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56 The PCB Magazine • February 2014 the easiest and most appropriate control of de- velopment is adjusting the processor through- put speed. The optimum development condi- tions for films and chemicals in most applica- tions are provided in the technical data sheets provided by the supplier. Looking again at the film cross-section (Fig- ure 5), one can see that the silver halide crystals that were struck by light have been converted to metallic silver. Unexposed crystals remain unaffected by the development step. Although we have an image at this point, it is not yet per- manent. To make the image permanent, the film must go through a fixing step (Figure 6) to remove the remaining silver halide crystals. During fixing, ammonium thiosulfate is used to convert these crystals to several soluble silver salts which are removed from the emulsion. The metallic silver image is unaffected by this step. Within reason, it is not possible to give a film too much fixing. Under-fixing will produce films which in time will turn gray in clear areas. In extreme cases of poor fixing, the film will exit the processor ex- hibiting patches that appear milky white where they should be clear. After fixing, the metallic silver image re- mains where there was exposure. The gelatin binder is still in place throughout the entire structure; it is not removed at any time in this process. A clarification of terms may be in order. The word "development" means different things in dry film resist processing and in phototool pro- cessing. In phototool processing, development amplifies a chemical change that started dur- ing exposure. In dry film resist processing such initial chemical change and its amplification all happen during exposure. In dry film resist processing development means the removal of unexposed resist (i.e., for negative working re- sist), a step that can be compared to "fixing" in phototool processing where unexposed silver halide is removed. PHOTOTOOLS, PART C continues Figure 4: development of silver halide film. Figure 5: cross-section after development.