Cu-(Sn-Bi) SLID Bond Microstructure Under Different Temperatures

Bi is introduced to a common Cu-Sn metal system for solid-liquid interdiffusion (SLID) bonding, allowing for lower process temperature. Cu-(Sn-Bi) SLID bonds were realized by using a eutectic Sn-Bi sandwiched between Cu layers. The microstructure of the bonds at different temperatures was studied to understand the behavior of the metal system and the role of Bi. Cross-sectional microscopy using scanning electron microscope and energy dispersive X-ray analysis was performed to identify the microstructure of different bonds. The results showed a higher concentration of Sn at the edges and more Bi concentration in the middle of the chip. The bond-lines generally had a layered structure of Cu / Cu-Sn IMC / Cu, with pockets of Bi enclosed by Cu-Sn bridges connecting the top and bottom Cu layers. The samples bonded at the highest temperature, 300 degrees C, showed a dissimilar structure, with Bi inclusions in the Cu layers.