Page 17 - PCMI Journal July 2018

David Allen | Emeritus Professor | Cranfield University | UK Photochemical Machining: Where has it come from, and where is it going?
The successful production of images via photography led to the concept that book and newspaper production could also be achieved through a photographic process that became known as photoengraving.
The definition of photoengraving [Borth] states that it is “any of several processes for producing printing plates by photographic means. In general, a plate coated with a photosensitive substance is exposed to an image, usually on film; the plate is then treated in various ways depending upon whether it is to be used in a relief (letterpress) or an intaglio (gravure) printing process”. The earliest commercial photoengraving companies appear to have been formed in the early 1900s.
Other early developments led to British Patent 565 of 1852, in which William Fox Talbot described a photoetching process for etching copper with ferric chloride solution through a photoresist stencil made from bichromated gelatin developed in water. Fox Talbot’s process was eventually used in the production of operational PCBs in 1943 by Dr Paul Eisler in Austria. These electrical circuit products were required in high volumes and increasing demand led to many developments in etching technology to improve the efficiency and yield of mass production.
US Patent 378,423 assigned to John Baynes in 1888 describes the etching of materials from two sides through similar and dissimilar registered photoresist stencils. It therefore became possible to form recesses (including fold-lines) and perforations simultaneously. These features figure prominently in parts made by PCM. The ideas of Baynes were extended by J.J. Snellman in British Patent 561,524 of 1944, applying PCM to the production of punches and dies. His innovations included the use of uniform width etch bands and incorporation of tabs for retention of etched parts within the metal sheet.
Unfortunately, the early photoresist formulations had a very limited shelf-life and characteristics varied from batch to batch as formulations were typically based on bichromated natural products such as albumen (from eggs), shellac (from female Kerria lacca bugs), gum arabic (from two species of acacia trees), gelatins (from mammal hides and fish skins) and casein (from milk).
Copper can be processed with fish gelatin photoresist as it can tolerate the high burn-in temperatures of 260-288°C required to harden the resist. It was introduced in 1892 by an American, W.H. Hyslop. A polyvinyl alcohol photoresist was used after 1945, requiring lower burn-in temperatures of 220-230°C compatible with the PERI (Photo-Engravers Research Inc.) powderless etch process for copper. [Schaffert et al] described best-practice copper photoengraving processes in the USA in 1949.
Other metals were also photoengraved: magnesium was introduced in the 1930s in Germany where they called it Elektron metal and a zinc patent (US Patent 2,180,293) was granted to W.H. Finkeldey in 1939.
Issue 131 July 2018 PCMI Journal 16