Effect of surface finish metallization on interfacial bonding behavior of sintered Ag joints

Yang Fan ,Wu Weizhen ,Du Jianjun ,Li Mingyu

1.School of Mechanical Engineering and Automation,Harbin Institute of Technology Shenzhen,Shenzhen 518055,China;2.Sauvage Laboratory for Smart Materials,School of Materials Science and Engineering,Harbin Institute of Technology Shenzhen,Shenzhen 518055,China

Abstract In this work,the sandwich joints were joined by low temperature pressureless sintering Ag paste.The morphology and thermal behavior of Ag nanoparticle paste was characterized and analyzed.The sintered Ag joints with different metallization were prepared and tested.The joints with Ag metallization exhibited superior shear strength and interface bonding ratio.However,the joints with Cu metallization showed lowest shear strength and interface delamination.The interfacial microstructures were observed and the diffusion kinetics between Ag and Au atoms were both calculated.The excessive diffusion of Ag atoms towards the Au layer deteriorated the interface bonding ratio and shear strength.This work will help understand the bonding mechanism between sintered Ag and other metallization.

Key words Sintered Ag,interface,Au metallization,Cu oxidation

0 Introduction

SiC and GaN are emerging as potential candidates of Sibased power modules due to their superior characteristics such as high frequency,switching speed,carrier migration velocity and thermal resistance[1–2].The electronic and mechanical properties of Si chip degraded when the working temperature exceeds 150 while SiC and GaN could service stably under 300.Technical challenges of heat dissipation and thermal resistance were imposed on the interconnection materials.

Low temperature sintering Ag technology is the most promising solution for SiC and GaN die attachment.Micro/nano Ag particles melt and coalesced at much lower temperature than that of its bulk counterpart while the sintered Ag maintain the melting of 961.The sintered Ag also exhibited excellent thermal and electric conductivity,mechanical strength and reliability[3–4].Schwarzbauer and Kuhnert first reported that micro Ag paste could be utilized as interconnection materials[5].In the later decades,pressure-assisted or pressureless sintering Ag were studied comprehensively in respect of their sintering processing methods and reliability[6].In spite of great achievements of applying sintered Ag in third semiconductor packaging,the interfacial bonding behaviors between surface finish metallization and sintered Ag were still barely reported.In this work,different surface finish metallizations (Au,Ag,Cu)were prepared and bonded with nano-Ag particles.The mechanical strength,microstructural morphology and diffusion kinetics of heterogeneous elements were observed and analyzed.

1 Experimental procedures

We prepared a nano-sized Ag paste as the bonding material.The Ag loading was approximately 85%.Ethylene glycol was used as solvents.The weight ratio of the Ag particles to the solvent was fixed to be 85:15.They were blended by a paste mixer (RM300SA,SINOMIX Science &Technology Co.,Ltd.) and the paste was obtained.

In the sintering experiments,the Cu substrates with a dimension of 3 mm × 3 mm were coated with electroplated Ni(1 μm) and Ag (1 μm) layers and selected as the upper pads in Ag and Au metallization experiments.The Cu substrates with a dimension of 5 mm × 5 mm were coated with electroplated Ni (1 μm) and Ag (1 μm) layers and electroplated Ni (5 μm) and Au (2 μm) layers,which were selected as the bottom pads in Ag and Au metallization experiments,respectively.The bare Cu substrates with a dimension of 3 mm × 3 mm and a dimension of 5 mm × 5 mm were selected as upper and bottom pads in Cu metallization experiments.Ag paste was applied on the bottom pads by screen printing to ensure uniform printing thickness.A 60 μmthick SUS 304 stainless steel mask was used for printing the Ag paste;the metal mask opening measured 4 mm × 4 mm.After printing,the upper pads were mounted on the paste and the sandwich-like joints were placed in the oven.The heating rate,sintering temperature and dwelling time were set as 10 ℃/min,250 ℃ and 1 h.All the experiments were performed in the ambient atmosphere.After sintering,the samples were cooled to room temperature.

The thermal property of the Ag paste was investigated by the thermogravimetric analysis (TGA,TA Instruments)with a heating rate of 2 °C/min in air.Scanning electron microscope (SEM,S4800,Hitachi) was used to observe the cross-sectional microstructure and fracture surface of the sintered joints.The shear strengths of joints were tested by a shear tester (MFM 1 200,TRY Corp.).The shear height from the substrate was set to 100 μm and the shear speed was 200 μm/s.

