High Electrical and Thermal Conductivity of Nano-Ag Paste for Power Electronic Applications

doi:10.1007/s40195-020-01083-3

Abstract

Abstract:

The nano-Ag paste consisted of Ag nanoparticles and organic solvents. These organics would be removed by evaporation or decomposition during sintering. When the sintering temperature was 300 °C, the resistivity of sintered bulk was 8.35×10^-6 Ω cm, and its thermal conductivity was 247 W m^-1 K^-1. The Si/SiC chips and direct bonding copper (DBC) substrates could be bonded by this nano-Ag paste at low temperature. The bonding interface, sintered microstructure and shear strength of Si/SiC chip attachment were investigated by scanning electron microscopy, transmission electron microscopy and shear tests. Results showed that the sintered Ag layer was porous structure and tightly adhered to the electroless nickel immersion gold surface of DBC substrate and formed the continuous Ag-Au interdiffusion layer. The shear strength of Si and SiC chip attachments was higher than 35 MPa when the sintering pressure was 10 MPa. The fracture occurred inside the sintered Ag layer, and the fracture surface had obvious plastic deformation.

Key words: Nano-Ag paste, Sintering process, Interfacial microstructure, Chip attachment, Shear strength

Hong-Qiang Zhang, Hai-Lin Bai, Qiang Jia, Wei Guo, Lei Liu, Gui-Sheng Zou. High Electrical and Thermal Conductivity of Nano-Ag Paste for Power Electronic Applications[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(11): 1543-1555.

Figures/Tables 17

Fig. 1 a SEM image of Ag nanoparticles and the thickness of PVP coating; b TGA-DSC curve of Ag nanoparticles heated in air at 10 °C/min

Fig. 2 a Image of chip attachment sintered by nano-Ag paste; b measurement of shear strength of chip attachment

Fig. 3 a Thermal decomposition of nano-Ag paste (heating rate 10 °C/min, air atmosphere); b FTIR spectra of the nano-Ag paste sintered at different temperatures

Fig. 4 Resistivity of nano-Ag paste sintered at different temperatures for 15 min

Table 1 Comparison of resistivity of nano-Ag paste reported by different companies

Silver particle paste	Sintering parameters	Resistivity (Ω cm)
Heraeus’ mAgic paste	280 °C/-	8.00E-05
DOWA silver nano-paste	180 °C/90 min	5.00E-05
NBE’s nano-paste	280 °C/-	3.80E-05
Harima’s NH4000 paste	250 °C/60 min	2.50E-06
Harima’s NPS-HB paste	150 °C/90 min	1.40E-05
ALPHA Argomax^? paste	190-300 °C/-	2.50E-05
This study	275 °C/15 min	2.31E-05
This study	300 °C/15 min	8.35E-06

Table 2 Thermal conductivity calculation method of the sintered Ag layer [34, 36, 37]

Calculation method	Equation	Parameters
Wiedemann-Franz (W-F) calculation	λ = LTσ	L: proportionality constant 2.44 × 10^-8 W Ω K^-2
		σ: electrical conductivity
		T: temperature
Thermal diffusivity calculation	λ = αCρ	α: thermal diffusivity C: specific heat
Thermal diffusivity calculation	λ = αCρ	ρ: sample bulk density
Porosity calculation	λ = λ₀(1-ξ)^3/2	ξ: the porosity λ₀: the thermal conductivity of solid material

Fig. 5 Thermal conductivity of Ag paste measured by different evaluation methods

Fig. 6 Surface morphology evolution of the nano-Ag paste with different sintering temperatures (sintering time 15 min, in air)

Fig. 7 Influence of sintering temperature on the porosity of the sintered Ag

Fig. 8 Surface morphology evolution of the nano-Ag paste with different sintering time (sintering temperature 300 °C, in air)

Fig. 9 Influence of sintering time on the porosity of the sintered Ag

Fig. 10 Cross-sectional microstructures of SiC chip attachment: a overview of chip attachment; b SiC chip/sintered Ag layer interface; c sintered Ag layer; d sintered Ag layer/ENIG surface

Fig. 11 Elemental distribution of cross-sectional chip attachment: a overview of chip attachment; b Ni; c Ag; d Au; e P; f Cu

Fig. 12 TEM images of the sintered Ag layer/ENIG surface interface: a low-magnification image; b-d high-magnification image; e high-resolution TEM image of Ag/Ni(P) interface; f high-resolution TEM image of sintered Ag layer/Ag-Au interdiffusion layer interface

Fig. 13 Effects of the sintering pressure on shear strength of chip attachment (sintering temperature 300 °C, sintering time 15 min)

Fig. 14 Fracture morphology of sintered a-b Si chip attachment and c-d SiC chip attachment (sintering temperature 300 °C, sintering time 15 min, pressure 3 MPa)

Table 3 Shear strength of chip attachment sintered by Ag paste at 300 °C

Chip attachment	Sintering parameter	Shear strength(MPa)	Refs.
Die-to-DBC (Ni/Ag surface)	300 °C/20 MPa	20	[42]
Cu-to-Cu joint (Ag surface)	300 °C/0 MPa	22	[18]
Cu-to-Cu joint (Ag surface)	300 °C/5 MPa	50	[18]
SiC-to-AlN joint (Cu surface)	300 °C/30 min	41	[28]
SiC-to-Si₃N₄ joint (Ag surface)	300 °C/0.4 MPa/60 min	21	[43]
Cu-to-Cu disk joint	300 °C/5 MPa/5 min	36	[44]
Cu-to-Cu disk joints	300 °C/2.5 MPa	15.5	[45]
Cu-to-Cu disk joints	300 °C/10 MPa	55	[27]
Bare Cu joint	300 °C/60 min/0 MPa	20.7	[46]
SiC-to-DBC joint (Au surface)	300 °C/10 min	15	[47]
Si-to-DBC joint (Ni/Au surface)	300 °C/10 MPa/15 min	45	This work

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