A Study on the Fluxless Soldering of Si-Wafer/Glass Substrate Using Sn-3.5 mass%Ag and Sn-37 mass%Pb Solder

Abstract

UBM-coated Si-wafer was fluxlessly soldered with glass substrate in N₂ atmosphere using plasma cleaning method. The bulk Sn-37 mass%Pb and Sn–3.5 mass%Ag solders were rolled to the sheet of 100 \\micron thickness in order to achieve bonding to Si-wafer by fluxless 1st reflow process. The oxide layer on the solder surface was analyzed by AES (Auger Electron Spectroscopy). After 1st reflow the Si-wafer with a solder disk was plasma-cleaned, and soldered to glass by 2nd reflow soldering process without flux in N₂ atmosphere. The thickness of oxide layer decreased with increasing plasma power and cleaning time. The optimum plasma treatment condition in this study was 500 W for 12 min and at this condition, 100% bonding ratio for Sn–3.5 Ag and over 80% bonding ratio for Sn–37Pb solder were achieved. The intermetallic compound of continuous Cu₆Sn₅ was observed along the Si-wafer/solder interface but discrete Cu₆Sn₅ along the glass/solder interface and the different shapes of Cu₆Sn₅ were caused by different thickness of Cu as a pad. The fracture of the tensile test specimen occurred at not only solder/UBM and solder/TSM interface but also in Si-wafer and glass substrate.