Abstract: In the disclosed method of applying electrical insulation, a housing with a porous wall is placed around a conductor in spaced relationship thereto, and a slurry is pumped into the space between the housing and the conductor. This slurry comprises a liquid and particles suspended in the liquid having good electrical insulating properties. The liquid component is allowed to discharge from the housing via its pores, but the solid particles are filtered out within the housing so as to build up from the internal surface of the housing a deposit of the particles. This pumping and filtering action is continued until the space between the housing and the conductor is substantially filled with the deposit.

Abstract: The invention provides a method for preparing a printed circuit board with plated-through-holes, using a specifically prepared photoresist material and a combination of exposure, development and etching means. This method is particularly useful for the preparation of a printed circuit board with plated-through-holes bearing a high density circuit pattern.