A Bismuth-free In-containing Lower-Temperature Lead-Free Solder Paste for Wafer-Level Package Application that Outperforms SAC305

A mixed solder powder paste technology has been used to develop a bismuth-free indium-containing solder paste (MTS) for lower temperature soldering. The paste has been used in board-stacking bonding with a 200°C peak temperature profile for mobile phones [1]. The joint of the wafer-level package experienced high stress/strain during thermal cycling, which led to the joint cracking right beneath the interface joining the wafer bonding pad and solder. MTS was used to assemble a wafer-level package (WLP256, full-array, 0.25mm diameter SAC305 balls and 0.4mm pitch) with the 200 to 240°C peak temperature reflow profiles with a constant paste-to-ball ratio of 1:4. Hybrid joints were created using the minimum 200°C peak temperature, which had two distinct areas: (1) the mixing zone dominated by the In-containing paste on the PCB side and (2) the area immediately above the mixing zone which retained the initial SAC305 morphology. In is present in multiple forms within the mixing zone, including
$\text{InSn}4(\gamma)$
phase, Sn(In) solid solution, and
$\text{In}4\text{Ag}_{9}$
or
$\text{In}4\text{Ag}9/\text{Ag}_{3}\text{Sn}$
composite precipitates. The full-mixing homogeneous joint was formed by raising the peak temperature to
$210^{\circ}\mathrm{C}$
, yielding a similar microstructure to the traditional SAC305 joint of the networked Ag-containing precipitates embedded in the Sn matrix. The
$\text{InSn}4$
phase was not observed in the full-mixing homogeneous joint while Sn(In) solid solution and
$\text{In}4\text{Ag}_{9}$
or
$\text{In}4\text{Ag}9/\text{Ag}_{3}\text{Sn}$
composite precipitates remained. TCT (temperature cycling) was run under a −40/125°C temperature range and a 20-minute dwelling profile. Regardless the reflow profiles, MTS outperformed SAC305 in TCT by 11% or greater. The 210°C peak temperature profile resulted in a more than 30% improvement compared to the SAC305 paste. The joint failure was localized to the solder joint below the wafer-solder interface, similar to the SAC305 joint.