2 Results and discussion

Fig.1 a shows the morphology of Ag nanoparticles(NPs).The spherical NPs exhibited stable property without any uncontrolled aggregation.The size distribution was in the range of 100?300 nm and the mean particle size was 189 nm.During the nucleation and grain growth of Ag NPs,the coating layer PVP would attach on the surface of NPs,which induced the steric hindrance effect and prevented the Ag NPs from agglomeration.

Fig.1 Microstructure chatacteristics of Ag NPs (a) The morphology of Ag NPs (b) The size distribution of Ag NPs

Fig.2 shows the thermal property of Ag NPs.The peaks were observed between 158 nm and 174 nm,which were identified as the combination of an exothermic peak and an endothermic peak.The former one was ascribed to the sintering process of Ag NPs and the later corresponded to the decomposition of organic coating layer.For metal,the exothermic peak was supposed to represent the nucleation,recrystallization and the atom diffusion.During the sintering process of Ag NPs,the reduction of surface energy and lattice distortion energy,recrystallization and the growth of sintering neck were all corresponded to the atom diffusion,which implied that the exothermic peaks corresponded to the sintering process.Besides,the decomposition of PVP occurred simultaneously and PVP was finally decomposed to CO2and NO2[7].

Fig.2 Differential scanning calorimetry result of Ag NPs

Shear tests were performed on the sintered joints with different metallization as shown in Fig.3.The Ag joints exhibited superior mechanical strength of 75.8 MPa in average while the Au and Cu joints only had mean shear strengths of 29.9 and 9.9 MPa,respectively.According to the traditional soldering,all these kinds of metallization could offer good wettability and bonding strength.The microstructural observation would figure out the problems mentioned above.

Fig.3 Shear strength of sintered joints on different metallization

Fig.4 shows the cross-sectional microstructure of Ag joints.The Ag plated pads were bonded by sintered Ag without cracks.The pores distributed uniformly in the sintered Ag matrix.Fig.4 b and c provided more details about the interfacial area.The upper and bottom zones exhibited similar microstructure.The Ag NPs sintered well with the Ag plating layer due to the homogeneous atom diffusion.

Fig.4 The cross-sectional image of Ag joints (a) Overview(b) Magnified view of upper interface (c) Magnified view of bottom interface

Au joints exhibited a different interfacial morphology from that of Ag joints in Fig.5 a.The Fig.5 b had a similar microstructure with that of Ag joints,but the Fig.5 c implied that the Ag-Au interface was much worse than Ag-Ag interface.The Ag NPs diffused on the Au surface while the necks were much smaller than those of Ag-Ag interface.Sintering was a process of atom diffusion and the interfacial bonding could be recognized as the interdiffusion of Au and Ag atoms.

Fig.5 The cross-sectional image of Au joints (a) Overview(b) Magnified view of upper interface (c) Magnified view of bottom interface

The diffusion kinetics of Au and Ag atoms could be calculated Arrhenius equation:

whereDis the diffusion coefficient,D0is the frequency factor,Qis activation energy,Ris molargas constant andTis temperature.Table 1 shows the diffusion coefficient of Au and Ag atoms in Au and Ag matrix at 250 ℃.During the sintering process,the Ag atoms diffused fast into the Au layer,even faster than the interdiffusion rate of Ag atoms.The diffusion rate of Au atoms towards Ag was slow.Hence,the excessive diffusion of Ag atoms towards the Au layer would cause that the neck became thinner and the debonding phenomenon even occurred.

Table 1 Diffusion coefficient calculation

Fig.6 shows the cross-sectional image of Cu joints.Cracks existed at the bonding interface.At the upper interface,Cu oxide formed between Cu substrate and sintered Ag.This would significantly degrade the mechanical strength of joints.Interfacial debonding was observed at the bottom interface.Based on the phenomenon above,the Cu-Ag interdiffusion lagged behind the Cu oxidation.With the further volume contraction of sintered Ag,the interface between Ag and Cu oxide was broken.This leads to the decrease of shear strength of Cu joints.

Fig.6 The cross-sectional image of Cu joints (a) Overview(b) Magnified view of upper interface (c) Magnified view of bottom interface

3 Conclusion

The effect of metallization on interfacial bonding behavior of sintered Ag joints were systematically observed and discussed.

(1) Ag joints exhibited the shear strength (75.8 MPa)and dense interfacial bonding.

(2) The Au joints had a 29.9 MPa and a relatively loose bonding interface due to the excessive diffusion of Ag atoms towards the Au layer.

(3) The Cu joints exhibited only 9.9 MPa,which was ascribed to the interfacial oxidation and debonding.

Acknowledgement

The authors acknowledge the financial support of the Shenzhen Science and Technology Plan Project under Grant No.JCYJ20180306172006392.