Elsevier

Journal of Alloys and Compounds

Volume 777, 10 March 2019, Pages 578-585
Journal of Alloys and Compounds

Pressureless sintering of nanosilver paste as die attachment on substrates with ENIG finish for semiconductor applications

https://doi.org/10.1016/j.jallcom.2018.10.294Get rights and content

Highlights

  • Interfacial delamination of pressureless sintered Ag on the ENIG finish was solved.

  • Rapid non-densifying diffusion of Ag atoms leads to interfacial delamination.

  • Grain size, angle, and (111) texture contribute to non-densifying diffusion.

  • A modified sintering profile could reduce non-densifying diffusion successfully.

Abstract

In order to overcome interfacial delamination of pressureless sintered nanosilver as die attachment on the ENIG finish, we tried to clarify the densification and interfacial diffusion behavior of sintering nanosilver paste on Au finish. Two different electroless nickel immersion gold (ENIG-1, ENIG-2) and one electroplated nickel gold (electrolytic Au) were compared in the aspects of mechanical, thermal performance, and microstructures. Delamination was found at the as-sintered Ag in the vicinity of Ag/Au interface in the ENIG-1. It was likely that too much Ag atoms was diffused into the ENIG-1, the left Ag atoms of the Ag-NPs in the vicinity of the interface are not sufficient to be densified as a robust sintered Ag joint at the higher temperatures. Electron backscattered diffraction (EBSD) results confirmed that the grain size, the high angle grain boundary, and the (111) texture lead to the rapid non-densifying diffusion of the Ag atoms with the ENIG. We proposed a modified sintering profile and the rougher ENIG-2 with thinner Au finish to reduce the non-densifying diffusion. The performance, and microstructures of the sintered Ag on the ENIG-2 confirmed that it is reasonable to enhance the sintered Ag on ENIG by reducing the rapid non-densifying diffusion at above 150 °C.

Introduction

Wide bandgap (WBG) semiconductors are widely used in power electronics because of their high thermal conductivity, low permittivity, high wide band gap. In order to meet the application of wide bandgap semiconductor in high temperature, the bonding materials of high temperature stability had attracted much attention [[1], [2], [3]]. Hence, nanosilver paste is one of the promising materials as die attachment for power electronics, due to its high melting temperature, high thermal conductivity, high electrical conductivity, and superior reliability [[4], [5], [6], [7]].

The nanosilver paste is usually sintered at low temperature to bond WBG power chips with direct-bond-copper (DBC) substrates in a pressureless way for assembling power modules [8,9]. The DBC substrate usually needs a surface finish, e.g., electroless nickel immersion gold (ENIG), to prevent oxidation of the copper sheets of the DBC substrate and enhance its solderability [[10], [11], [12]], especially for aerospace applications requiring high reliability [13,14]. In our previous work, however, it is interesting that the shearing strength of the pressureless sintered Ag joint in the case of the ENIG finish was less than 8 MPa [15], which is much lower than that of the pressureless sintered joint with the silver finish and the copper finish [16], even though silver could be bonded well with gold by solution diffusion theoretically [17]. At present many defects, e.g., delamination, could be present at the interface between pressureless sintered Ag and ENIG finish, leading to weak bonds [15,18,19].

In order to overcome the problem, high hydrostatic pressure sintered was used in the sintering of nanosilver paste on the ENIG. For example, Kim et al. [20] increased the shear strength of the sintered die attachment on an ENIG substrate to 22 MPa using a hydrostatic pressure of 20 MPa. Nishikawa et al. proposed preparing the sintered Ag with the ENIG by preheating the nanosilver paste at 130 °C for 300 s, and then heating it at 300 °C for 600 s with the pressure of 10 MPa in air. Their maximum strength of the sintered Ag in the case of the ENIG could reach 30 MPa [21]. It seems that high hydrostatic pressure could be an effective way to increase the bonding strength of the sintered Ag on the ENIG. However, such high hydrostatic pressure demands expensive processing equipment and complex operation to get rid of the concerns on eliminating potential damages to extremely thin WBG power chips [22]. It is crucial to bond WBG power chips with DBC substrates with the ENIG, which is a most used gold finish in power electronic systems for aerospace and communication, by sintering of nanosilver paste in a reliable pressureless way.

In this paper, we tried to clarify the densification and interfacial diffusion behavior of sintering nanosilver paste on Au finishes. Then the mechanism could guide us to improve the quality of the pressureless sintered nanosilver die attachment on the ENIG finish. It would be useful to spread the usage of the nanosilver paste for power electronics in harsh environment, especially for aerospace applications.

Section snippets

Experiment

The silver paste was provided by NBE LLC. The nanosilver paste was made by adding selected organic surfactant, binder, and thinner into 30 nm nanosilver particles [23,24]. It is worth noting that dispersant keep nanosilver particles from aggregating, binder prevent cracks in the connection layer during the sintering process and improve the mechanical strength of joint connection, and thinner improve the viscosity and fluidity of the nanosilver paste. 3 × 3 mm2 chips with commercial Ag

Weak bonding and interfacial delamination on ENIG

We found that the average die-shear strength of sintered Ag joint by the conventional sintering profile [15] on the electrolytic Au, and the ENIG-1 was 20.8 ± 3.1 MPa and 7.5 ± 0.7 MPa, respectively. Fig. 2 shows the fracture micromorphology of Ag and Au mapping by EDS. The sintered Ag on the electrolytic Au failed in the joint. Fig. 2(b) shows that hillock-like Ag could be found on the fracture surface of ENIG-1. Both Ag and Au residues could be present on the fracture surface, as shown in

Conclusion

We studied the densification and interfacial diffusion behavior of sintering nanosilver paste on gold finishes in this paper. Two different electroless nickel immersion gold (ENIG-1, ENIG-2) and one electroplated nickel gold (electrolytic Au) were compared in the aspects of mechanical and thermal performance as well as microstructures.

Delamination was found at the as-sintered Ag in the vicinity of Ag/Au interface in the case of the ENIG finishes. We concluded that too much Ag atoms was diffused

Acknowledgments

This work was supported by the Science Challenge Project (No. TZ2018003), the National Science Foundation of China (No. U1637203), the Tianjin Municipal Natural Science Foundation (No. 17JCYBJC19200), and the National High Technology Research and Development Program of China (No. 2015AA034501). Dr. Yunhui Mei is the corresponding author of this work.

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