Thermal fatigue behaviors of SiC power module by Ag sinter joining under harsh thermal shock test

Chen Chuantong ,Zhang Hao ,Jiu Jinting ,Long Xu ,Suganuma Katsuaki

1.Institute of Scientific and Industrial Research,Osaka University,Osaka 567-0047,Japan;2.Senju Metal Industry Co.,Ltd,Tokyo 120-8555,Japan;3.School of Mechanics,Civil Engineering and Architecture,Northwestern Polytechnical University,Xi’an 710072,China

Abstract The excellent properties of SiC bring new challenges for the device packaging.In this study,the bonding strength,fracture behaviors and microstructural evolution of micron-porous Ag joint were elevated during thermal cycling (–50 °C–250 °C) in SiC/DBC (direct bonding copper) die attachment structure for different time.During harsh thermal shock test,the strength of sintered joint deceased gradually with the increase of cycling number,and the value just was half of the value of as-sintered after 1 000 cycles.Coarsening of Ag grains was observed in micron-porous joint with the structure inhomogeneity and defects increasing,which were the reasons of the strength decease.In addition,it was also found that the fracture behavior of sintered joints was changed from ductile deformation of Ag grain to brittle fracture of crack propagation after 1 000 cycles.This study will add the understanding in the mechanical properties of Ag sinter joining and its applications at high temperature.

Key words power module,high-temperature reliability,Ag sinter joining,low stress structure,thermal shock test

0 Introduction

Power devices have penetrated and impacted our daily life from a tiny battery charger to a large power supply system.Over decades,Si has made significant progress and occupied the first chair of semiconductor material.However,power devices based on Si material are rapidly approaching their theoretical limits of performance in terms of blocking voltage,operation temperature,and conduction and switching characteristics due to its material characteristics.It is unable to handle the increasingly improved power density in the near future.Therefore,it will be necessary to develop devices with new semiconductor materials to match the increased power density,reduce power losses in high-frequency systems and achieve higher efficiency.In recent years,silicon carbide (SiC) has emerged as the most prominent candidates for the replacement of Si[1–3].Since SiC can handle heavy power density and is expected to generate massive heat,which will result in a severe operating temperature on the order of 250 °C.The massive heat generation leads to issues for thermal management and packaging of the whole device,causing a massive impact on device performance and long-term reliability problems,for instance thermal stress caused by different materials and hightemperature oxidation of packaging materials,will directly lead to the failure of SiC device[4–6].

At present,Ag sinter paste joining might be the most suitable candidate for die attaches[7–9].Ag paste,consisting of Ag particles and organic solvent,can be sintered below 250 °C due to the high surface energy of small particles.After sintering,the Ag particles merger into a uniform porous structure due to Ag self-diffusion.To face the severe challenges caused by harsh operating conditions in SiC devices,the understanding in the mechanical properties of microporous Ag and its applications at high temperature[10–11].

In this study,SiC power module structure by Ag sinter layer was designed and its high-temperature reliability was investigated.SiC chips were bonded to DBC substrate using Ag sinter paste at 260 °C for 10 min without any pressure.The joints structure was subjected to thermal shock testing in temperature range of–50 °C to 250 °C for duration up to 1 000 cycles.The bonding strength,fracture behaviors and microstructural evolution of micron-porous Ag joint were elevated during thermal cycling for different time.

1 Experiments

The micron-sized flake particles (AgC239,Fukuda Metal Foil and Powder Co.Ltd) and submicron-sized Ag spherical particles (S211A-10,DAIKEN CHEMICAL CO.,LTD.) were used as the silver fillers of hybrid micron paste as a weight ratio of 1:1.These mixed particles were then mixed with an ether–type solvent (CELTOL-IA,Daicel Corporation) using a hybrid mixer (HM-500,Keyence Corporation) to fabricate silver paste.The hybrid particles provided a suitable choice to fabricate a high density Ag sinter layer.In previous study,the hybrid particles paste provide a stable bonding structure with a strength of approximately 40 MPa at 250 °C under 0.4 MPa[12].The ethertype solvent will volatilize from the temperature about 80 °C,and there was an exothermic peak at 185℃ for the CELTOL-IA solvent in air[13–14].

