Preparation of B2O3-ZnO-SiO2 Glass and Sintering Densification of Copper Terminal Electrode Applied in Multilayer Ceramic Capacitors

YUE Yi ,LI Hong＊ ,CAO Xiuhua ,ZHANG Xuehui ,HUANG Jun ,HUANG Xuye ,ZHANG Yongqiang,XU Ruipeng,XIONG Dehua

(1. State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China; 2. Guangdong Fenghua Advanced Technology Holding Co.Ltd., Zhaoqing 526000, China)

Abstract: B2O3-ZnO-SiO2(BZS) glass containing CuO with excellent acid resistance,wetting properties,and high-temperature sintering density was prepared by high temperature melting method and then applied in copper terminal electrode for multilayer ceramic capacitors (MLCC) applications.The structure and property characterization of B2O3-ZnO-SiO2 glass,including X-ray diffraction,FTIR,scanning electron microscopy,high-temperature microscopy,and differential scanning calorimetry,indicated that the addition of CuO improved the glass′s acid resistance and glass-forming ability.The wettability and acid resistance of this glass were found to be excellent when CuO content was 1.50 wt%.Compared to BZS glass,the CuO-added glass exhibited excellent wettability to copper powder and corrosion resistance to the plating solution.The sintered copper electrode films prepared using the glass with CuO addition had better densification and lower sintering temperature of 750 ℃.Further analysis of the sintering mechanism reveals that the flowability and wettability of the glass significantly impact the sintering densification of the copper terminal electrodes.

Key words: B2O3-ZnO-SiO2(BZS);low melting glass;MLCC;densification;copper terminal electrode

1 Introduction

With the increasing demand for multi-layer ceramic capacitors (MLCC) in the market,electronic information products are requiring high frequency,low power consumption,miniaturization,superior energy storage and low cost[1-3].MLCC is composed of ceramic dielectric,internal electrode and terminal electrode[4-7].The low temperature sintered copper electronic paste for terminal electrode is mainly composed of metal copper powder,inorganic glass powder and organic carrier[8,9].Many studies have investigated the effects of sintering atmosphere,temperature,rate,and time on performance of MLCC[10-15].Glass powder,as the binder phase,plays a decisive role.The preparation of MLCC terminal electrode has high requirements for the adhesion and compactness of the glass phase.A suitable glass system is needed to encapsulate the copper powder and act as a spreader so that the copper terminal electrode of the MLCC becomes a copper-rich top layer (for electroplating) and a glass-rich bottom layer (for porcelain bonding).

Nowadays,glass powders used for preparing electrode paste generally have several disadvantages,such as a high glass transition temperature (Tg,this refers to the temperature at which the physical properties of glass in a glassy state undergo a reversible change as the temperature is raised or lowered.BelowTg,the material is in a solid glassy state,while aboveTg,it exhibits rubbery elasticity of glass),poor wettability at high temperatures,and low corrosion resistance to electroplating solutions[13,16,17].These drawbacks result in copper powder not being compact and becoming susceptible to oxidation during the sintering process of the terminal electrode,as well as causing diffusion of the coating to the inner electrode during the electroplating process.These phenomena greatly limit the performance of MLCC.

Zhang[18]compared several lead-free glass compositions and found that SiO2-B2O3-ZnO-BaO glass,which melts at 900 ℃,exhibits good wettability with the substrate.The use of this glass to coat copper powder results in a sintered copper film that has a smooth surface and is closely adhered to the substrate.Nobuo Nishioka[6]added TiO2to Si-B-Na-Ba glass to improve the corrosion resistance of electroplating solution,and V2O5to enhance the melting fluidity of the glass and the wettability of the copper substrate.However,the sintered film of copper electrode paste using this new glass exhibits low densification compared to normal glass.A V Dmitrieva[5]used glass containing heavy metal elements such as palladium and bismuth to encapsulate the electrode,but found that oxidation and reaction between palladium and bismuth led to disconnection between the inner and terminal electrodes during sintering.The use of heavy metals raises environmental concerns and should be carefully considered.Ren[19]compared the application of four different low temperature lead-free glass powders in MLCC copper electrode paste and found that B-Zn-Na-Cu glass had the best overall performance,but small cracks appeared at the terminal electrode after high-temperature welding.Here,an attempt is made to develop a new low melting glass with an optimum composition for electronic device applications and analyze its mechanism of action in the sintering process of terminal electrodes.Traditionally,low-melting-point glasses used for MLCC copper terminal electrodes include barium borosilicate glass and zinc borosilicate glass,which have relatively good wettability with copper[20-23],but have weak acid resistance and require a high temperature to form a sintered dense film.In order to improve the acid resistance and wettability,a component was designed to reduce theTgpoint of the B2O3-ZnO-SiO2(BZS) glass,by adding other oxide powder and a small amount of CuO to improve the acid resistance and wettability,and reduce the sintering temperature[24].The transition temperature and thermal expansion coefficient of the BZS glass were measured,and wettability of the BZS glass to evaluate its physical properties.

