Temperature Cycling Reliability Assessment of Die Attachment on Bare Copper by Pressureless Nanosilver Sintering

A nanosilver-sintering-enabled die-bonding technique has been widely applied for attaching power semiconductor devices onto the surfaces of noble metals such as silver or gold. Recently, high bonding strength (> 35 MPa) of die attachment has been demonstrated by sintering on bare copper surface. In this work, the temperature cycling reliability of joints processed by nanosilver sintering on bare copper surface was evaluated. Joints of silicon dice on copper were fabricated by pressureless sintering of nanosilver paste in forming gas. All the joints survived 1000 temperature cycles in the temperature range of -40 °C to 125 °C and with a cycling period of 60 min. Die-shear test results showed that the average die-shear strength of joints had no significant drop after 1000 cycles and remained over 30 MPa. X-ray tomography and scanning electron microscopy (SEM) were applied to detect the cracks or delamination generated during cycling. X-ray images show no significant change on the delamination percentage of die-attach area during cycling. Cross-sectional SEM images of the joints show no delamination formed at the silver/copper interface after 1000 cycles. We believe that strong metallurgical bonding and low interdiffusion rate at the silver/copper interface contribute to such great temperature cycling reliability.