Laser Drilling & Plasma Descum Employed In The Process of Wafer-Level Chip Scale Package(WLCSP)

Abstract

Laser drilling is widely employed for PCB/FPCB production, especially for ABF/RDL substrates used for 5G telecommunication. Compared with the mechanical drilling which is usually used for the via diameter over 200um, laser drilling can fulfill the smaller via diameter amid 15um and 200um. Plasma descum and wet cleaning are the common post-processes after the laser drilling to obtain the better via quality by removing drilling residue and debris. But traditional nano-second/pico-second lasers used in laser drilling may not drill well on the protective layer or redistribution layer(RDL) of the wafer-level chip-scale-package(WLCSP). Challenges including: (1) Bottom metal layer damaged or insulation layer peeling from the metal layer attributed to the severe heat-affected-zone(HAZ). (2) Via diameter smaller than 30um is unachievable. (3) Positional error below 5um can’t be secured while drilling at the faster speed. This paper will demonstrate using the femto-second laser with the inherent feature of cold ablation to drill vias on the ABF layer and then carrying out the post-treatment of plasma descum to fulfill the requirements of higher taper angle and faster throughput. Advantages include: (1) Flesible drilling capability, via diameter is programmable in the range of 15um to 200um. (2) Undamaged and residue-free on the bottom metal layer. (3) Smooth via sidewall. (4) Continuous drilling can reach the speed of 3000 via/sec.