2 Experimental

2.1 Preparation of BZS glass samples

New glass compositions were designed based on the work of previous researchers[25].Table 1 lists the glass compositions.The specified compositions of B2O3(99.9% pure),ZnO (99.5% pure),SiO2(99.0% pure),and other raw material powders were weighed and mixed thoroughly.The glass samples were produced by placing the mixed powder in an aluminum crucible,melting and retaining at 1 000 ℃ for 30 min under an air atmosphere,then pouring onto an iron plate.After grinding the glass into powder,the glass structure was analyzed using Fourier transform infrared spectroscopy(Nicolet 6700) and X-ray diffractometry (Empyrean).

Table 1 Chemical compositions of BZS glass/wt%

2.2 Experimental procedure of BZS glass samples

The glass transition temperature (Tg),a key thermodynamic property of glass,was determined using a differential scanning calorimeter (STA449F3).For the analysis,the bulk glass was pulverized into a fine powder using an agate mortar.50 mg of the powder and an alpha-alumina reference sample were placed in separate platinum dishes,and measured after being left to stand on a specified heat sink.The measuring conditions were controlled as follows: temperature elevation rate of 5 ℃/min,measuring temperature range RT-1 000 ℃,and an air atmosphere.

In order to simulate the changes of the fluidity of the glass powder during the sintering process,the glass powder was pressed into a cylinder with a height of 1 cm and a diameter of 0.5 cm through a mold,and then placed it on top of the ceramic substrate for sintering,with the temperature elevation rate 5 ℃/min,sintering time 30 min and sintering temperature 750 ℃.The fluidity of the glass liquid was analyzed by observing the wetting angle formed on the top of the ceramic substrate after melting.

The glass blocks were cut into 25 mm × 5 mm ×5 mm glass strips using an internal circle cutter for the thermal expansion coefficient test.

2.3 Preparation process of copper terminal paste

Table 2 lists the components of the copper terminal electrode paste.Glass samples pulverized using a ball grinder into particles with a diameter of 2 μm were used as the glass frit.By dosing,mixing,and kneading each sample with three roll mills,copper terminal pastes were produced with evenly distributed components.The terminal of the MLCC chip was dipped into the prepared copper terminal paste and the sample was subsequently held at 150 ℃ for 10 minutes in order to form a thin layer after firing.Next,using an inert atmospheric sintering furnace to fire the dry layer to obtain a fired copper electrode.The temperature profile is shown in Fig.1.The surface and crosssections of the fired samples were observed by using a scanning electron microscope (TM4000,Hitachi) to evaluate the film structure and density.

Fig.1 The heat-treat schedules for copper terminal paste

Table 2 Components of copper terminal eletrode paste/wt%

3 Results and discussion

3.1 Characterization and analysis of BZS glass

Fig.2 shows the XRD pattern and the infrared scanning pattern of seven groups of glass samples.As seen from the XRD pattern,there are no sharp peaks in the seven groups of glass,indicating good glassforming performance and no obvious crystallization.From the infrared spectrum,it is clear that the FTIR curve shapes of the matrix glasses with different CuO contents are basically the same,showing five absorption bands located near the wavenumbers of 550,688,911,1 087,and 1 392 cm-1,respectively.The vibration of the [AlO6]cluster is near 550 cm-1,and the 685 to 688 cm-1wavenumber corresponds to the B-O-B bending vibration in [BO3].The wavenumber of 970 to 991 cm-1corresponds to the asymmetric vibration of Si-O-Si in [SiO4].The asymmetric stretching vibration of Si-O-Si in [SiO4]corresponds to the wavenumber around 1 087 cm-1.The vibration of B-O in [BO3]corresponds to the wavenumber of 1 374 to 1 392 cm-1[26-28].As seen from Fig.2,CuO does not appear in the glass structure as a glass network former,and the increase or decrease of CuO content will not affect the glass structure.

