Shinji Fukuda

Defects in nickel plating layers on copper-metallized substrates induced by thermal cycles

The reliability of electronic substrates at high temperature is a significant issue for high-power semiconductor modules. To improve the reliability, it is essential to understand and reduce thermal-cycling-induced surface roughening of metal layers on the substrates. We observed the surfaces and cross sections of nickel plating layers on copper-metallized silicon nitride substrates by active metal brazing after the substrates were subjected to thermal cycles of −40 to 250°C. No cracks were observed on the surfaces of the nickel layers on substrates that were subjected to 100 and 200 thermal cycles. However, voids or cavities were observed just under the nickel surfaces on those substrates. We considered that those voids or cavities were formed by thermal stress induced by thermal cycling and that they could be one of the origins of cracks in the nickel layers.

Effect of mechanical properties of the ceramic substrate on the thermal fatigue of Cu metallized ceramic substrates

The effects of temperature cycling from −40°C to 250°C on the active metal brazing (AMB) substrates were investigated using silicon nitride ceramics or aluminum nitride ceramics with different fracture toughness and strength. Visual inspection of the substrates after 1000 cycles hardly detected failure in the Si<inf>3</inf>N<inf>4</inf>-AMB substrates with the high fracture toughness of 8.0 MPa·m^1/2. By contrast, the Si<inf>3</inf>N<inf>4</inf>-AMB substrates with the lower fracture toughness of ca. 5 MPa·m^1/2 exhibited delamination of the Cu layer at only 20 or 50 thermal cycles. Detachment of the Cu layer occurred at seven cycles when the AlN-AMB substrates with fracture toughness of 3.2 MPa·m^1/2 were employed. It was found that the endurance to thermal fatigue of AMB substrates was controlled by the fracture toughness of ceramic substrates rather than the fracture strength.

Residual stress in copper paste films on alumina substrates

The mechanical properties of metal conductor layers strongly influence the reliability of high-power electrical modules. In this study, the microstructure, elastic modulus, and residual stress during temperature cycling of screen-printed sintered paste films were evaluated to develop guidelines for designing metal conductor layers to the module. The number of pores decreased and the elastic modulus increased for paste films sintered at higher temperatures. These films deformed plastically at lower temperatures when heated from room temperature; those that had been sintered at the highest temperature of 800 °C showed the highest maximum compressive stress, which was still approximately one third smaller than that of copper electroplated films. All films developed creep deformation above 200 °C during both heating and cooling processes. The substrate under the film was considered to affect the residual stress in the elastic-deformation area owing to its coefficient of linear thermal expansion and to not affect the residual stress in the creep-deformation area.

Highly thermal-fatigue resistant Si 3 N 4 substrates with excellent mechanical and thermal properties

Thermal fatigue of active metal brazing (AMB) substrates was investigated under severe heat cycle condition from −40°C to 250°C using developed silicon nitride ceramics (Si<inf>3</inf>N<inf>4</inf>) with high thermal conductivity and excellent fracture toughness as well as conventional Si<inf>3</inf>N<inf>4</inf> and aluminum nitrides (AIN). No peeling off of copper layer was observed even after 1000 cycles for the AMB substrates with various Si<inf>3</inf>N<inf>4</inf>, whereas the AIN-AMB substrates exhibited copper delamination only after 50 cycles. Bending strengths of the AMB substrates with conventional Si<inf>3</inf>N<inf>4</inf> decreased gradually with thermal cycles. By contrast, degradation in the bending strength of the AMB substrates with developed Si<inf>3</inf>N<inf>4</inf> was negligible. It was found that the developed Si<inf>3</inf>N<inf>4</inf>-AMB substrate with superior mechanical and thermal performance could endure harsh thermal environments.

Effects of copper particle size on creep deformation in copper paste films

Copper paste films develop creep deformation above 200 or 250 °C similar to copper sputtered and electroplated films. We evaluate the effects of copper particle size on the creep strain rate in the paste films. As the copper particle size changed, differences in the residual stress clearly appeared above 400 °C. For small particle sizes, the compressive stress reached 0 MPa faster during heating and the tensile stress increased more slowly during cooling above 400 °C. This result indicated that the creep strain rate of copper paste films became increased when the particle size decreased.