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Phototools are either silver or diazo. The objective of this column is to familiarize you with diazo phototools. Less common, and more expensive, are chrome-on-glass phototools. Diazo phototools are transparent even in the darker, unexposed areas and they come with a characteristic amber color.
Photospeed is one characteristic feature of phototools. Laser plotter films have a relatively high photospeed. By contrast, diazo films, which are exposed by contact printing using a silver film to define the pattern, are much slower.
Diazo films are usually positive working. A negative working film reproduces a black line on a clear background as a clear line on a black background. A positive working or duplicating film reproduces a black line as a black line.
Diazo films have some properties that make them uniquely suited for use as phototools that must be visually registered to boards. The diazo phototool is visually transparent, with the image a light yellow color, to allow easy registration to a pre-drilled board. Yet, that image is opaque to the ultraviolet radiation to which the photoresist is sensitive. Also, the surface of a diazo phototool is significantly tougher than the gelatin used in silver halide films, making it more resistant to the handling a phototool receives during a production run.
Editor's Note: This column originally appeared in the December 2013 issue of The PCB Magazine.