Fig.2 XRD (a),FTIR spectra (b),and DSC curves (c) of glass

Fig.2(c) shows the DSC curves of three groups of glass,and the thermal data of glass samples are shown in Table 3.

Table 3 Thermal data of glass samples

TheTgof the seven groups of glass is about 520℃,and the glass melting temperature (Tf,this refers to the temperature at which a material undergoes a transition from the solid glassy state to the liquid state within the range of the glass transition temperature.Above theTfpoint,the material behaves as a liquid.TheTfpoint is typically higher than theTgpoint,indicating that the material requires a higher temperature to fully convert to a liquid state) is about 535 ℃.In the later sintering experiment,the required temperature is between 650 and 850 ℃,so the seven groups of glass can be completely melted.The thermal expansion coefficient of the seven groups of glass is not much different.

3.2 Wettability and acid resistance of BZS glass

The glass powder was pressed into a column and sintered at 750 ℃.The cross-sectional morphology of the glass powder after sintering and the hightemperature microscope characterization of the glass powder are shown in Fig.3.

Fig.3 The side view morphology of the glass powder after sintering and the high-temperature microscope characterization of the glass powder.(α：wetting angle)

Fig.3 clearly shows that the addition of CuO changes the color of the sintered glass from black to light blue,and the area of the glass liquid spreading decreases with increasing CuO content (as observed in the morphologies of Cu#1 and Cu#2).However,as the CuO content continues to increase,the blue color of the glass gradually deepens,and the area of the glass liquid spreading gradually becomes larger.This phenomenon is also evident in the side view of the sample.Initially,the addition of CuO reduces the fluidity of the sintered glass,resulting in a sudden increase in the wetting angle from 45.8° to 69.7°.However,as the CuO content increases,the fluidity of the glass gradually increases,and the wetting angle decreases from 69.7° for Cu#2 to 50.3° for Cu#7.This indicates that the addition of CuO reduces the fluidity of the borate glass,but as the CuO content increases,the fluidity of the glass liquid eventually increases.

Fig.4 shows the SEM image of the glass after sintering and etching.As seen in Fig.5,Cu#1 exhibits crack defects and poor acid resistance after acid etching.On the other hand,the morphology of glasses containing copper oxide (Cu#2 to Cu#7) improves with increasing copper oxide content.Starting from Cu#2,the glass morphology no longer shows cracks like Cu#1,and Cu#2,Cu#3,and Cu#4 exhibit excellent acid resistance after acid etching.However,starting from Cu#5,noticeable gullies appear in the morphology,and the size and number of gullies increase with increasing copper oxide content.This is due to excessive copper oxide content leading to easier crystallization of the glass,and the resulting crystals being corroded after acid etching,leaving gullies.

Fig.4 SEM images of glass after sintering and etching

Fig.5 SEM images of each sintered sample at different sintering temperatures

In summary,the appropriate addition of CuO can not only effectively improve the wettability of glass,but also adjust the acid resistance of glass;however,too much CuO content will lead to glass crystallization,which will leave gullies after corrosion by electroplating solution.

3.3 Sintering densification of copper terminal paste

Undoubtedly,the appropriate temperature is essential for promoting sintering between copper powder particles.However,a temperature that is too low can result in incomplete sintering,while a temperature that is too high can lead to over burning and oxidation[11,29,30].In addition,a suitable glass melt with good wettability can provide an environment that facilitates wetting and mass transfer during copper powder sintering.However,a high viscosity of the glass melt can impede copper powder sintering mass transfer,while a low viscosity may not be able to fill the pores between the copper powder particles,resulting in the formation of holes that can affect sample densification.The literature suggests that the addition of CuO can reduce the viscosity and melting activation energy of borate glass[11].It is hypothesized that the degree of densification of the copper terminal electrode is determined by a balance between the temperature and viscosity of the glass melt.When the temperature and viscosity reach equilibrium,the degree of densification can be maximized.Glass powder,copper powder and organic carrier were mixed in a certain proportion.The copper terminal paste was evenly printed on the ceramic substrate by screen printing,and sintered at different temperatures.The surface morphology during sintering was observed by SEM.Fig.6 is the scanning morphology of each sintered sample at different sintering temperatures.

