Plasma Treatment for Fluxless Flip-Chip Chip-Joining Process

Flip-Chip technology is a well-established solution to increase the number of connections between a chip and a PCB. Unfortunately, with large die and high bump density, flux residues cleaning is increasingly challenging. Fluxless soldering is becoming more attractive given that flux residues cleaning step can be avoided leading to a more environment friendly process while reducing water consumption and chemical waste. Hydrogen radicals are known as a reducing agent to remove metal oxide. We present here assembly tests performed in an industrial-like environment where a hydrogen-based plasma treatment is used to suppress bumps oxide in replacement of flux chemical. The plasma treatment is performed in a vacuum capacitively coupled plasma chamber with a gas mixture containing a percentage of hydrogen. We use large 20 x 20 mm2 chips and associated organic substrate, which bumps (80µm diameter and 185.6µm pitch) and pads are made of a tin-based lead-free solder. The plasma treatment is perform on both the chip and the substrate prior to assembly using furnace mass reflow. We have successfully demonstrated the assembly of several dies using a standard mass reflow furnace. In the idea of process industrialization, re-oxidation kinetic shown a process window of 48 hours between plasma treatment and chip-joining, as shown by chip-pull, optical microscopy inspection and deep thermal cycling reliability test.