Study on the performance of silver paste sintered sealing joints for hermetic packaging

Lin Liting,Zhang Yongfei,Li Xin

School of Materials Science and Engineering,Tianjin University,Tianjin,300350,China

Abstract The traditional hermetic packaging methods,which require high temperatures and high voltages,would have adverse effects on electronics devices.Based on current-assisted sintering technology,we proposed a new hermetic packaging method using silver paste.Comparing the microstructure of the joints sintered with different currents,we found that with the increasing sintering current from 4.7 kA to 5.3 kA,the density of the joint increases,which improves the hermeticity and bonding strength of the joints.The results of thermal cycling tests showed that the sealing joints sintered at 4.7 kA with larger porosity are more prone to fail because pores tend to be sources of defects under thermal cycling.After 250 ℃ high thermal storage,there is the coarsening of the pores and the delamination near the Ag/Au interface,degrading the sealing performance of the sintered joint.Besides,the sealing joints sintered at 5.3 kA exhibit superior reliability when exposed to high temperatures.

Key words silver paste,electric-current-assisted sintering,hermetic package,reliability

0 Introduction

Electronic components,applied in aerospace,oil,and gas extraction,are usually operating at harsh environment.For example,moisture and ionic impurities would corrode circuit elements and interconnections inside the electronic components[1–3].Hermetic packaging plays an important role in providing a barrier against corrosion and preventing electronic equipment from deterioration of function and reliability[4].

Anode bonging was used as a traditional attaching technique in hermetic devices however the device needs to be exposed in extremely harsh processing environment[5].Previously,soldering was the most interesting choice for hermetic lid sealing[6–8].Among the solder alloys,the Au-Sn eutectic solder was one of widely used interconnection materials,owing to its favorable mechanical strength,superior corrosion resistance,and hermeticity[9–12].However,high process temperature (over 300 ℃) and low operating temperature (below 280 ℃) of Au-Sn eutectic solder limit its appli cation[3,13,14].

Recently,silver paste has been emerging as a novel interconnection material for electronicapplications because of its low process temperature (250 ℃) and high operating temperature (approximately 700 ℃) can avoid adverse impact on electronic components[15–16].To replace the traditional Au-Sn eutectic solder,the sintering technology of silver paste has attracted lots of attention in hermetic packaging[3,17].However,most of studies on hermetic package focused on traditional hot-press sintering,which is relatively complicated and time-consuming.

Rapid sintering of silver paste by current can achieve strong joints in seconds[18].Instead of the hot-press sintering method,electric-current-assisted sintering (ECAS) has advantages of shorter holding time,denser sintered joint,and markedly improving reliability.Lu et al.[19]studied the characterizations of silver joints by ECAS,and found that current magnitude and current-on time had a significant influence on the density and mechanical strength of sintered joints.Mei et al.[20]investigated the fundamental mechanism of ECAS of silver paste.They concluded that the densification of sintered joints was governed by sintering temperature,and the joints would be further densified when the sintering temperature exceeded 400 ℃.

In this study,we proposed a novel hermetic sealing method based on ECAS of silver paste.To study the effect of current on the quality of the hermetic packaging,we recorded sintering temperature profiles at different currents and analyzed the differences in the microstructure joints and leakage rate of the joints sintered at different currents.Through the results of the high temperature storage test and thermal cycling test,we have investigated the reliability of the joints in detail.

1 Experiments

In this paper,the lid and the cavity employed in hermetic packaging were both made of Kovar alloy plating with electroplated Ni/Au layer,as shown in Fig.1 a.The thickness of Ni and Au layer was 4 μm and 1.3 μm,respectively.X-series silver paste (from NBE Technologies LLC) was used as connection material.

