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The use of a Co-W-P metallization substrate in SiC power modules is expected to improve high temperature reliability because Co-W-P metallization has been found to induce strong bonding strength to both sintered Ag joints and encapsulation resins. To progress in the development of this technology, the successful applicability of Co-W-P metallization into the module fabrication process is of critical importance. In this paper, the effects of annealing a Co-W-P metallization substrate in the die attach process onto its resin adhesion was studied with representative annealing conditions for Ag sintering. Initially, mild annealing at 200 °C for 1 h was applied and showed strong resin adhesion greater than 15 MPa at 225 °C and an ideal cohesion fracture mode of resin, the same as that found in that of the fresh Co-W-P case. However, more severe annealing resulted in lower resin adhesion. For example, annealing at 280 °C for 1 h resulted in a poor resin adhesion below 15 MPa, as well as a delamination fracture mode between the resin and the Co-W-P metallization. This mechanism was investigated with the use of SEM–EDS and XPS analysis. It was observed that annealing at 200 °C induced a slight oxidization of Co, but Co(OH)2 for the chemical reaction to resin, still remained on the Co-W-P surface. On the other hand, annealing at 280 °C formed an alternative main component (CoO). The shortage of Co(OH)2 on the top surface created by severe oxidization was found to induce poor resin adhesion. The results from this research are significant to future designs and applications of a module fabrication process using Co-W-P metallization substrates, as well as to the fundamental understanding of adhesion behavior on Co-W-P metallization.

One of the applications of wide band gap semiconductors is high temperature operation. That application requires high temperature compatible (i) joining materials such as sinter Ag, (ii) encapsulation resins such as imide type primers or molding compounds, and (iii) metallization for those materials. Ag metallization, the best candidate metallization for sinter Ag materials, has difficulty in bonding to encapsulation resins. Conversely, Ni/Au-flash metallization enables strong resin adhesion but also demonstrates poor reliability for sintered Ag joints. There is no single metallization compatible to both sintered Ag and encapsulation resin for high temperature application. This paper reports on a single metallization, electroless plated CoW metallization, which has demonstrated the capability to achieve both (i) high-temperature reliability (250 °C for 500 h) for sintered Ag joints and (ii) high-temperature adhesion (at 225 °C) for encapsulation resins. Such results have not been achieved with either Ag or Au metallization. The shear strength of sintered Ag joints on CoW metallization exceeded 40 MPa. TEM observation revealed excellent bonding between the sintered Ag and the metal Co of the CoW metallization. Furthermore, CoW metallization also showed strong resin adhesion (about 21 MPa) at 225 °C. XPS analysis identified metal Co for bonding to sinter Ag and, Co(OH)2 and WOx for bonding to resin on the top surface of CoW metallization layer. The foregoing results indicate that CoW may well represent a new metallization process for the fabrication of high reliability and high-temperature compatible SiC power modules.

Using an electroless nickel/electroless palladium/immersion gold (ENEPIG) surface finish with a thick palladium–phosphorus (Pd-P) layer of 1 μm, the intermetallic compound (IMC) growth between the ENEPIG surface finish and lead-free solders Sn-3.5Ag (SA) or Sn-3.0Ag-0.5Cu (SAC) after reflow soldering and during solid-state aging at 150°C was investigated. After reflow soldering, in the SA/ENEPIG and SAC/ENEPIG interfaces, thick PdSn4 layers of about 2 μm to 3 μm formed on the residual Pd-P layers (~ 0.5 μm thick). On the SA/ENEPIG interface, Sn was detected on the upper side of the residual Pd-P layer. On the SAC/ENEPIG interface, no Sn was detected in the residual Pd-P layer, and Cu was detected in the interface between the Pd-P and PdSn4 layers. After 300 h of aging at 150°C, the residual Pd-P layers had diffused completely into the solders. In the SA/ENEPIG interface, an IMC layer consisting of Ni3Sn4 and Ni3SnP formed between the PdSn4 layer and the nickel–phosphorus (Ni-P) layer, and a (Pd,Ni)Sn4 layer formed on the lower side of the PdSn4 layer. On the SAC/ENEPIG interface, a much thinner (Pd,Ni)Sn4 layer was observed, and a (Cu,Ni)6Sn5 layer was observed between the PdSn4 and Ni-P layers. These results indicate that Ni diffusion from the Ni-P layer to the PdSn4 layer produced a thick (Pd,Ni)Sn4 layer in the SA solder case, but was prevented by formation of (Cu,Ni)6Sn5 in the SAC solder case. This causes the difference in solder joint reliability between SA/ENEPIG and SAC/ENEPIG interfaces in common, thin Pd-P layer cases.

