Evaluating the Registration of Mirror-Image Photoresist Stencils

For the production of photoctched metal parts, by etching through an adherent photographic stencil, the minimum size of hole that can be made and the control of hole size are both limited by undercut. A valuable method of improving the perfonnance is to coat both sides of the metal with photoresist, and expose between a registered pair of mirror-image masks. By the time the etchant has perforated the metal sheet, it will only have been necessary to etch for half the depth required by singlesided etching, and the effect of undercut will be correspondingly reduced. Acritical factor influencing the quality obtainable by doublesided etching is the precise registration of the mirror-image masks.