Toshiaki Morita

Study of Bonding Technology Using Silver Nanoparticles

We investigated a new bonding technique utilizing nano-scaled particles for use in high-temperature environments. The results of our investigations revealed that the method could be used to form bonds by simultaneously applying heat and pressure. Moreover, compared to a conventional Pb–5Sn-solder bond, a nanoparticle-based bond suffered no degradation in bonding strength over an elevated-temperature holding period of 1000 h at 250 °C, and its discharge characteristics were improved (i.e., increased) threefold. It is possible to extend this bonding technique to mounting components in devices that operate in high-temperature environments, e.g., it can be used to mount components such as silicon carbide (SiC) devices, which are expected to be applied in environments with temperatures exceeding 250 °C.

A novel metal-to-metal bonding process through in-situ formation of Ag nanoparticles using Ag2O microparticles

The metal-to-metal bonding has been successfully achieved via the bonding process using Ag metallo-organic nanoparticles at a bonding temperature of around 300-, which can be alternative to the current microsoldering in electronics assembly using high-temperature solders. However, further reduction of bonding temperature and/or bonding pressure is needed. In the present research, a novel bonding process through in-situ formation of Ag nanoparticles instead of the filler material of the Ag metallo-organic nanoparticles has been developed. The Ag nanoparticles can form by the reduction of Ag2O particles. In this study, the Ag2O particles were mixed with triethylene glycol as a reducing agent to form a paste for bonding. The Au coated cylindrical specimens were bonded using the paste. The Ag nanoparticles formed at around 130 to 160 through the reduction process of Ag2O particles with triethylene glycol. The Ag nanoparticles were immediately sintered each other due to a great surface energy per volume. A transmission electron microscope observation revealed that the sintered Ag metallurgically bonded to the Au substrate at around 160 and a dense Ag layer formed after further heating. The tensile strength of the joint bonded at 250 under a bonding pressure of 5MPa was around 60MPa

Low-Temperature Bonding Using Silver Nanoparticles Stabilized by Short-Chain Alkylamines

In this paper, we report on low temperature bonding using silver nanoparticles stabilized by short-chain alkylamines with different numbers carbon between 8 to 12. The bonding can be achieved by simultaneous heating and pressurization. The shear strength increased as the decomposition temperature of the stabilizer decreased. In the case of silver nanoparticles stabilized with n-octylamine (having the lowest decomposition temperature among the three stabilizers), the shear strength was 24 MPa for bonding at 400 °C. The difference in the shear strength is probably related to the amount of residual organic material in the sintering layer; the residual organic material would hinder the sintering of the silver nanoparticles, which was suggested by thermogravimetric analysis (TGA) results.

Effects of Solvents in the Polyethylene Glycol Serieson the Bonding of Copper Joints Using Ag2O Paste

The effects of reducing solvents on the bonding process using silver oxide paste in a copper joint were investigated. Three solvent types were tested: diethylene glycol (DEG), triethylene glycol (TEG), and polyethylene glycol (PEG). The strength of the joints was assessed by fracturing, which occurred at the interface of the copper oxide layer and the copper substrate in DEG and TEG samples and at the bonded interface in the PEG sample. Analysis of the samples revealed that, in the DEG and TEG samples, the copper substrate was oxidized during the bonding process, which compromised the shear strength of the joints. In contrast, the PEG sample exhibited nonuniform sintering of the silver layer while retaining good shear strength. It was found that the combination of DEG and PEG produced optimum shear strength in the copper joint, as PEG suppressed the growth of copper oxide and DEG promoted the formation of a dense sintered silver layer. The bonding strength achieved was higher than that of the gold-to-gold joint made using standard Pb-5Sn solder.

