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84 SMT007 MAGAZINE I JULY 2021 Article by Karl Dietz Editor's note: This article was originally published by Karl Dietz as a column in 1998 and is being reprinted here with prior permis- sion om the author. Allow me to digress a little in this column from the theme of fine line printed wiring boards to the world of photochemical machining or "chemical milling." Some readers may con- sider this a rather obscure topic. However, this industr y is wor th studying since it has advanced technologies which it shares with PW B fabr ication. is "niche technology," as the German title [1] implies, deals with ver y demanding precision requirements and fine geometries. During a recent visit to photo- chemical machining shops, I have seen more 2-3 mil (50-75 micron) feature work than in any "high tech" PWB shop. Pho to chemical machining ha s steadi ly expanded into electronic packaging. Examples are lead frames [2] , EMI shielding, and flexible circuits that require etching or plating of cir- cuits as well as etching of Kapton® polyimide layers. is industry is using materials famil- iar to PWB fabricators, such as photoresists, developers, etchants, and strippers. Process equipment may be identical to PWB fabrica- tion, but you might find more tank process- ing than conveyorized spraying, and there are industry-specific process alterations, such as "barrel" plating. A 1996 survey of photochemical machin- ing companies [3] showed that most of them had annual sales revenues in the range of $1-5 million (1997 survey: $1-10 million), which is quite low compared to PWB fabricators. For lack of volume and standardization, these niche businesses were barely in a position to drive the development of equipment, pro- cesses, or chemicals to serve their needs, but had to adopt and tailor offerings from suppli- ers of other, larger industries. It is therefore no surprise to find that owners of photochemi- cal machining shops are colorful personalities who love to tinker and invent. Karl's Tech Talk Visiting Photochemical Machining FROM THE ARCHIVES Karl Dietz 1941–2020