Power cycle reliability of Cu nanoparticle joints with mismatched coefficients of thermal expansion

The power cycle reliability of Cu nanoparticle joints between Al₂O₃ heater chips and different heat sinks (Cu-40 wt.%Mo, Al-45 wt.%SiC and pure Cu) was studied to explore the effect of varying the mismatch in the coefficient of thermal expansion (CTE) between the heater chip and the heat sink from 4.9 to 10.3 ppm/K. These joints were prepared under a hydrogen atmosphere by thermal treatment at 250, 300 and 350 °C using a pressure of 1 MPa, and all remained intact after 3000 cycles of 65/200 °C and 65/250 °C when the CTE mismatch was less than 7.3 ppm/K, despite vertical cracks forming in the sintered Cu. When the CTE mismatch was 10.3 ppm/K, the Cu nanoparticle joint created at 300 °C endured the power cycle tests, but the joint created at 250 °C broke by lateral cracks in the sintered Cu after 1000 cycles of 65/200 °C. The Cu nanoparticle joint created at 350 °C also broke by vertical cracks in the heater chip after 1000 cycles of 65/250 °C, suggesting that although sintered Cu can be strengthened to tolerate the stress by increasing the joint temperature, this eventually causes the weak and brittle chip to fracture through accumulated stress. The calculation results of stresses on the heater chip showed that the stress can be higher than the strength of Al₂O₃ when the CTE mismatch is 10.3 ppm/K and the Young's modulus of the sintered Cu is higher than 20 GPa, suggesting that the heater chip can be broken.