Electroless plating of a 5G copper antenna on polyimide patterned with laser-induced selective activation and curing of metal–organic catalyst

An improved electroless copper plating method for fabricating 5G/Sub-6 GHz antennas is reported. A catalyst, palladium ion/4-vinylpyridine (Pd2+/4VP) complex, which not only initiated electroless copper plating but also promoted the adhesion of the copper layer, was dip-coated onto a polyimide (PI) substrate. Selective curing and activation of the Pd2+/4VP complex catalyst were induced by a pulsed UV laser. Wherever the laser scanned on the PI substrate, the palladium ions were reduced to palladium atoms, and the free radicals created from the decomposition of tert-butyl peroxybenzoate (TBPB) initiated the copolymerization of 4VP and PI, which enhanced the adhesion of the catalyst on the substrate. The unreacted Pd2+/4VP was removed by ethanol and collected for reuse, hence a much-reduced material cost. By adjusting the laser power, scanning speed, defocusing distance, and electroless plating time, the optimized process condition for Cu circuit fabrication without damaging the PI substrate was found. A high-quality copper circuit pattern was achieved with a laser power of 100 mW, a scanning speed of 330 mm/s, a defocusing distance of 2.0 mm, and an electroless plating time of 40 min. The copper patterns exhibited excellent adhesion and maintained good conductance after the bending fatigue tests. This laser treatment approach is simple, cost-effective, and applicable to non-planar 3D substrates, and the usefulness of this method was demonstrated by fabricating a 5G/Sub-6 GHz antenna with good device performances.