Fig.6 SEM images of copper electrode (Cu#4)

It is apparent from Fig.5 that at 650 ℃,although the copper powder particles of Cu#1 start to come into contact,they remain separate and do not blend with each other.While Cu#3 also mainly consists of independent copper powder particles,and some parts have even begun to fuse.Cu#5 and Cu#7 exhibit many obvious gullies,but it is noteworthy that the part of Cu#7’s copper powder in contact with each other are significantly closer,and even copper phase particles with a size significantly larger than 2 microns appear in some places,as shown by the circle in the figure.This demonstrates that the contact between copper powders is close,and mass transfer occurs.Two or more small copper powder particles are sintered and fused into large copper phase particles.This illustrates that at 650 ℃,with the increase in CuO content,it is easier to initiate mass transfer between copper powder particles.

At 700 ℃,Cu#1 still predominantly consists of individual copper powder particles,whereas the copper powder particles in Cu#3 have begun to closely contact and produce sintered necks.Although the morphology is not dense,it shows a tendency to densify.Cu#5 has formed a dense morphology at this temperature,and the copper powder particles have started to fuse with each other.At 700 ℃,it performs the best compared to other samples.The fusion between Cu#7 copper powders is the most complete,indicating that with the increase of CuO content,mass transfer between copper powders is easier to initiate at 700 ℃.However,numerous holes and gullies in the sample cannot be ignored.Due to the low viscosity of the glass melt,it flows to the bottom of the sample,failing to provide a wet sintering environment for the copper powder,leading to the formation of many holes.Although increasing CuO content reduce the melting activation energy of glass powder and make the copper powder easier to sinter than the sample with lower CuO content,it cannot compensate for the defects caused by the low viscosity of glass melt.

When the temperature reaches 750 ℃,the advantage of high viscosity glass melt is evident.The samples Cu#1 and Cu#3 exhibit a very high degree of densification due to the suitable temperature and moderate viscosity.Each copper powder particle is in close contact with each other,and there are almost no gaps.Additionally,the proportion of largesized copper phase particles significantly increases,indicating complete sintering of the copper powder and the appearance of a highly densified morphology.Although high temperature can promote the sintering between copper powder particles,resulting in a large number of dense sintering for Cu#5 and Cu#7 copper powder particles,the further reduction of the viscosity of the glass melt cannot be ignored.As the temperature increases,the viscosity of the glass melt decreases further,making it easier to flow and enrich at the bottom of the sample,thus slowing down or even preventing the sintering process,which is visible from the gullies in Fig.6.As the temperature rises to 800 ℃,the benefits of high temperature become evident.High temperature promotes the sintering of copper powder.Due to the low content of CuO,the glass melt of Cu#1 and Cu#3 maintains a suitable viscosity,allowing them to maintain a high degree of densification at this temperature.With the high temperature providing ample energy for the sintering of copper powder,many copper powder particles are sintered completely before the glass melt is lost.As a result,even Cu#5 and Cu#7,with lower viscosity glass melts,exhibit a higher degree of densification.However,Cu#7 still has obvious holes and gullies,indicating that the loss of glass melt is still present.When the temperature was raised to 850 ℃,the promoting effect of high temperature on the sintering of copper powder had reached its saturation point.Further heating would not provide additional energy for the sintering process,and the size of copper particles had also reached a maximum of nearly 5 microns.The disadvantage of low viscosity of glass melt was again evident.Cu#1 and Cu#3 were able to maintain an extremely high degree of densification due to their suitable viscosity.Although most of the morphology of Cu#5 was dense,there were still holes,and the morphology of Cu#7 was more obvious.Between the copper powder particles,copper phase strips with a high degree of fusion were formed,similar to “small bones,” but there were gaps between the copper phase strips that lacked both a conductive phase and a glass phase bond.The high temperature caused the glass phase to be completely lost.Cu#4 glass exhibits excellent wettability and acid resistance.Furthermore,the electrode paste composed of Cu#3 glass showed a high degree of densification at 750 ℃.Based on these findings,it is concluded that Cu#4 is the optimal glass component.

