T. Sugahara

Low-pressure sintering bonding with Cu and CuO flake paste for power devices

Low-temperature sintering bonding has been proposed as an alternative technique for the soldering to overcome such high operating temperature in wide-gap semiconductor power devices. Ag nanoparticle sintering is one of the candidates in die-attach bonding, but there are certain obstacles for mass production mainly due to the high cost of silver. In addition, metal nano-particle paste including Ag nanoparticle paste bonding needs to apply certain high pressure of MPa order. For mass productions, it is necessary to decrease the applying pressure during the bonding process. In the present study, the authors make flake-shaped Cu based particles by using mechanical milling for improving the contact area between the particles to decrease the required pressure. The die-bonding with Cu flake pastes was carried out at 300 °C with a formic acid. Resulting die-shear strength exceeds 15 MPa for bonded at 300 °C for 60 minutes low pressure (0.4 MPa). Moreover, Cu flake pastes with polyethylene glycol (PEG) solvent showed solid interface layer like bulk Cu. Thus, the Cu flake PEG paste is one of the most promising bonding materials with the remarkably high strength of the sintered bonding.

Effects of additional Ni and Co on microstructural evolution in Sn-Ag-Bi-In solder under current stressing

With continuously shrinking process rules of electronic devices, the decreasing dimension of interconnections results in increasing electrical current density, and recalls the risk of massive electromigration (EM) of metal elements in solder joints. This paper reports that minor additions of Ni and/or Co to Sn-Ag-Bi-In lead-free solder to improve the EM behavior and microstructural evolutions in the solder material subjected to high electric current density of 10 kA/cm2 over 200 hours. With additional Co elements, EM resistance of solder joint was improved as twice before with microstructural evolution by it.

Silver sinter joining and stress migration bonding for WBG die-attach

Power electronics play an important role in the generation-storage-distribution conversion cycle of the electric energy. Wide band gap semiconductors such as SiC and GaN have attracted many researchers recently because both of their excellent energy conversion efficiency and of capability of device downsizing. The performance of interconnection, especially die-attach, has one of the essential roles for achieving high performance. Among various proposals for die-attach materials and processes, Ag sinter joining is a promising method for power semiconductors as well as for power LEDs, providing excellent heat-resistance to achieve stable joint structures beyond 200 °C. This paper summarizes the present status of the Ag sinter joining and of the new bonding approach with Ag films.

Silver sinter joining for WBG die-attach

Power electronics with wide band gap (WBG) semiconductors have been expected to play an important role in the generation-storage-distribution conversion cycle of the electric energy. The performance of interconnection, especially die-attach, has one of the essential technologies for achieving high performance of WBGs. Among various proposals for die-attach materials and processes, Ag sinter joining is becoming a promising method for power semiconductors, providing excellent heat-resistance to achieve stable joint structures beyond 200 °C. This paper summarizes the present status of the Ag sinter joining including the new bonding approach with Ag films bonded both at low temperature and at low pressure.

High temperature SiC power device realized by electroless plating diffusion barrier for Ag sinter die-attach

Ag sinter die-attach is utilized for bonding SiC Schottky-barrier diode (SBD) dies on Cu lead frames metalized by Ni/Pd/Pt/Ag electroless plating. After Al wiring, the assembled structure was mold-packaged by imide-based high temperature thermosetting resin. The produced devices are then subjected to environmental tests of high temperature storage at 250°C, and of thermal cycling between −50°C and 250°C. The metallization layers at the bond interface remain unchanged after the harsh reliability tests because of the underlying Pt diffusion barrier layer, proving thermal stability of the bond structure up to 250°C.

