IEEE COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY SOCIETY

This talk will review the soldering process, with flux and without flux, followed by solid-state bonding and flip-chip interconnects using pure silver (Ag) columns or joints. In nearly every soldering process in electronic industry, flux, acidic rosin, is used to remove oxides in order to achieve good bonding. Fluxless bonding process, that does not need flux, is made possible by proper design of bonding media and environment. The Ag-In system will be presented to demonstrate the fluxless principle. In contrast with such soldering process, solid state bonding does not involve any molten phase. Ag is chosen for this study because it has the highest electrical conductivity and thermal conductivity among all metals. It is shown that Ag can be bonded directly to copper (Cu) at 250°C. It can also be bonded to gold. Using Ag as bonding medium, Si chips coated with thin Cr/Au have been bonded to Cu substrates. The resulting structure looks like Si/Cr/Au/Ag/Cu. At present, nearly all large Si chips are packaged in flip-chip configuration, with solder bumps connecting the active surface of the chip to the packaging substrate. These bumps are made of Tin(Sn)-rich solder. When the bump diameter shrinks down to 50μm, many problems, such as intermetallic growth, bump bridging, resistance, etc., may arise. These are caused by fundamental limitations and cannot be overcome by process modifications. So, we turned to pure Ag and have demonstrated 15μm Ag interconnect columns between Si chips and Cu with a pitch of 40μm. This presentation will discuss ten potential advantages of the Ag flip-chip technology, when compared to the popular solder-based flip-chip interconnects, in detail.