Bonding and reliability assessment of 30 μm pitch solder micro bump interconnection with various UBM structure for 3D chip stacking

With the increased demand of multifunction in electronic device, downscaling of interconnection pitch presents an important role for the next generation electronics with high performance, small form factor, low cost and heterogeneous integration. In the current types of interconnects, solder micro bumps have received much attention due to its low cost of material and process. For fine pitch solder micro bump interconnections, selection of under bump metallurgical material is a crucial issue because the solder micro bump joints with different kinds of UBM material will present varied reliability performances. However, which structure of solder micro bump joint shows the better reliability properties is not concluded yet until now. In this study, three-dimensional (3D) chip stacking using 30μm pitch interconnects with lead-free solder bumps and two types of UBM material is described. The reliability of solder micro bump interconnection with varied UBM material is also discussed. Assembly of the chip-on-chip test vehicle with a micro bumps diameter of 18 μm and a pitch of 30 μm was conducted. There were more than 3000 micro bumps with Sn2.5Ag solder material on both the silicon chip and carrier. Two kinds of UBM layer on Si chip were selected in this study: one was single copper layer with a thickness of 8 μm and the other was Cu/Ni layer with a total thickness of 8 μm. The UBM was electro-plated on Al trace and then the Sn2.5Ag solder with a thickness of 5 μm was deposited. During bonding process, the micro joints were formed at a peak temperature of 250 °C and the microgaps between chips were then filled by a capillary underfill cured at 150°C for 30 min. In this study, we evaluate the effect of fluxless bonding on the joining ability of solder micro bumps. The influence of underfill on the reliability of solder micro bump interconnections was estimated also. Subsequently, the chip-stacking modules were inspected by an X-ray and a scanning acoustic microscope (SAM) to determine the quality of micro joints including bonding accuracy, formation of interconnections and the percentage of voids within the underfill. Afterwards, the moisture sensitivity level 3 pre-conditioning test and temperature cycle test for 1000 cycles were performed to evaluate the reliability of solder micro bump interconnects. The results of reliability test revealed that the introduction of underfill could apparently enhance the reliability performance of micro joint under mechanical evaluation.