The amount of the solvent was maintained at approximately 10 wt%.SiC dies (3 mm × 3 mm × 0.5 mm) and DBC substrates (Cu/Si3N4/Cu,30 mm × 30 mm × 2 mm) were used as a test specimen to simulate practical joints.Both the die and the substrate were coated with a 200 nm Ti and a 2 μm Ag metallization layers via sputtering method.The specimens were heated on a hot plate in ambient air at 260 °C for 10 min.The sintering process did not require assistance of extra pressure.Finally,the joints were left in ambient to cool to room temperature.The thermal shock test was performed by thermal cycling chamber (TSE-11-A-S,ESPEC,Osaka,Japan).The test conditions were performed from–50 °C to 250 °C with 30 min at each extreme temperature.The experimental procedure,sintering profile and schematic diagram of sintered joint were exhibited in Fig.1.

Fig.1 Experimental procedure,sintering profile and schematic diagram of sintered joint (a) SEM image of Ag particles(b) SiC die attached joint structure (c) Sintering curve (d) Thermal shock test condition (?50?250℃ for 1 000 cycles)

The shear strength of the bonding joints was evaluated by shear tests (Dage4000 bond tester,Nordan DAGE,Aylesbury,UK) at 100 μm/s.The shear height was fixed to 50 μm from the surface of the substrate.Cross-section of joints were prepared using an ion-milling polishing system(IM4000,Hitachi High-Technologies Corporation,Tokyo,Japan),and the microstructure was characterized by fieldemission scanning electron microscopy (SU8020,Hitachi High-Technologies Corporation) and energy-dispersive Xray spectroscopy (EDS).

2 Results and discussions

2.1 Initial microstructure and die shear strength

Fig.2 a shows the cross-sectional microstructure of the sintered silver joint structure,which presents a micron porous structure with uniform pore distribution.The hybrid Ag sinter paste can be sintered well even in a short sintering time of 10 min without pressure during the sintering.Fig.2 b exhibits the maximum,minimum and the average shear strength changes of the die-attach joints during thermal cycles ranging from–50 °C to 250 °C.The bonding strength of sintered joint decreased gradually with increasing number of thermal cycles.After 100 and 300 cycles,the value declined slightly but still kept over 30 MPa.However,such strength considerably dropped to 23.8 MPa and 17.57 MPa after 500 and 1 000 cycles,respectively.Despite degradation of bonding strength,such micron-porous sintered Ag joint still had an more excellent thermomechanical reliability than the traditional high-temperature alloys[15–16].The strength of Au88Ge12 solder will decreased greatly to less than 20 MPa only after 400 cycles from–55 °C to 250 °C[17–18],and the shear strength of Pb-free solder (SAC) cannot survive for a long time thermal cycling even from–55 °C to 125 °C[19].

Fig.2 Cross section of initial sintering structure and shear strength changes of the die-attach joints during thermal cycles ranging from ?50 °C to 250 °C (a) Cross section (b) Shear strength changes

2.2 Microstructure evolution

Fig.3 presents the cross-section microstructural evolution of micron-porous sintered Ag layers during thermal cycling.The initial sintered joint (Fig.3 a and b) was a micron-porous structure,and the pore distribution was uniform.After 100 cycles (Fig.3 c),the sintered Ag grains become coarsening the thickness of sintered layer become small,and the Cu interfaces on DBC was a little curved.The following thermal cycling induced to the coarsening behaviors of Ag grain,aggravation of deformation at Cu interface,and crack formation.As shown in Fig.3 e,Ag grains were further coarsening and porous structure become inhomogeneous after 500 cycles.Somewhere the sintered structure become densified with the decreasing porosity or even changed into a bulk-like Ag structure without pores.Somewhere the structure become very loose so that 20-30 μm cracks was formed and crossed the whole joints.Moreover,the deformation of Cu interfaces was more severe so that the interfaces became bumpy.The increase of defects agreed with the decrease of strength and ductilebrittle transition of fracture behaviors.Thus,the sintered Ag joints experienced a process of earlier coarsening of Ag grains,an inhomogeneity of porous structure and a deformation of Cu interface during thermal cycling from–50 °C to 250 °C.As for the nano-porous structure,there were obvious longitudinal cracks after harsh thermal cycling,but the microstructure was still uniform[20].The different pore size,distribution and porosity would induce to different distribution of thermal stress in the whole joint and thereby influenced the different thermal fatigue resistance[21]

