particles were processed to flake copper particles. The flake copper powders were recovered from the solution, washed and dried under vacuum.
2.3.
(1)
3.
Results and discussion
3.1.
Synthesis of spherical and flake copper powders
In this work, spherical copper particles were obtained by reaction of [Cu(NH3 )4 ]2+ complex with hydrazine hydrate in ammonia chloride solution. In this process, the following chemical reaction occurred: Cu2+ + 4NH3 → [Cu(NH3 )4 2+
(2)
2[Cu(NH3 )4 ]2+ + N2 H4 → 2Cu + N2 + 4NH3 + 4NH4 +
(3)
Fabrication of thick films for BME-MLCC
The thick film pastes were formulated having the composition of as-prepared copper powders as the functional material, ZnO–SiO2 –B2 O3 glass frit as the permanent binder, a mixture of ethyl cellulose and solvent, i.e. ethylene glycol acetate, as a vehicle. The weight ratio of the composition was kept at 65:10:25. The pastes were rolled several times by a threeroll mill and coated on the termination of MLCC chip, whose specification was 0805Y5V, and dried, then, copper end terminations were fired at the peak temperature of 910 ◦ C for 10 min in a nitrogen atmosphere, with a controlled level of oxygen.
2.4.
RS L
Characterization
The behaviors of powder particles and microstructure of copper thick films were directly observed with the scanning electron microscopy on a XL30␦DX-4i (Philips). The crystal structure was characterized by X-ray diffraction (Philips). Thermogravimetry (TG), differential thermogravimetry (DTG) and differential thermal analysis (DTA) were carried out at scanning rates of 10 ◦ C/min in a flowing air atmosphere (Q600, SDT, TA Inc., USA). The purity of powder was determined with inductively coupled plasma spectrometer (ICPS) (PE Optima 3000). The specific areas of powder particles were measured
In aqueous ammonia media, ammonia reacts with Cu2+ according to Eq. (2) and [Cu(NH3 )4 ]2+ complex was reduced to obtain fine copper powder as seen in Eq. (3). It was obvious that Eq. (3) occurred in basic solution. If pH value was low, reaction could not occur smoothly. Ammonia chloride/aqueous ammonia buffer was employed to keep constant pH values at 8–8.5, in where the dispersability of powder particles increased. It was very important to choose appropriate dispersion agent for preparing fine metallic particles with chemicalreduction method. Dispersion agent could effectively control particle size and dispersability according to electrostatic repulsion effect, space hindrance effect and interfacial tension effect. We could distinctly find out that the micron-sized monodispersed non-agglomerated single spherical copper particles have been synthesized when sodium tartrate was employed as an inorganic dispersing agent. In an optimal condition, as-synthesized particles have an excellent dispersability and uniform single particle size of 3.5 ± 0.5 m by SEM analysis as given in Fig. 1a. By ball milling precursor spherical copper powders, the thin flake copper particles with a uniform size of 9 ± 2 m were formed as shown in Fig. 1b. The powder particles have a uniform size, less specific area and suitable ratio of diameter–thickness. By size distribution analysis, one could see that the powder particles have an excellent dispersabil-
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1129–1133
1131
Fig. 3 – Thermal analysis of flake copper particles.
Fig. 1 – SEM photographs of spherical (a) and flake (b) copper particles.
ity and normal size distribution. With the help of dispersing agent, the soft agglomeration between particles was destroyed by strong mechanical power from ball milling, resulting in appearing flake monodispersed particles. The ICPS analysis shows that purity of flake copper particle is more than 99.95%.
3.2.
XRD of flake copper particles
Fig. 2 showed the XRD pattern of the flake copper particles. From the pattern, it was obvious that the diffractogram exhib-
Fig. 2 – XRD spectrum of flake copper powders.
Fig. 4 – Influence of specific area on oxidizing temperature.
ited the characteristic peaks of crystalline metallic copper ˚ = 2.089, 1.809, 1.279, corresponding to (1 1 1), (2 0 0), (fcc). d (A) (2 2 0) plane, respectively, which is very close to that given by JCPDS file no. 4-836. The intensity of peaks was higher than those of its precursor spherical particles.
3.3.
