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Karl's Tech Talk
By Karl Dietz
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Phototools, Part A
The objective of this column is to familiarize you with artwork generation, the film types, and properties of silver and diazo films. Some technologies, no longer practiced, are mentioned to illustrate innovation and change over several decades.
It may be useful to explain the terms artwork and phototool. Historically, a “master artwork” was generated manually or by photolithographic means, or a combination thereof. This artwork was not used in the PWB exposure process to avoid damage to the artwork through handling and abrasion, but it was used to create copies, called phototools, which were used during exposure. Phototools may be called templates of the circuit pattern, with opaque and transparent areas through which UV radiation is transmitted in the transparent areas to form the resist pattern in the photoresist.
Today, the phototool is directly formed from digital data in the photoplotter, so that “artwork” is a term used for the “data.” The initial data are CAD data coming from the designer. The fabricator adds and modifies information in the CAM station. The CAM data encompass more information than just the circuit pattern: They include drill patterns, AOI data, legend pattern information, and soldermask pattern information. The circuit pattern information drives the photoplotter to generate the phototool. In the case of laser direct imaging (LDI), it drives the laser, omitting the generation of a phototool. Likewise, if legend print is inkjetted, the CAM data drive the inkjet, bypassing the generation of a screen for screenprinting.
The designer provides the circuit pattern for a “one-up” or single pattern. The CAM front-end engineer then steps, or repeats, the pattern for optimal use of a given laminate size. Additional features are then added (e.g., fiducials or registration targets, or non-functional copper areas for improved dimensional stability or better current distribution during plating, or copper areas around registration holes).
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Editor's Note: This column originally appeared in the October 2013 issue of The PCB Magazine.
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