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Simplification of the Nanosilver Sintering Process for Bonding Large-Area Semi-conductor Chips: Reduction of Hot-Pressing Temperature below 200°C

Fig. 1. Relationship between die-shear strength and pressure of drying for 10 10 mm2 large area die-attachments. Inset figures are SEM images showing the microstructures of sintered silver under corresponding drying pressures.
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 larger than 1 cm2, an external pressure of a few Mpa's is reported necessary at the sintering temperature of about 250°C. This hot-pressing process in excess of 200°C can add significant complexity and costs to manufacture and maintenance. In this study, we ran a fractional factorial design of experiments aimed at lowering the temperature at which pressure is required for the die-attach process. In particular, we examined the feasibility of applying pressure during the drying stage of the process at temperatures below 180°C followed by the sintering stage without pressure. The experiments helped 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. Furthermore, the positive effect of pressure applied during drying on the bonding quality was demonstrated. The relationships between die-shear stress, microstructure of sintered silver joint and applied pressure during drying stage are shown in Fig. 1. Based on the results, a simpler process, consisting of pressure-drying at 180°C under 3 MPa pressure, followed by sintering at 275°C under atmospheric pressure was found to produce attachments with die-shear strengths in excess of 30 MPa.