Transient-Liquid-Phase Bonding of Granulated Cu–Sn Bumps With a 4-μm Fine Pitch

It is difficult for Cu–Sn transient-liquid-phase (TLP) bonding to achieve fine bump pitches because liquid Sn extrudes laterally under bonding pressure and may cause electrical short of adjacent bumps. To solve this problem, this article reports a new Cu–Sn bonding method that uses granulated Sn layers instead of conventional continuous Sn layers. To form Sn granules, a reflow method is developed by exploiting a new phenomenon that sequential oxidation and reduction of melted Sn changes it to granulated structures. During Cu–Sn bonding, the granulated Sn allows fast Cu diffusion and rapid Cu–Sn reaction, which consumes Sn quickly and avoids liquid Sn extrusion. The Cu–Sn reaction forms intermetal compounds (IMCs) with a coral-like structure and large porosity, further alleviating the Sn extrusion by accommodating the melted Sn. Using this method, fine-pitch Cu–Sn bump bonding with a
$2.9~\mu \text{m}$
diameter and a
$4~\mu \text{m}$
pitch has been successfully achieved with high bonding yield and low resistivity.