Evaluation of Copper Oxide-Based Interconnecting Materials

Copper (I) oxide (Cu2O) and copper (II) oxide (Cu2O2) were investigated for their bondability as an interconnecting material. It was found that copper (II) oxide is more suitable than copper (I) oxide because the former is reduced at a lower temperature (345oC) when molecular hydrogen is used as a reducing agent. A combined analysis of theoretically-calculated reduction paths, thermogravimetric/differential thermal analysis (TG-DTA) curves, and shear test results revealed that lower activation energy and higher stabilization energy for the reduction contribute to better mechanical and thermal properties of an interconnection.

Metal–metal bonding process using copper oxide nanoparticles

Abstract This work performs metal–metal bonding using CuO nanoparticles prepared with salt base reaction in aqueous solution. A colloid solution of CuO nanoparticles was prepared by mixing Cu(NO3)2 aqueous solution and NaOH aqueous solution. Submicrometre sized leaf-like aggregates composed of CuO nanoparticles were produced at a Na/Cu ratio of 1·7 and at 20°C, though Cu2(OH)3NO3 was also obtained. An aging process, which is a process composed of preparation of the particles at 20°C and then aging them at 80°C, provided transformation from Cu2(OH)3NO3 to CuO with no damage of the leaf structure. The shear strength, which was required for separating discs bonded using the particles as a filler at 400°C in H2 gas, was 32·5 MPa at the maximum for the particles prepared at the Na/Cu ratio of 1·7 with the aging process. These results indicated that the formation of leaf-like aggregates of CuO particles with high purity led to efficient metal–metal bonding.

Reliability of sintered silver layer obtained using silver-oxide paste in power cycling test

To investigate the reliability of a sintered silver bonding layer obtained using silver-oxide paste, a power cycling test was performed. The module obtained using silver-oxide paste achieved 73,400 power cycles in a test with Tjmax = 150 °C (ΔTj = 120 °C), while a soldered (Pb3.5Sn1.5Ag) power module failed at 24,600 cycles. After the cycling test, a crack was observed in the Pb3.5Sn1.5Ag solder layer but not in the sintered silver layer. These results reveal that the sintered silver layer obtained using silver-oxide paste can treble the lifetime of a power module.

A Void Free Soldering Process in Large-Area, High Power Insulated Gate Bipolar Transistor Modules

We have developed a new void free process for making the solder joint between the chip mounted AlN substrate and the metal substrate in large-area, high power insulated gate bipolar transistor (IGBT) modules. This new process consists of two steps. First, Ar+ were used to clean the surfaces of Ni plated film on a metal and AlN substrates which were then coated with 0.5-µm-thick Ag film. Second, 50 wt% Pb–Sn solder was sandwiched between the two substrates and heated to 503 K in a vacuum for 5 min before being cooled in a N2 atmosphere. By using this process, the area percentage of voids in a soldering area up to 130×190 mm2 can be reduced to less than 0.1%. IGBT modules made by this process were also found to exhibit satisfactory current-voltage characteristics.

Low Temperature Sintering Bonding Process Using Ag Nanoparticles Derived from Ag2O for Packaging of High-Temperature Electronics

A novel bonding process using Ag2O paste composed of Ag2O particles and a reducing agent has been proposed as a Pb-free alternative of high melting point solders in electronics packaging. Ag2O paste formed Ag nanoparticles through the redox reaction in the bonding process and in-situ formed Ag nanoparticles sintered immediately. While the bonding process using Ag metallo-organic nanoparticles, which have been proposed, was unfavorable to the bonding at 250 degree Celsius or lower in terms of requiring removal of stable organic shells, the bonding process using Ag2O paste demonstrated the possibility of further low-temperature bonding.

New Method for Estimating Impact Strength of Solder-Ball-Bonded Interfaces in Semiconductor Packages

A method for estimating the impact strength of a solder ball junction in encapsulated semiconductor packages was developed. This method reveals the impact force in a solder ball unit and the time required for a solder ball to break. The bombardment absorbed energy required for the ball to break is evaluated as impact strength. This method can be used to quantitatively evaluate the impact strength of solder ball junctions, which is difficult to do using conventional shear and tensile strength estimation methods.