Bonding of 5 μm Cu-Sn Micro Bumps Using Thermal Reflow and Solid-State Pre-Bonding

Copper–tin (Cu–Sn) transient liquid-phase (TLP) bonding has attractive advantages and has been widely used in three-dimensional (3-D) integration to establish electrical connections between stacked chips. However, TLP bonding has technical difficulties in bonding Cu–Sn bumps with small diameters and fine pitches due to the bonding pressure-induced lateral extrusion of the melted Sn layer. As bonding of micro bumps with diameters smaller than 5
$\mu \text{m}$
is highly desired in future applications, this article reports the TLP bonding of micro bumps using improved reflow and pre-bonding. Due to the large surface-to-volume ratio of micro bumps, the reflow conditions dramatically influence the reflow characteristics, and systematical investigations are performed such that an Sn layer with an optimal thickness has been obtained to avoid lateral extrusion. A new bonding scheme has been developed by combining a solid-state pre-bonding and a TLP final bonding. The new bonding method and the Sn thickness control reflow avoid liquid Sn extrusion under bonding pressure. Using these techniques, Cu–Sn bonding of bumps with a diameter down to 5
$\mu \text{m}$
has been successfully achieved with a high bonding yield.