Fig.1 Schematic of (a) metal structure (b) process of specimen preparation (c) process of ECAS and sealed specimen

The lid and cavity were ultrasonic cleaned in alcohol.According to the sealing area on the top surface of the cleaned cavity,hollow square silver paste with a thickness of 100 μm was stencil printed onto the cleaned lid.The asprinted silver paste was placed on a hot plate to pre-dry at 90 ℃ for 20 minutes to remove the solvent in air,so as to avoid the overflow of silver paste during sintering[20].After pre-drying,the cavity was picked and placed on a silver layer.The sample preparation is shown in Fig.1 b.The assembled specimen was rapidly sintered under the loading of 10 MPa by the setup shown in Fig.1 c.The sealed specimen is also shown in Fig.1 c.An infrared radiation (IR)camera was used to record the temperature profile of silver paste layer during the process of ECAS.

With the same current-on time of 2 500 ms,sintered silver joints were fabricated by ECAS at 4.7 kA,5.0 kA and 5.3 kA，respectively.The microstructure of sintered joints was characterized by the scanning electron microscope(SEM,Hitachi,S4800) to reveal the effects of current magnitude.The sealed specimens were detected for leakage rate according to the MIL-STD-883E standard,which was used to evaluate sealing performance.The reject limit of leakage rate for the specimen used in this paper is 5 ×10–3Pa·cm3·s–1,considering the volume of the cavity.To evaluate the bonding quality,the shear strength of the joint was measured by XYZTEC Condor150 tester at shearing speed of 4 × 10–4m/s.To further study the reliability of hermetic seals,the thermal cycling tests from –55℃ to 125 ℃,and the high temperature storage tests at 250 ℃ were carried out[21].In addition,we employed the C-SAM (C-mode Scanning Acoustic Microscope) to detect the deterioration of sealing joints.

2 Results and discussion

2.1 Evaluation of bonding quality

2.1.1 Microstructure characterization

Fig.2 shows the temperature profiles of sintered joints fabricated at different currents,i.e.,4.7 kA,5.0 kA,and 5.3 kA,with the same current-on time of 2 500 ms.It is obvious that different profiles have similar trends.The heating process can be divided into two stages: rapid rise stage and slow rise stage.With the increase of temperature,the removal of organics in silver paste leads to the decrease of resistance of silver paste[22–23].Consequently,Joule heat due to thermal resistance decreases rapidly,leading to slow down the heating rate.On the other hand,the peak temperature increases with the current magnitude increasing.As Joule's law of electric heating described,the higher current can generate more Joule heat.Thus,the current is 4.7 kA,5.0 kA and 5.3 kA,and the corresponding peak temperature can reach 370.0 ℃,403.5 ℃and 431.2 ℃,respectively.

Fig.2 Temperature profiles of sintered silver joints

The current which determines the sintering temperature is greatly crucial for the formation of joint.Fig.3 shows the microstructure images of the cross-section of joints sintered at different currents.It is obvious that the densification of sintered silver joint enhances with the current magnitude.The rising of temperature would accelerate silver atomic motion,promoting the formation of sintering necks amongst silver particles[24]and further realizing the densification of joint.

Fig.3 Microstructures of cross-section of joints sintered at different current (a) 4.7 kA (b) 5.0 kA (c) 5.3 kA

As shown in Fig.4,the current magnitude increases from 4.7 kA to 5.3 kA,the porosity decreases from 17.58%to 5.11%.It indicated that the use of higher current can effectively improve the density of the joint.In addition,it has been reported that the yield stress of sintered silver layer decreases with the increase of temperature[25].Mei et al[20]found that when the sintering temperature exceeds 400 ℃,the densification of sintered silver joints will enhance by eliminating some continuous and large pores owing to plastic deformation under the loading of the electrode.As shown in Fig.2,the peak temperature of the joint sintered at 5.0 kA can reach 403.5 ℃.Under such high temperature and loading of 10 MPa,the sintered silver joint will be further densified through plastic deformation.Compared with the microstructure of the joint sintered at 4.7 kA,there are only dispersed and small pores in the joints sintered at 5.0 kA and 5.3 kA.The increases of density can improve the bonding strength and sealing performance of sintered joints.

