PCB007 Magazine

PCB-Feb2014

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52 The PCB Magazine • February 2014 by Karl Dietz Karl dietZ conSultinG llc Phototools, Part C c o l u m n karl's tech talk 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 de- rived 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 crys- tals 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 ap- plications 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 bro- mide, silver chloride, and silver iodide. They are typically cubic or triangular in shape with edges approximately 200–300 nm long. A crystal of this size will contain about 10 million atoms. To each crystal are added a few atoms of a sensitiz- ing material, such as gold or sulfur, to form a sensitivity center. During exposure, the crystal absorbs a pho- ton of light, and the energy is used to form an atom of metallic silver at the sensitivity center. The absorption of more photons increases the number of silver atoms formed. When the num- ber of metallic silver atoms exceeds a threshold of between four and 10, the crystal is said to have a latent image. It is this latent image that allows the entire crystal to be turned to metal- lic silver during development. The amount of exposure the film receives must be adjusted to give optimum results. If the original is negative appearing or the film is exposed on a plotter, too much exposure will yield lines and features (dark areas) that are too wide. Too little expo- sure will produce features that are too narrow and have low optical density. Select an exposure which accurately reproduces the line width of your original as measured with a microscope. Figure 2 shows the structure of a typical sil- ver halide film. The various layers include an overcoat to provide protection against scratches and abrasions. Most films also contain a small amount of matte in this layer to permit faster and more uniform drawdown in a vacuum Figure 1: Silver halide crystals.

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