Pressureless sintering of nanosilver paste as die attachment on substrates with ENIG finish for semiconductor applications
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.
References (46)
Wide bandgap semiconductor materials for high temperature electronics
Thin Solid Films
(1999)- et al.
Applying Anand model to low-temperature sintered nanoscale silver paste chip attachment
Mater. Des.
(2009) - et al.
Effect of surface finish on interfacial reactions of Cu/Sn–Ag–Cu/Cu(ENIG) sandwich solder joints
J. Alloys Compd.
(2008) - et al.
Characterization and reliability study of low temperature hermetic wafer level bonding using in/Sn interlayer and Cu/Ni/Au metallization
J. Alloys Compd.
(2009) - et al.
Correlation between interfacial microstructure and bonding strength of sintered nanosilver on ENIG and electroplated Ni/Au direct-bond-copper (DBC) substrates
J. Alloys Compd.
(2016) - et al.
A novel multiscale silver paste for die bonding on bare copper by low-temperature pressure-free sintering in air
Mater. Des.
(2018) - et al.
Thermally stable high temperature die attach solution
Mater. Des.
(2016) - et al.
Microstructure evolution during 300°C storage of sintered Ag nanoparticles on Ag and Au substrates
J. Alloys Compd.
(2014) - et al.
Silver nanoporous sheet for solid-state die attach in power device packaging
Scr. Mater.
(2014) - et al.
Microscale Ag particle paste for sintered joints in high-power devices
Mater. Lett.
(2015)
Atomistic modeling of segregation and bulk ordering in Ag–Au alloys
Surf. Sci.
Order-Disorder Transitions in the System Ag2−xAuxS (0≤x ≤ 1)
J. Less Common Met.
Beyond the Fisher model of grain boundary diffusion: effect of structural inhomogeneity in the bulk
Acta Mater.
Continuum framework for dislocation structure, energy and dynamics of dislocation arrays and low angle grain boundaries
J. Mech. Phys. Solids
The work function and electronic structure of coherent Ag–Au interfaces
Solid State Commun.
Nanosintering
Nanostruct. Mater.
How to determine surface roughness of copper substrate for robust pressureless sintered silver in air
Mater. Lett.
Evaluation of mechanical interlock effect on adhesion strength of polymer–metal interfaces using micro-patterned surface topography
Int. J. Adhes. Adhes.
A wide bandgap silicon carbide (SiC) gate driver for high-temperature and high-voltage applications
High-temperature electronics - a role for wide bandgap semiconductors?
Proc. IEEE
Sintered nanosilver paste for high-temperature power semiconductor device attachment
Int. J. Mater. Prod. Technol.
Thermomechanical reliability of low-temperature sintered silver die attached SiC power device assembly
IEEE Trans. Device Mater. Reliab.
Low temperature joining technology - a high reliability alternative to solder contacts
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