Fig.4 Porosity of joints sintered at different currents

2.1.2 Bonding strength and hermeticity performance

Fig.5 shows the shear strength of joints sintered at different currents.It was found that the joints sintered at 4.7 kA exhibits the lowest value of 28.7 MPa.The shear strength significantly increases,when the joint sintered at a high current.The shear strength is 46.5 MPa and 52.1 MPa,corresponding to joints sintered at 5.0 kA and 5.3 kA,respectively.

Fig.5 The shear strength of joints sintered at different currents

To quantitative hermeticity characterization,we carried out the helium leakage rate testing.The leakage rate of specimens fabricated at different currents is listed in Table 1,and 6 specimens were tested for each process.The leakage rate of sealed specimens sintered at 4.7 kA exceeds the reject limit (5 × 10?3Pa·cm3·s?1),which cannot meet the requirement of hermeticity.On the contrary,the leakage rate of specimens fabricated at 5.0 kA and 5.3 kA is far below the reject limit,which shows the excellent sealing performance.

Table 1 1The leakage rate of specimens fabricated at different currents

The results of shear tests and leakage rate tests also demonstrated the microstructure of sintered joints plays a vital role in bonding and hermetic performance.

2.2 The result of thermal cycling tests

In order to evaluate the thermomechanical reliability of these sealing joints,thermal cycling tests have been performed in this paper.As shown in Fig.6,the temperature range is from ?55 ℃ to 125 ℃ with a 15minutes dwell time at each extreme temperature.Three specimens of each process were prepared to test.The sealed specimens were tested for leakage rate every 100 cycles until the 400th cycle.

Fig.6 Temperature cycling profile

Fig.7 a shows the effect of the number of the temperature cycles on leakage rate of sealed specimens fabricated at different currents,i.e.,4.7 kA,5.0 kA,and 5.3 kA,with the same current-on time of 2 500 ms.Fig.7 b shows SAM images of sintered silver joints under different thermal cycles,which were used to detect the deterioration of sealing joints during thermal cycles.Because differences in the acoustic resistance of the substance lead to differences in gray scale in the SAM image,the uniform and consistent color indicates that there are no obvious defects such as delamination in silver paste layer.Obviously,the leakage rate under different currents increases with the number of thermal cycles increasing but there are differences in the trend of change and the degree of increase.As shown in Fig.7 b,in the initial stage,all sealing rings sintered at different currents have no detectable defects.After 200 cycles,there are some defects in the upper right corner of the sealing ring sintered at 4.7 kA,which results in a small increment of leakage rate,as shown in Fig.7 a.After 300 cycles,half of the original connection delamination area in the joint sintered at 4.7 kA appears to delamination.Correspondingly,the leakage rate increases abruptly,which indicates that the hermeticity of the sealing joint sintered at 4.7 kA has deteriorated completely after 300 cycles.In the contrast,until 300 cycles,there arestill no detectable defects in the sealing ring sintered at 5.0 and 5.3 kA.Thus,the leakage rate is almost constant.Until 400 cycles,some obvious defects have appeared in the sealing rings sintered at high currents.

Fig.7 The results of thermal cycling tests (a) Leakage rate(b) C-SAM images of joints sintered at different currents

2.3 The result of high temperature storage tests

According to previous studies,there may be the grain growth,coarsening pores and the increase of porosity in sintered silver joints during serving at high temperature environment,which are responsible for the degradation of bonding performance[26].In order to evaluate the influence of thermal exposure for hermeticity,the sealed specimens have aged at 250 ℃.Fig.8 shows that the leakage rate of sealed specimens fabricated at different currents.It’s interesting to note that the sealing joint sintered at 4.7 kA increases from 5.5 × 10?3Pa·cm3·s?1to 1.1 × 10?1Pa·cm3·s?1after aging at 250 ℃ for 300 h.Conversely,the leakage rates of the joints sintered at higher current have little change during aging.

Fig.8 Leakage rate at different aging times

The microstructures of sintered joints after thermal exposure have been analyzed to reveal the influence of aging time.Fig.9 shows the microstructures of joints sintered at different currents after aged for 300 h.Compared with Fig.3,the sintered porous sliver has coarsened after aging,which can be seen as a further sintering process.Additionally,the pore size also increased.When the sintered silver joints were exposed to high temperature,grains grew to reduce the interface energy of grain boundaries[27],which caused the coarsening of the sintered porous silver.At the same time,pores also migrated along with the coarsening of the sintered silver.Consequently,the pore size increased after aging.Compared to the discrete small pores,the aggregated large pores made it easier for gas molecules to diffuse into the cavity,which might lead to the increase of leakage rate.