Fig.6 shows the SEM image of the copper terminal electrode prepared using Cu#4 glass powder sintered at 750 ℃.The cross-sectional images of the samples in Fig.6 demonstrate that the MLCC fabricated using Cu#4 glass achieves a high degree of densification,with the electrode and the ceramic sintered closely without any glass phase enrichment at the interface between the ceramic and the terminal electrode.This indicates that the copper phase will remains connected to the inner electrode.High-power observations of the surface morphology in Figs.6 reveal that the copper powder and glass powder sinter tightly,with the glass melt filling the gaps between the copper powder particles well,thereby promoting the formation of a dense structure on the surface of the terminal electrode without generating any crystallization or voids.The full view in Fig.6(f) also illustrates that the sintered terminal electrode of the sample demonstrates good densification.

To investigate whether new glass groups were generated after sintering the copper terminal electrode,infrared spectroscopy tests were conducted.As shown in Fig.7,the infrared spectrum of copper powder exhibited a characteristic peak at 620 cm-1,corresponding to the bending vibration of the Cu-O bond.The glass powder showed characteristic peaks near 688,983,and 1 378 cm-1,corresponding to the bending vibration of B-O-B in [BO3],the asymmetric vibration of Si-O-Si in [SiO4],and the vibration of B-O in [BO3],respectively.The infrared spectrum of the sintered copper terminal electrode revealed characteristic peaks near 620,671,971,and 1 340 cm-1.It is evident that the characteristic peaks of the copper terminal electrode are the simple superposition of those of the copper and glass powders,with no new peaks appearing.It was concluded that sintering the copper terminal electrode does not generate new glass groups.

Fig.7 FTIR spectra of copper powder,glass powder and copper terminal electrode

3.4 Sintering densification mechanism of copper terminal paste

Fig.8 illustrates the sintering densification mechanism of the copper terminal electrode.The schematic diagram before sintering shows the paste printed on the ceramic substrate,where the blue spheres represent the glass powder and the red spheres represent the copper powder.The glass powder and copper powder are uniformly distributed in the paste block.

Fig.8 Copper terminal electrode sintering densification mechanism diagram

In the early stage of sintering,the particles in the copper terminal paste underwent rearrangement,generating bonding at the points of contact and gradually eliminating large pores.However,the total surface area of the solid-gas changed little during this process.In addition,the glass powder in the copper terminal paste began to soften and exhibited a tendency to envelop the copper powder[31,32].

In the middle stage of sintering,mass transfer occurred between the copper powder particles,resulting in an increase in grain boundary and further narrowing of the pores.However,the pores were still interconnected like tunnels.Meanwhile,the glass powder was completely melted and started to fill the pores between the copper powder particles,enhancing the wettability of the copper powder and creating an optimal sintering environment for the copper powder particles.

In the final stage of sintering,the glass powder was fully melted and evenly coated the copper powder particles,creating an optimal sintering environment.Copper powder particles continued to grow,resulting in the highest achievable strength and theoretical density of the Cu terminal electrode[33-35].Thanks to its excellent fluidity and wettability,the melted glass filled the gaps between the copper powder particles,further improving the densification of the Cu terminal electrode.Moreover,it flowed down to the bottom of the terminal electrode to form a glass layer that bonded the Cu terminal electrode and the ceramic,thereby enhancing the adhesion between them.The sintering condition at this stage is considered the most ideal state.

4 Conclusions

a) The addition of CuO to boron-zinc glass can significantly enhance the acid resistance of the glass after sintering,and can also improve the sintering density of the glass as a bonding phase.

b) The most suitable bonding phase composition is found to be BZS glass with 1.5 wt% CuO.The wetting angle between this glass powder and the ceramic substrate after sintering at 750 ℃ is less than 60°,and no cracks are observed in the sintered glass powder after acid etching.The resulting electrode exhibited a highly dense morphology.

c) The densification mechanism can be divided into three stages: first,the glass melt wraps around the copper powder,followed by the copper powder being wrapped in the glass;then,the molten glass fills the gaps between the copper particles,providing ideal sintering conditions for the copper powder.Finally,the copper powder is fully sintered,resulting in a desirable sintering morphology.

Conflict of interest

All authors declare that there are no competing interests.