Effect of plasma cleaning on fluxless plasma soldering of Pb-free solder balls on Si-wafer

An Ar-10vol%H-2 plasma was applied as a cleaning medium on UBMs (Under Bump Metallizations) to improve the bondability of solder balls prior to plasma reflow. Each UBM area comprised four layers deposited on a Si-wafer. Sequentially, the deposit comprised 0.4 mum of Al, 4 mum each of Ni and Cu, and 20 nm of Au from bottom to top of the metallization. Two compositions of lead-free solder balls (Sn-3.5 mass%Ag and Sn-3.5 mass%Ag-0.7 mass%Cu) and a lead containing one (Sn-37 mass%Pb) as a reference were selected for the experiment. The solder balls, of 500 pm diameter, were placed on the UBM's and fluxlessly soldered under Ar-10%H-2 plasma (with or without prior plasma cleaning). Additionally, air reflow with flux was also performed for comparison. Experimental results showed that the spreading ratios of the solders by plasma reflow after plasma cleaning were 20-40% higher than those plasma reflowed without cleaning. The shear strengths of the solder balls processed by the plasma-cleaned plasma reflow showed around 58-65 MPa, which is 60-80% and 15-35% higher than that of plasma reflow without plasma cleaning, and that of fluxed air reflow, respectively. From this study, plasma cleaning of UBM's using Ar-10%H-2 gas was shown to be quite effective to improve the bond strength of solder balls.