Fig.9 Microstructures of joints after aged for 300 h (a) Sintered at 4.7 kA (b) Magnification of interface (c) Sintered at 5.3 kA (d) Magnification of interface (e) Sintered at 5.3 kA (f) Magnification of interface

Furthermore,as shown in Figs.9 a and 9b,after aging for 300 h,there was obvious delamination near the Ag/Au interface of sintered joint fabricated at 4.7 kA.The same phenomenon occurs in sintered silver joints fabricated at 5.0 kA and 5.3 kA,as shown in Figs.9 d and 9f.The free energy of Ag-Au solutions is lowest at 50% of their respective concentrations[28].This produces a driving force for Ag-Au interdiffusion.During the aging process,Ag atoms near the interface in the sintered silver joint would continue to diffuse into the Au layer,meanwhile Au atoms will diffuse into the Ag layer.However,the Au plating layer has completely dissolved into the Ag layer at the interface.However,the diffusion flux of Ag to Au is not the same as of Au to Ag[29].According to the following Arrhenius equation,the diffusion coefficient between Ag and Au at an aging temperature of 250 °C is calculated.

whereDis diffusion coefficient (cm2/s);D0is frequency factor (cm2/s);Qis activation energy (J/mol);Ris universal gas constant (8.31 J/mol·K);Tis absolute temperature (K).The parameters of the Arrhenius equation and diffusion coefficient between Ag and Au are shown in Table 2.

As shown in Table 2,at 250 °C (523 K),the diffusion coefficient of Ag in Au is 3 orders of magnitude higher thanthat of Au in Ag.Besides,the diffusion coefficient of atoms through the surface is greater than that of the interior diffusion[30],and the pores in the sintered silver joint provide more diffusion paths for Ag atoms.During the high-temperature aging,Ag atoms would continuously diffuse into the Au layer through the pore surface.Therefore,the diffusion rate of Ag into Au is much greater than that of Au into Ag.As a result,during the high-temperature aging process,delamination would appear near the interface,reducing the sealing performance of the joint.

Table 2 Paraments of Arrhenius equation and atomic diffusion coefficient between Ag and Au.

The coarsening of the pores and the delamination near the bonding interface would reduce the sealing performance of the sintered silver joint,increasing the leakage rate,as shown in Fig.8.Because coarsened pores were formed by the aggregation of discrete pores,and the initial porosity and pore size of the joint sintered at 4.7 kA was the largest.As a result,the aggregated pores size of joint sintered at 4.7 kA were also the largest.In addition,due to the highest initial porosity of the joint sintered at 4.7 kA,it provides more paths for Ag atoms to diffuse into the Au layer and accelerates the diffusion rate,which results in delamination near the bonding interface more severe.Hence,the sintered silver sealing joints fabricated at lower current with larger porosity are more prone to deteriorate when exposed in high temperature.

3 Conclusions

In this paper,the rapid sintering of silver paste by ECAS was used for hermetic packaging.The effects of current magnitude have been studied.Thermal cycling tests and high temperature storage tests were conducted to evaluate the reliability of sealing joints.Some important conclusions are summarized as follows:

(1) Reliable sealing joints used in hermetic packaging can be obtained by silver pasteelectric-current-assisted sintering.

(2) When the joints are sinter at 5.0 kA and 5.3 kA,the densification of the microstructure is further enhanced and the leakage rate is far below the reject limit.

(3) In the thermal cycling test,the joints sintered at 5.0 kA and 5.3 kA show superior reliability after 300 cycles because less pores reduce stress concentration.

(4) In the high temperature storage test,without pores coarsening and delamination,the joints sintered at 5.0 kA and 5.3 kA still survive after 300 h owing to lower porosity.