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The application of Co-W-P plating technology in high-temperature package structure is advantageous from a point of structural reliability because Co-W-P metallization is known to deliver strong bonding to both high-temperature-compatible Ag-sintered joining and high-temperature-compatible encapsulation resins. However, Co-W-P, unlike a noble metal, has a potential risk of surface oxidation in the module fabrication process. This surface oxidation can result in a decrease in resin adhesion. In this paper, the effects of W content (7 wt%, 11 wt%, 21 wt%) in Co-W-P metallization on both the oxidation resistance and the resin adhesion were studied. The resin adhesion on the annealed Co-W-P metallization with a high W content (21 wt%) was found to be sufficiently strong even after 250 °C anneal for 1 h. This resin adhesion strength was not present in other Co-W-P metallization tests. SEM–EDS analysis revealed that the oxidization of the Co-W-P-metallized surface during the anneal process proceeded more slowly in the case of the Co-W-P metallization with a doping 21 wt% W. XPS analysis revealed that Co(OH)2, necessary for a chemical reaction with the resin, exists mainly on the Co-W-P-metallized surface in the case of doping 21 wt% W, even after 250 °C anneal. XRD analysis revealed its structure to be a characteristic Co-W solid solution, unlike the structures found in other Co-W-P metallization. The findings in this study are significant for the promotion of Co-W-P metallization in the module fabrication process, as well as to the fundamental understanding of oxidation resistance and adhesion behavior on Co-W-P metallization.

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.

For the surface finishing of copper layer on printed circuit boards, autocatalytic electroless deposition of nickel-phosphorus alloy and immersion deposition of gold (ENIG, Ni-P/Au) or autocatalytic electroless deposition of Ni-P, palladium and immersion deposition of Au (ENEPIG, Ni-P/Pd/Au) are widely used. The Ni-P/Pd/Au surface finish is suitable not only for lead-free soldering but also for wire bonding [1]. For soldering, it was reported that hydrogen plasma improved bonding behavior and realized a flux-free process [2]. Electrochemically deposited metal films include hydrogen atoms. The hydrogen adsorbed in metal films causes various phenomena such as atomic diffusion, alloying, crystal growth, void formation. We have been investigating the atomistic state and behavior of hydrogen in electrodeposited metals by thermal desorption spectroscopy [3, 4]. In this study we measured changes in hydrogen amounts in the Ni-P/Au and Ni-P/Pd/Au surface finishes [5]. We used two different electroless Pd baths using formic acid (Ni-P/Pd/Au) and sodium phosphinate (Ni-P/Pd-P/Au) as reducing agents. The Ni-P films contained large amounts of diffusible hydrogen that desorbs at room temperature. During autocatalytic electroless deposition of Pd and Pd-P on Ni-P films, the greater part of the hydrogen in the Ni-P films desorbed. Although no hydrogen evolution accompanied immersion Au deposition, the hydrogen amounts in Ni-P/Au, Ni-P/Pd/Au, and Ni-P/Pd-P/Au films were much greater than in films without a Au layer. Moreover, at room temperature, they remained almost unchanged for one month after deposition, indicating that the thin Au layer (< 0.1 μm) prevents hydrogen desorption from the metal films. The high hydrogen concentration in Ni-P/Au films improved solder wettability. [1] Y. Oda, M. Kiso, S. Kurosaka, A. Okada, K. Kitajima, and S. Hashimoto, Proc. Internal. Microelectronic and Packaging Soc., IMAPS (2008). [2] T. Hagihara, T. Takeuchi, and Y. Ohno, Proc. 10th Electronics Packaging Technology Conf. (IEEE), 595 (2008). [3] N. Fukumuro, S. Kojima, M. Fujino, Y. Mizuta, T. Maruo, S. Yae, and Y. Fukai, J. Alloy. Comp., 645, S404 (2015). [4] N. Fukumuro, K. Tohda, and S. Yae, this symposium. [5] Y. Oda, Y. Sagara, N. Fukumuro, and S. Yae, J. Surf. Finish. Soc. Jpn., 70, 163 (2019).

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.

We investigated a new metallization layer has both functions in high temperature reliability with Ag sinter joining and in high temperature adhesion with insulating resin. As a result, it was found that CoW metallization layer works effectively from above two perspectives. Initial shear strength of sintered Ag joints on CoW plated layer was about 46 MPa, which is better joint strength than that of Au or Ag plated layer. Furthermore, the shear strength of insulating resin on a CoW plated layer was about 21 MPa at 225 °C, which is almost the same adhesion strength as that of Ni or Ni/Au plated layer.

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.

To improve the performance of 3D electronic chips, dense I/O and interconnects are required. Increasing the density of interconnects requires smaller pitch micro-bumps. However, when scaling down microbumps several challenges have to be taken into account. Lithography of dense and high aspect ratio bump, wet etching of seed and barrier layer, solder volume and intermetallics (IMC) formation are some of the challenges that needs to be addressed. With reducing bump dimensions, solder volume decreases as well, converting Sn to complete IMC during the Thermo-Compression-Bonding (TCB) process. Full IMC formation increases stress in the joint, leading to crack formation and a brittle connection. Beside concerns about the IMC layer, the UBM (under bump metallization) consumption by the solder has to be addressed as well. Therefore, it is important to select the right UBM and solder to have enough Sn and UBM left in the joint for the time the product is working at a specific temperature [1].

PurposeTo report on research on the alternative surface finish “direct gold on copper”, including reaction mechanism, methods of deposition and end uses.Design/methodology/approachExamines the deposition reaction of the electroless flash gold plating bath, and the effects of the copper surface roughness and deposition time on the deposit and solderability characteristics.FindingsDirect immersion gold is only partially immersion and mostly electroless in deposition mode. The surface is applicable to soldering for both leaded solder and lead‐free solders. The surface is also wire bondable.Originality/valueThe paper offers details of a new alternative surface finish for use in printed circuit board fabrication as well as in packaging applications. The paper shows the electroless deposition mode of the process. The finish is ideally suited where Rf losses must be minimized. It is suitable for soldering as well as for wire bonding.