Photoplotter

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A photoplotter is an electro-mechanical-optical machine that exposes a latent image on a media, usually high-contrast monochromatic (black-and-white) photographic film, using a light source under computer control. Once the exposing step is complete, the media is processed in a film processor using a developer solution, along with fixing, washing, and drying.

Nearly any conceivable image can be formed. Photoplotters are used primarily for the production of PCB's (printed circuit boards) and IC packaging. Other application areas include chemical milling and specialized graphic arts. Photoplotting is the first step of making photolithography masks for printed circuit boards. In the PCB industry, these masks are called photoplots and are generally limited to features of 20 µm or more. Integrated circuits are made in a similar fashion utilizing photomasks with sub-micrometer feature sizes; photomasks are traditionally made by photoreducing photoplotter output.

The first photoplotter was introduced by the Gerber Scientific Inc. (now Ucamco, see http://www.ucamco.com) in the 1960s. Early machines used a xenon flash lamp, and projected an image mounted in a rotating aperture wheel onto the photosensitive surface of the film or glass plate. The imaging head assembly traversed over the surface of the media without touching it to produce draws and flashes. Draws are vectors or arcs created by continuous illumination as the imaging head moves over the photosensitive surface. A flashe create single simple graphic in a location by shining light through an aperture of the appropriate shape at a fixed location.

Modern photoplotters are generally raster-scan devices that use a laser beam focused to one or more spots, and modulated at multi-megahertz rates, to form the image. Initially, green argon-ion lasers and blue helium-cadmium lasers were frequently used. Current models utilize a red helium-neon laser, red laser diodes or even red LEDs (light-emitting diodes).

Photoplotters are closely related to imagesetters. Photoplotters differ from their imagesetting counterparts in the type of controller used to produce the image, and in the resolution and absolute accuracy of the image, with photoplotters meeting much more stringent specifications than imagesetters.

The most recent development related to photoplotting is LDI (Laser Direct Imaging) which utilizes a high-power laser or Xenon lamp to directly expose photoresist on a coated substrate instead of exposing photographic film. This eliminates the handling of photographic film. LDI machines currently sell for prices in the one-half million U.S. dollar range.

The input of photoplotters is a vector graphics file, typically in Gerber Format. Some photoplotters also accept bitmap formats such as TIFF.

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