Fig.3 Cross-section microstructural evolution of micron-porous sintered Ag layers during thermal cycling (a) and (b) initial cross section and its magnified view (c) and (d) cross section and its magnified view after 100 thermal shock test (e) and (f)500 thermal shock test (g) and (h) 1000 thermal shock test

Fig.4 shows the interfacial stability of sintered joint during thermal cycling.Fig.4 a and Fig.4 b show the initial SiC die/sintered Ag interface and sintered Ag/DBC substrate interface,respectively.Fig.4 c and Fig.4 d show the SiC die/sintered Ag interface and sintered Ag/DBC substrate interface after 1 000 thermal shock test,respectively.According to EDS analysis,after 1 000 thermal shock test,Ag atoms did not diffuse across the Ti layers and into the sintered Ag during this test.The results indicated the Ti layers at SiC die/sintered Ag interfaces was stable.However,the case was completely different at sintered Ag/DBC substrate interfaces.A clear delamination occurred between Ti layers and Cu layer on DBC substrate and a new thicker layers appeared between Ti layers and sintered Ag.The EDS results showed this layer was composited by Cu and O elements,thus it is a Cu oxidation.The formation of Cu oxidation was mainly caused by the break of Ti layers.When Ti layers disappeared somewhere,Cu element would diffuse into the sintered Ag through these defects and then react with oxide in the pores of sintered joints to form a thick layer of Cu oxidation.Moreover,the break of Ti layers was related closely to the serve deformation of Cu layers on substrate.

Fig.4 Interfacial stability of sintered joint (a) initial SiC die/sintered Ag interface (b) sintered Ag/DBC substrate interface (c) and (d) interface after 1000 thermal shock test

2.3 Fracture behaviors

The corresponding fracture behaviors of sintered Ag joints were also evaluated to investigate the decreasing of strength during thermal cycling process.Fig.5 a,b,c and d show the fracture surface after shear test for the initial Ag sinter joint,after 100,500 and 1 000 cycles,respectively.As for initial and 100 cycles joints (Fig.5 a and b),the fracture sites were inside of sintered joint,and the surface was a little coarse.The magnification view (Fig.5 e and f) shows the fractures were completely covered by ductile and severely deformed Ag grains.Thus,the failure of sintered joint was caused by the ductile fracture of Ag grains.However,the case was different after 500 cycles as shown in Fig.5 c.Although the joint fractured at inside of bonded Ag layer,most surface area become smooth and was covered by a large number of cracks.The magnification view shown in Fig.5 g presents that the deformation of Ag grains was not obvious in the smooth area.In addition,it was also found that the fracture surface appeared many cracks and which increased with the thermal shock cycle number.The cracks correspond to the vertical cracks at the cross section after thermal shock as shown in Fig.3.

Fig.5 SEM images of fracture surface of sintered Ag initial and thermal cycling joints under two magnifications (a) and (e)initial joint (b) and (f) 3 00 cycles (c) and (g) 500 cycles (d) and (h) 1000 cycles

After 1 000 cycles,the fracture surface was completely covered by the increasing number of cracks,and Ag grain also did not deform.Thus,the fracture of sintered joint changed from ductile deformation of Ag grain to brittle fracture due to the formation and propagation of crack after 1 000 cycles.Moreover,the changes of fracture surfaces accorded with degradation of bonding strength in Fig.2.Initially,the fracture of sintered joint was caused by tensile deformation of Ag grains.Since many cracks appeared in the sintered Ag,the die shear strength was decreased after thermal shock test,and thus the small shear strength may not lead to the plastic deformation at Ag sinter necking location.The shear strength may smaller than that the yield strength of sintered Ag after thermal shock test.