It is known that the Co alloy deposit has high electromigration resistance and thermal diffusion resistance to Cu. We could prepare electroless deposit of Co-W-P with different W contents on Cu substrate by changing the Co-W-P bath parameter. After heat treatment under 200-400°C with Air or N 2 conditions, Cu diffusion to the surface was measured. Co-W-P layer has excellent barrier property for Cu in N 2 heat treatment. However, Cu diffused to the surface when whole of Co-W-P (W=0 and 11wt.%) layer was oxidized in high temperature with air condition. The results indicate that oxidized layer of Co-W-P deposit has no barrier effect for Cu. In Co-W-P deposit in the condition of high W content (W=23wt.%), an unoxidized layer still remained and only a small amount of Cu was detected on the surface. We confirmed that Co-W-P (W=23wt.%) was difficult to oxidize and Cu diffusion was suppressed by preventing oxidization of the Co-W-P deposit.

The fracture behaviors and Ag–Au joint interface evolution of sintered micron-sized Ag particles paste joined on an electroless nickel/electroless palladium/immersion gold (ENEPIG)-plated direct-bonded aluminum (DBA) and direct-bonded copper (DBC) substrates are evaluated during an extreme thermal shock test (TST) from −50–250 °C. The die shear strength of the as-sintered Ag joint on the DBA substrate is evaluated to be 33.5 MPa at a sintering temperature of 200 °C without any assisted pressure, which decreased gradually with an increase in the thermal cycling number. The shear strength declined slightly but remained at approximately 20 MPa; subsequently, it decreased considerably to 11.2 MPa after 1000 cycles. Coarsening of the sintered Ag layer is observed as the microstructure inhomogeneity and vertical cracks increased after 1000 cycles. In addition, the Al layer induced a greater undulate deformation, resulting in a sintered Ag layer exhibiting partial compression and tension after a TST. The sintered Ag layer became dense with a significant decrease in porosity at the compression parts and large horizontal cracks appeared at the tension parts. Both of horizontal cracks and vertical cracks led to fracture mode change and shear strength decrease. The Ag–Au joint interdiffusion layer became thicker during thermal shock with the gradual diffusion of the Au atoms into the sintered Ag layer. The die shear strength of the as-sintered Ag joint on the DBC substrates is evaluated as 34.4 MPa at 200 °C sintering but decreased to 0 MPa after 250 thermal shock cycles owing to the stress-induced delamination between the Ag and Au interdiffusion layer and sintered Ag layer. This study provides insights into the interface reaction and evolution of sintered Ag on ENEPIG-finished DBA and DBC substrates for applications at high temperatures.

Robust bonding and thermal-stable sinter Ag joining on an Au finished substrate was first time achieved for use in SiC power modules. Five kinds of Ag paste, including different solvents and Ag fillers, and four kinds of Au plating processes (ENIG, ENIGEG, ENEPIG and ENEPIGEG) were used to optimize the initial shear strength of Ag–Au joints. Here, EN means electroless Ni plating, EP means electroless pure palladium plating, IG means immersion gold plating process and EG is the process of electroless gold plating. Micro-scaled Ag flake paste showed the best die shear strength at 33.9 MPa by in-suit formation of Ag nanoparticles, accomplished with a sintering temperature of 250 °C without pressure in air. With the use of Electron backscatter diffraction (EBSD) analysis, the ENEPIG plating process exhibited an Au surface with greater Au (111) grain orientation and larger grain size, which both of that benefits bonding with Ag paste. The thermal-stable sinter Ag joining on ENEPIG was evaluated by aging at 250 °C up to 1000 h. The shear strength had slightly increased to 36.5 MPa after 1000 h. The mechanism of robust bonding and thermal-stable during high temperature aging were systematically analyzed via Scanning Electron Microscope (SEM), Energy dispersive X-ray spectroscopy (EDS), Transmission Electron Microscope (TEM) and X-ray diffraction (XRD), which was attributed to the large Au grains size, the favorable surface condition of the ENEPIG, and the Pd barrier layer which prohibited Ni diffusion into the Au layer during sintering and aging.

Sintered Ag is well known for die-attach materials, suitable for Ag metalized interfaces with a self-healing function of generated cracks. A remaining risk of sintered Ag bonding may be possible degradation of interfacial strength at high temperatures. Molding process is thus important for supporting the die-attach in the encapsulated power module once certain adhesion strength is assured between a lead-flame and mold resin. We propose a prominent interface structure using novel bonding materials for electronic power modules targeting high-temperature operation.