Simplification of the Nanosilver Sintering Process for Large-Area Semiconductor Chip Bonding: Reduction of Hot-Pressing Temperature Below 200

摘要

Die attach by low-temperature sintering of nanoparticles of silver is an emerging lead-free joining solution for electronic packaging because of the high thermal/electrical conductivity and high reliability of silver. For bonding small chips, the attachment can be achieved by a simple heating profile under atmospheric pressure. However, for bonding chips with an area ${>}{1}~{rm cm}^{{{2}}}$, an external pressure of a few MPa is reported necessary at the sintering temperature of ${sim}{250}^{circ}{rm C}$. This hot-pressing process in excess of 200$^{circ}{rm C}$ can add significant complexity and costs to manufacturing and maintenance. In this paper, we conduct a fractional factorial design of experiments aimed at lowering the temperature at which pressure is required for the die-attach process. In particular, we examine the feasibility of applying pressure only during the drying stage of the process when the temperature is still at 180$^{circ}{rm C}$. The experiments help to identify the importance and interaction of various processing parameters, such as pressure, temperature, and time, on the bonding strength and microstructure of sintered nanosilver joints. In addition, the positive effect of pressure applied during drying on the bonding quality is observed. With the results, a simpler process, consisting of pressure drying at 180$^{circ}{rm C}$ under 3 MPa pressure, followed by sintering at 275$^{circ}{rm C}$ under atmospheric pressure, is found to produce attachments with die-shear strengths in excess of 30 MPa.