Page 18 - PCMI Journal July 2018

David Allen | Emeritus Professor | Cranfield University | UK Photochemical Machining: Where has it come from, and where is it going?
These last two metals cannot tolerate high burn-in temperatures so are processed with cold-top enamels based on shellac with a burn-in range of 100-150°C, first described by Ernst Doelker of Switzerland in British Patent 183,817 of 1923.
The cold enamel process required considerable skill in controlling its consistency [Smethurst] and reliability [Loening, 1948]. Further developments in photoengraving metals continued and were fully described in 1969 [Wallis and Cannon].
With the experience gained from surface etching of copper, magnesium and zinc for production of photoengraving plates for printing, it was relatively easy to progress onto the two-sided etching of stainless steel, carbon steel, iron-nickel alloy and brass foils (known today as PCM) for the production of parts used in various engineering applications.
The manufacture of etched and filled glass graticules required a Bakelite (phenol-formaldehyde resin) resist able to withstand aggressive HF solutions or HF vapour. Imaging was achieved via an additional top-coating of bichromated gum arabic [Loening, 1950].
Kodak marketed the highly successful KPR family of negative-working pre-sensitised photoresists based on poly(vinylcinnamate) in the 1950s. This catalysed the start of the PCM industry as we know it today as the resist composition was consistent and reliable [DeForest]. Kodak also published many brochures on PCM techniques and various etchants for metals, including electrolytic etchants.
Shipley developed positive-working photoresist formulations based on the Kalle Kopierlacke diazo- salt formulations of the 1940s from Germany and Du Pont Riston dry film photoresist (DFR) was introduced in 1968. To lower environmental impact, Dynachem developed the first aqueous processable DFR, Laminar A, in 1971.
The driving force for the development of PCM was to keep pace with the demands of new technologies involving:
• the manufacturing of lead frames (as every integrated circuit needs to be fitted onto a conductive platform prior to its protective encapsulation), screens, monitors, medical, environmental and aerospace devices
• miniaturisation requiring high resolution capabilities
• the use of novel metals and alloys increasing efficiency and reducing waste
• flexible manufacturing requiring rapid production and variable batch size.
Commercial developments in the USA
Buckbee-Mears (now defunct) started as a photoengraving company in 1907. However, one of the company leaflets from 1979 (Figure 2) shows a major emphasis on lead frames and components used in microelectronics and optoelectronic systems.
Issue 131 July 2018 PCMI Journal 17