Barium Strontium Titanate Overlay Capacitor Fabrication on Printed Circuit Board

A technique has been developed for making varactors in the form of parallel plate overlay capacitors, using barium strontium titanate (BST) polymer composite thick film. The BST-polymer composite film recipes were developed using commercial powder and two types of polymer. An existing recipe using cyclic olefin copolymer (COC) was adapted for spray coating, while a new recipe based on poly methyl methacrylate (PMMA) was designed for spray coating as well. The use of standard air brush to deposit BST-polymer composite, and the planarization of printed circuit board (PCB) using soldermask allows for film deposition which has not previously been reported. A bandpass filter on PCB was designed for high frequency measurements and BST composite material electrical characterization. The measured tunability of 3% was achieved for the BST/COC composite overlay capacitor on PCB filter and 5% for the BST/PMMA filter.