Application of Laser Assisted Soldering for Replacing Massive Reflow in Solder Ball Mounting Process

This study analyzes the feasibility of introducing laser assisted soldering (LAS) into solder ball mounting (SBM) process. The vertical cavity surface emitting laser (VcSEL) adopted in this LAS system radiates an infrared (IR) laser beam with a wavelength of 980 nm over a 240 mm × 95 mm area. The workable range of LAS applicable to SBM process has been determined by varying laser power and radiating time. Among the tested conditions, the most favorable condition is 12,000 W for 1.5 s. Before SBM, the average warpage of packages used in this experiment was 240.3 μm. The average warpage of them has increased to 271.9 μm after SBM using LAS, but it has increased to 350.4 μm after SBM using massive reflow (MR). For LAS, X-section result of solder ball showed that the intermetallic compound (IMC) layer between the ENEPIG pad and the solder was undetectable by scanning electron microscopy (SEM) inspection, but for MR, the IMC layer thickness was thicker than 2 μm. The reason why the solder balls mounted with LAS have 11.9% stronger ball shear strength than that with MR is expected to be because IMC layer was not detected. Temperature cycle (TC) test results also showed that solder balls mounted by using LAS had thinner IMC than MR even after 1000 cycles of TC test. Therefore, LAS can be considered as a good alternative to MR in SBM process because it has advantages such as improved warpages, bond properties, and shorter process time.