Exploring direct laser reflow techniques for forming stable and reliable solder bump interfaces on semiconductor substrates

Abstract:

The purpose of this study is to introduce and qualify a contactless laser assisted reflow (LAR) process for creating stable and reliable solder bump interfaces (C4) on semiconductor substrates offering a potential solution to minimize the CO₂ footprint of conventional oven-reflow processes. This work is a continuation of the studies by Fettke et al. [1] and focuses on evaluating the lifespan of the generated solder joints.

Silicon (Si) chips and flexible FR4 substrates were used as test vehicles to evaluate the effects of the laser assisted reflow process on a solder ball interface consisting of solder preforms made of SAC305 and Sn42Bi58. The 100 µm and 400 µm solder spheres were placed on the pads using a selective ball
dropping process. For performance comparison, reference samples were prepared and conventionally reflowed in an oven. After the reflow processes, samples underwent various analyses to evaluate and compare the soldering quality. 500-hour thermal cycling test was performed on the samples. Shear tests, X-ray, cross-sectional polishing, scanning electron microscopy (SEM) and optical microscopy were employed to examine the formation of the intermetallic compounds (IMCs) and the metallurgical properties of the solder bumps.

Finally, industrialization proposals, possible application and future prospects will be discussed.

Keywords — laser reflow, laser-assisted bonding, LAB, LAR, LCB, solder bump, solder joint, copper pillar