After a long thermal cycle,there was an increasing number cracks in the whole joint.The growth and propagation cracks determined the mechanical properties of sintered joint,thus leading to the failure of joint much bellow yield strength of sintered Ag.Thus,the Ag grain defamation-controlled joints had a higher strength while the strength of sintered joints controlled by crack propagation was lower.It is concluded that the sintered joint experienced a significant decreasing of bonding strength and a ductile-brittle fracture transformation during thermal cycling.The fracture fatigue behaviors of SiC power module by Ag sinter joining under harsh thermal shock test will add the understanding in the mechanical properties of Ag sinter joining and its applications at high temperature.

2.4 Thermal stress mechanism

The sintered Ag joints were composited by heterogeneous materials with different coefficient of thermal expansion (CTE).Thus,the CTE mismatch would result in thermal stress in the joint from–50 °C to 250 °C.Fig.6 shows three-dimensional finite element analysis (FEA)modeling to simulate the distribution of thermal stress in the temperature range from–40 °C to 200 °C.The results of elastoplastic simulation show that a maximum compressive stress appeared during the heating process (Fig.6 b) and a maximum tensile stress during the cooling process (Fig.6 d)accumulated on the edge of sintered Ag layer and DBC substrate.Because the yield strength of Cu is about 70 MPa,it will be deformed under the larger thermal stress[22–25].The serve deformation of Cu layers will result in defects like crack or even delaminating with Ti layers,and then Cu will diffuse through crack and into the sintered Ag layers.Therefore,the failure of Ti layers in the interfaces was caused by inhomogeneous distribution of thermal stress during thermal cycling.Moreover,compared with thermal aging at 250 °C,micron-porous sintered Ag after thermal cycling (–50 °C–250 °C) not only have a more obvious coarsening degree of during process but also become inhomogeneous.It implied that the migration of Ag grains during thermal cycling was much larger than the Ag thermal diffusion during aging at 250 °C.This phenomenon might be explained by the enhanced stress-induced Ag migration.Many studies have reported that the diffusion of metallic atoms was enhanced by the thermal stress.The best examples were the growth of Ag hillock under the thermal stress in electrical packaging.The theory can be as described in the following equation[26-27].

Fig.6 Three-dimensional finite element analysis model (a) von Mises stress distribution of SiC power module by Ag sinter(b) stress distribution of sintered Ag layer during heating (c) and (d) stress distribution during cooling

WhereCis the atomic concentration,Ωis the atomic volume,kBis Boltzmann’s constant,T is the absolute temperature,D0is the Ag self-diffusion coefficient,Qis the activation energy,and σ is the hydrostatic stress.The equation indicated that gradient of σ served as a driving force for atomic diffusion,and the atoms move from a position with higher stress toward one with lower stress.Thus,the migration Ag atoms were more significant in the thermal fatigue from–50 °C to 250 °C,causing the existence of bulklike structure and loose porous structure in a sintered joint.

3 Conclusions

(1) The SiC/DBC joint structure by Ag sinter joining was evaluated under the thermal shock test from–50 °C to 250 °C.The die shear strength decreased from 35.4 MPa initially to 17.57 MPa after 1 000 cycles.

(2) The deceasing of strength was accorded with the fracture transition from ductile deformation of Ag grain-controlled mode to brittle crack propagation-controlled mode.

(3) The harsh thermal cycling process induced to the evolution of microstructure including the coarsening of Ag grains,the structural inhomogeneity and the increasing of defects like crack or voids.

(4) The failure of whole joint bellow yield strength was caused by structural deterioration .Moreover,CTE mismatch was caused by the thermal stress during thermal cycle served as a driving force of the Ag migration.

Acknowledgement

This work was also partly supported by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (Grant No.19121587),and was also supported by the Natural Science Foundation of Shaanxi Province (No.2021KW-25).