Pressureless sintering of nanosilver paste at low temperature to join large area (≥100 mm2) power chips for electronic packaging

doi:10.1016/j.matlet.2014.04.127

Materials Letters

Volume 128, 1 August 2014, Pages 42-45

https://doi.org/10.1016/j.matlet.2014.04.127 Get rights and content

Highlights

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Bonding large area chip (≥100 mm²) is achieved using pressureless sintering at 250 °C.
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The particle size of silver ranges from 0.02 to 2 µm.
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The adhesion strength of pressureless bonding reaches to 43.8 MPa.
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The sintered joint shows low porosity, high thermal conductivity, and low void ratio.

Abstract

We achieve robust bonding of large area power chip (≥100 mm²) based on pressureless sintering of silver particles at low temperature, i.e., 250 °C. The particle size of silver ranges from 0.02 to 2 µm and most organics can be evaporated rapidly from 70 °C to 180 °C. It is not necessary for metallic bond to be formed entirely by silver nanoparticles. Instead, silver microparticles also can rapidly grow up and aggregated together by atom diffusion from the silver nanoparticles. This point of view leads to a new way to bond the large area chips without additional pressure. The sintered silver joint shows high shear strength, low porosity, high thermal conductivity, and low void ratio.

Introduction

Nanosilver paste has been increasingly used in electronic packaging devices, especially for high-temperature applications [1], [2], such as high power insulated gate bipolar transistor. It has been applied as alternatives to the traditionally electrical and structural interfacial materials, i.e., solder alloys and conductive epoxies, because of their superior properties, such as high melting point (960 °C), low sintering temperature (275 °C), high electrical and thermal conductivity (4.1×10⁷ S m⁻¹ and 240 W m⁻¹ K⁻¹, respectively), and excellent reliability [3], [4].

Pressureless sintering of nanosilver paste for bonding small area chips [5], [6], i.e., 2×2 mm² and 6 mm diameter, had attracted attention in the past. However, chip size becomes larger and larger because of the trend of higher and higher power in power electronics [7]. Low-temperature sintering of the nanosilver paste for bonding large area (≥100 mm²) chips has not been realized because of poor joint quality, e.g., generation of many voids in the joint, which affects the adhesion strength and thermoelectric properties greatly. Therefore, it is necessary to develop a new kind of nanosilver paste for bonding large area power chips at low temperature without the help of additional pressure.

In this paper, we developed a new kind of pressureless nanosilver paste by mixing silver nanoparticles with silver microparticles under the help of some organics to bond large area (≥100 mm²) power chips with very high power density. It is not necessary for metallic bond to be formed entirely by silver nanoparticles. Instead, silver microparticles can rapidly grow up and aggregated together by atom diffusion from the silver nanoparticles, which are distributed in the gap between silver microparticles. The morphology of metallic bond between silver was observed. The shear strength, porosity, thermal conductivity and voids ratio are discussed.

Section snippets

Materials and methods

This silver paste [8] for pressureless sintering includes silver particles, dispersants, binders, thinners and solvents. The microstructures of the sintered specimen were analyzed by scanning electron microscopy (SEM). The transmission electron microscopy (TEM) was used to analyze the grain boundary structure of sintered silver joint. Thermogravimetric analysis (TGA) was also conducted to measure the added organic components in nanosilver paste. X-ray CT was used to analyze the internal

Results and discussion

The nanosilver paste is firstly analyzed by TGA. Fig. 1 shows that the organics in the paste are about 10.3 wt%. Fig. 2(a) shows the size of silver particles in the paste ranges from 0.02 to 2 µm. The average size of the silver particles is below 1 µm. The driving force for the sintering is the tendency of the silver particles to reduce its chemical potential or energy. On the basis of TGA analysis, the pressureless sintering progress of nanosilver paste could be divided into three stages (I–III).

Conclusions

In conclusion, an improved nanosilver paste is developed for bonding large area (≥100 mm²) power chip at low temperature of 250 °C without additional pressure. The size of silver particle in the paste ranges from 0.02 to 2 µm, and most organics can be rapidly vaporized from 70 °C to 180 °C. The shear strengths are 43.8 MPa without additional pressure and above 50 MPa under the pressure of 1.77 MPa, respectively. The porosity is 25.6% and the thermal conductivity is calculated as 263 W m⁻¹ K⁻¹. The

Acknowledgments

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 51101112) and the Tianjin Municipal Natural Science Foundation (under grant 13JCQNJC06600 and 13JCZDJC33600).

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Pressureless sintering of nanosilver paste at low temperature to join large area (≥100 mm2) power chips for electronic packaging

Highlights

Abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Conclusions

Acknowledgments

Mater Sci Eng, A

Mater Lett

Scr Mater

Mater Sci Eng, A

Mater Sci Eng, A

Appl Phys Lett

Nanotechnology

Pressureless sintering of nanosilver paste at low temperature to join large area (≥100 mm²) power chips for electronic packaging