Thermal analysis of flake copper powders
The thermal behaviors of metallic flake copper prepared by the above-method were analyzed by TG/DTG/DTA in an air flow. Fig. 3 showed the TG/DTG/DTA curves with a heating rate of 10 ◦ C/min. According to TG curve, the increase in weight began at 250 ◦ C, which was content with the results by DTG/DTA analysis. There was a distinct exothermal peak attributed to formation of Cu2 O at 262 ◦ C in DTA curve. The oxidation of copper became very rapid at 383.4 ◦ C corresponding to 15% weight gain by TG curve, which leveled off above 550 ◦ C, at which the CuO formed. Influence of specific area of flake copper powders on oxidizing temperature was investigated as given in Fig. 4. The oxidizing temperatures decreased proportionally as an increasing specific area. For the oxidation of copper at firing in air, the chemical adsorption of oxygen atoms on the copper particles was one of the critical factors because the physical adsorption would be easily desorbed at a so high temperature
1132
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1129–1133
Fig. 5 – Polarized light photo of cross-section of MLCC. Fig. 6 – SEM photograph of copper thick films. (more than 200 ◦ C). According to the monolayer adsorption theory (Langmuir equation), the adsorptive capacity of oxygen atoms was the function of specific area of particles, so the amount of oxygen atoms adsorbed on the surface of copper particles decreased as a decreasing specific area, resulting to the higher oxidizing temperature for forming Cu2 O. However, for copper particles, the lower specific area meant the lower flaky degree, which was unfavorable for particle’s homogeneous dispersion in organic vehicle and deteriorated the stability of pastes, despite its higher oxidation-resistance power. The suitable specific area of flake copper particles would be chosen based on the overall consideration.
3.4.
Copper end termination of BME-MLCCs
The flake copper particles could effectively manage the rheologic properties of thick film paste due to their platy shape. The coordination of flake copper particles with spherical copper particles effectively was a very important question for making conductive termination pastes. The cross-section of MLCCs packaged with pastes comprising as-synthesized flake copper particles was discussed by polarized light microscope, and the photo was given in Fig. 5, in which the connection between internal and terminal electrode, distribution of glass and interfacial layer were exhibited. As seen from this photograph, the distribution of glass was uniform in copper thick films. The connection of internal–terminal electrode was excellent. The gray transitional region occurred in the interface between electrode and chip due to the interfacial reaction. The proper attachment of the glass to the dielectric bodies, whose basic material is BaTiO3 , produced high adhesion strength. No separation of electrode appeared when the applied force was 1.8 kgf mm−2 . The average adhesion strength of electrode was 2.4 kgf mm−2 . The SEM photograph of copper thick films was given in Fig. 6. The densification of the fired copper thick film was high. It could be attributed to the so-called “framework” effects. In the printed thick films, the larger flake copper particles could be considered to be the “bricks”, and formed a similarly stable “framework”. On the other hand, the smaller spherical copper particles could be jammed into the “gaps” between “frameworks”. The compact structures in thick films should be formed, resulting in a high densification of the fired films.
The high densification could effectively avoid the leakage of electroplating solution, which may produce the failure of BMEMLCCs. The degree of densification was also important in determining the electrical resistivity of the fired thick film. Norton (1991) reported that the increased film density reduced the electrical resistivity as the degree of contact between particles was increased. Bulk resistivity of as-prepared thick films was 5.5 × 10−5 cm. 95% of terminal electrode covered by new solder showed that the thick films had excellent solderability behavior. Resistance behavior to soldering was very good. The results showed that no micro-crack appeared in MLCC at such a high impact temperature of 260 ◦ C, and 80% of terminal electrode was covered by new solder.
4.
Conclusions
In conclusion, spherical copper particles with a size of 3.5 ± 0.5 m were used as precursor to prepare flake copper powders by ball milling process. Flake copper particles with a uniform size of 9 ± 2 m were formed. The oxidizing temperature of flake copper powders increased as a decreasing specific area. The fired-on type films prepared by conductive pastes comprising as-synthesized flake particles have a high densification because of “framework” effects. The copper end termination of BME-MLCCs has an excellent electrical and mechanical behavior.
references
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