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Karl's Tech Talk
By Karl Dietz
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Phototools, Part C
The objective of this column is to familiarize the reader with silver halide phototools.
Halides are salts that contain an ion from the group of elements called halogens, Group 7 of the periodic table. The word halogen is derived from the Greek words for “salt” and “to generate.” The halides found in silver halide phototools are typically chloride or bromide and sensitive to UV radiation. When exposed to UV radiation, nuclei of metallic silver are formed and can be grown into larger silver crystals by treating them with a reducing chemical.
Silver halide films are much more versatile than diazo films and can be used in a broader range of applications than the diazo materials. High-speed films are typically 100,000 times faster than diazo films, allowing them to be used in low light and high-speed recording applications such as photo plotters, cameras, and step-and-repeat machines.
Figure 1 shows silver halide crystals. The silver halide crystals used in silver halide films are composed of a combination of silver bromide, silver chloride, and silver iodide. They are typically cubic or triangular in shape with edges approximately 200 to 300 nm long. A crystal of this size will contain about 10 million atoms. To each crystal are added a few atoms of a sensitizing material, such as gold or sulfur, to form a sensitivity center.
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Editor's Note: This column originally appeared in the February 2014 issue of The PCB Magazine.
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