The invention relates to a method of fabricating a flexible-rigid PCB which includes a flexible circuit substrate and a rigid circuit substrate. The flexible circuit substrate defines a rigid region and an exposed region and has a conductive pattern, such as conductive traces, formed on the exposed region. The method includes the steps of providing the flexible circuit substrate; printing a paste containing epoxy-silicone hybrid materials onto the conductive pattern; curing the paste; and building up the rigid circuit substrate on the rigid region of the flexible circuit substrate. Particularly, the paste having a specific composition is subjected to predetermined conditions of temperature and time in order to transform the paste into a peelable mask with heat resistance, chemical resistance and a contact angle greater than 20 degrees.