TABLE OF CONTENTS PWB manufacturing requirements can be divided into two classes: prototype and production quantities. Prototype boards have traditionally been constructed using a photographic etching process, although milling is becoming popular for single-layer prototypes due to the fast turn-around time and reduced setup costs. Production quantities are seldom milled due to a large processing time per board and are more often etched or deposited.
The photographic etching process is efficient for manufacturing large quantities of circuit boards, having been used now for several decades. Etching involves several steps. The first is the generation of artwork on transparent film. Various methods can be used by the designer to generate film, including RubyLithe, photoplotting, linotype, and computer printer transparencies. Alternatively, service bureaus are available which take a desktop CAD/publishing file which the designer creates (such as DXF or Gerber) and generate film using high precision equipment. The board manufacturer's responsibility is to replicate the film pattern as metalization on the desired substrate material. The manufacturer starts with a metalized board, drills via holes, replates the entire board to plate the via holes, applies photoresist to the board, exposes the photoresist to ultraviolet light using the transparent film, and chemically etches the unwanted metal from the board. Each of these steps brings with it additional factors which must be considered for accurate microwave filter design.
An important consideration in the etching process is the "etch factor." This is the amount by which the chemical etching process over-etches the board, as shown in...
