The study of the stability of Pd/PVP nanoparticles added with phosphoric acid and the activity to electroless Cu deposition

Palladium nanoparticles were synthesized simply by reducing Pd ions which were attracted to electron nitrogen atom in poly(N-vinyl-2-pyrrolidone) (PVP). This Pd/PVP aqueous system was developed as the activator for electroless copper deposition. Compared with commercial Pd/Sn colloid that was easily oxidized by dissolved oxygen and agglomerated in the solution, Pd/PVP activator was stable without any Pd aggregation for a long time. Pd/PVP activator showed high catalytic activity as Pd/Sn colloid on flat FR-4 substrate (glass fiber reinforced epoxy). From back-light test for printed-through-hole (PTH) process, we found that micro-etching process would reduce catalytic activity of Pd/PVP activator and voids in PTH occurred especially on glass fiber. Adding phosphoric acid to Pd/PVP activator could improve back-light performance, but Pd nanoparticles precipitated in a few days. In this study, we found that H3PO4 molecule was the cause of Pd agglomeration by forming hydrogen bond with PVP. Pd nanoparticles would precipitate if the concentration of H3PO4 were high in the solution. IR spectra and UV-vis spectra proved that Pd/PVP activator would react with H3PO4 molecules to form a complex by hydrogen bond, and DLS analysis also showed that Pd/PVP/H3PO4 nanoparticles formed a larger hydrolysis cluster than Pd/PVP nanoparticles. TEM images gave the information about particle size and shape of Pd nanoparticles, and more information about dispersion and distance of Pd nanoparticles and Pd clusters could be obtained by the model fitting of SAXS data. The results showed that Pd/PVP/H3PO4 nanoparticles formed a looser structure than Pd/PVP nanoparticles. Since there is higher Cu deposition on epoxy when we use Pd/PVP/H3PO4 nanoparticles as activator, in PTH process, Cu deposition on glass fiber is improved by Cu deposition on epoxy nearby. So back-light performance become acceptable for PCBs industry.