The use of surfactants and post supercritical CO₂ mixing on the electroless nickel metallization of blind holes in printed circuit board

Printed circuit board (PCB) with multi-layered circuit configuration has become an important technical practice in applications. The via between circuit layers play a key component role for the integration of the circuits. For further compaction of the circuit layers, the blind via hole would be an indispensible technique and deserve continuous breakthrough. During electroless nickel plating in blind hole, the blocked path renders the flow of electrolyte difficult inside the cavity. Hence, the replenishment of reaction reagents will be difficult without the bulk flow of electrolyte. The successful plating inside the blind via hole can be challenging. In this study, different types of surfactant were employed to reduce the surface tension of the electrolyte and improve its wetting inside the blind hole. In addition, an electroless plating using post supercritical CO₂ mixed electrolyte to further improve the process was also investigated. Blind holes in 800-qm thick PCB with three different sizes of 550, 250 and 150 qm were studied. Among the surfactants used in this study, the anionic surfactant sodium dodecyl sulfate (SDS) demonstrated the best improvement in blind hole plating while the cationic surfactant cetrimonium bromide (CTAB) caused the interruption of plating even in planar substrate. With the addition of 4 g/L SDS in the electrolyte, the hardness of the coating increased by 60% comparing with its pristine counterpart. From the composition measurement, no significant modification in alloying element content was observed. This hardness increase should be indebted to the presence of Ni₃P in the coating associated with the reduction of Ni upon the addition of SDS in the electrolyte. The results of mixing different surfactants and supercritical carbon dioxide in electroless plating solution exhibited the similar trend as that shown by conventional electroless plating solution. But with the addition of PEG, because this non-ionic surfactant helps the carbon dioxide gas dissolve into an electroless plating solution, it showed a better result. In the plating of blind hole, the use of SDS, particularly with concentration of 4g / L, demonstrated the best results and nearly 100% depth could be plated for three different sized holes. Compared with the conventional electroless plating, post supercritical process can have better performance in plating blind holes. When the supercritical carbon dioxide has been mixed, the plating bath has combined properties of gases and liquids. This feature reduces the surface tension of the bath, even in the atmospheric vent after some time, which renders the better plating performance.