Microstructure and shear strength on ultrasonic-assisted soldered the Si/Cu joint using Sn-3.5Ag solder

The thermal stress, resulting from the big difference of CTE between Si and Cu, could reduce the strength of joints. In this study, Silicon and Cu were soldered by a two-step process (pre-metallization and brazing), which could alleviate the stress. Firstly, a Al-Si metallization layer (consisting alpha-Al matrix and Si phase) was prepared on the surface of monocrystalline Si by using Al metal powder at 900 degrees C. Then the joint of pre-metallized Si and Cu was successfully achieved by ultrasonic-assisted soldering method with Sn-3.5Ag filler at 250 degrees C in air. The effects of ultrasonic time on microstructure and shear strength of the Si/Cu joints were investigated in details. It was found that the oxide layer was broken and gradually dissolved with the ultrasonic action, which was attributed to the high velocity (3055 m/s), high pressure (2.32 x108 Pa), and the high temperature (5700 K) generated by the cavitation bubble collapse. Besides, the Ag2Al compounds and Si phases at the solder/metal-lization layer interface were observed. And the Cu6Sn5 +Cu3Sn compounds were detected forming at the solder/ Cu interface. As the ultrasonic vibration duration time increased, Ag2Al compounds attached to the Al-Si layer gradually increased, and ultimately entered into the filler. Attentionly, the maximum strength of Si/Cu joint was 32.4 MPa with the ultrasonic time of 6 s, and the fracture occurred inside the monocrystalline Si. The results show that ultrasonic action affects the distribution of Ag2Al compound and Si phase, which has an important influence on the joint strength.