Problems of PCB microvias filling by conductive paste

The using of conductive paste for the through holes and the blind holes filling in PCBs were analyzed. Such processes seem to be easy and can be applied instead of common used electrochemical plating process. The performed investigations have shown that these processes are not so simple.To understand more problems connected with microvias filling, the influence of adhesive rheology and printing parameters for the quality of small diameter through holes filling and small diameter blind vias were investigated. The performed analysis was supplemented by electrical measurements as well as observation of microvia cross sections. The FR-4 laminate test samples with through holes with 0.3, 0.5 and 0.8 mm diameters as well as blind holes diameters 0.15, 0.3 and 0.45 mm were used in experiments.The average single fills resistances in the range 50 mΩ for through hole fillings diameter 0.5 mm and below 20 mΩ for blind holes diameter 0.3 mm was obtained. The adhesive fill resistance is stable after dump heat and thermal cycling tests.