Ag sinter joining and wiring for high power electronics

Sinter joining with Ag particle pastes has attracted much attention for wide band gap power assembly that requires high power and high temperature interconnection technology. The micron/submicron Ag particle hybrid paste, which is affordable with excellent performance, is one of the promising materials. Bonding conditions are very much mild such as 250 °C, low pressure less than 1 MPa and air atmosphere. The metallization layer structure was evaluated by changing interlayer structures. The SiC die-attached on a DBC structure (Cu/Si3N4/Cu) with the Ag hybrid paste exhibits excellent stability at 250 °C up to 1,000 hours in air, when a suitable layer structure was selected. Shear strength over 30 MPa was maintained. It was found that the interfaces both of a Ag layer and DBC and of SiC die and a Ag layer must be protected to prevent severe oxidation. A barrier layer of Ti was found to be effective to stop oxidation of the metallization such as Cu and Ni. Severe thermal cycling between −40 °C and 250 °C decreased strength while good strength beyond 10 MPa was maintained up to 1,000 cycles in air. The Ag hybrid paste can be also applied for wiring instead of Al or Cu wire-bonding. 3D printing of Ag hybrid paste provides heat-resistant wiring for SiC assembly. Microstructures and selected properties will be demonstrated.

Pressure-less plasma sintering of Cu paste for SiC die-attach of high-temperature power device manufacturing

Pressure-less sintering of Cu flake paste is achieved assisted by hydrogen plasma process toward die-attach technique of next-generation high-temperature power semiconductor devices. The sintered paste shows high bond strength aver 50 MPa, showing large abnormal grain growth at the bonding interface, with homogeneously distributed void of porous interconnection layer. Our results indicate that still the reduction of the surface oxide of Cu flake powders, and thus both the metal powder synthesis and the bonding process must be optimized to achieve a sound bonding interface.

Oxidation resistance and joining properties of Cr-doped Zn bonding for SiC die-attachment

Pure Zn exhibits excellent properties for high temperature lead-free solder such as the high melting point (419.5°C), surpassing tensile strength, high thermal and electrical conductivity, in addition to the reasonable cost. However, the brittleness and oxidation of pure Zn can potentially cause a reliability problem, when it is employed at high operation temperature expected for SiC power devices. In this study, the effects of minor Cr-doping in pure Zn die-attach is investigated, particularly on the enhanced reliability due to the improved joining microstructure and anti-oxidation at high temperature. As-cast Cr-doped Zn alloy indicates finer grain size in the microstructure than as-cast pure Zn, implying higher oxidation resistance as well. To evaluate the high temperature reliability of the Cr-doped Zn solder, the growth of intermetallic compound (IMC) at the interface with Cu substrate has been investigated at soldering and also during subsequent thermal aging. The aging test at 250 °C for 100 h the joining sample has proved no crack developed at Zn-Cr alloy/Cu interface, while a considerable number of cracks have observed inside the γ-Cu5Zn8 phase grown at pure Zn/Cu interface. The minor Cr addition thus remarkably suppresses IMC growth at the interface, and significantly improves the reliability of pure Zn die-attach method.

Heavy Ribbon Wire Bonding for Advanced Power Module Packages

Metal ribbon wiring attracts much attention for next-generation power-electronics interconnection technology, which requires wider capacity of electrical current in smaller package. The bonding methods of metal ribbons are to be optimized suitable for the larger bonding area than conventional thin string wires, and also for the more harsh operating conditions achieved by wide-gap semiconductors. We here use ultrasonic bonding for Al ribbon (1500×200 μm) on to electroless Ni immersion gold (ENIG) finished cupper substrate, and optimize the bonding process parameters to minimize the heat damage with sufficient bonding strength. Aging tests of the bonded specimens at 200 °C are ongoing, together with the interface microstructure observations.

Electromigration effect on mechanical shock behavior of Sn-Ag-Bi-In + Co solder joints for surface-mounted chip components

Mechanical shock behavior of SABI and SABI+Co has been investigated with Charpy impact tests after subjected to high current stress. Co addition plays a role as a refiner in the SABI solder system, and the joints with the refined solder exhibit 30% higher mechanical strength than those with original SABI. After 80 hours of 20 A current stresses applied at 160 o C, the fracture energy in Charpy tests decrease about 30% for SABI, while, SABI+Co keeps almost the same fracture energy. The refinement in solder materials useful for enhancing EM life time, is thus shown to improve the mechanical shock resistivity.