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Home , Photochemical machining, Photoengraving, Sheet metal

Photochemical : Where has it come from and where is it going? Emeritus Professor David Allen, Cranfield University, UK

Abstract The history and development of the (PCM) process is described together with predicted process developments that should lead to a more robust and versatile rapid manufacturing process for the future. Introduction Photochemical machining is generating a tremendous interest in production engineering circles due to its versatility, low cost [Tsang] and high resolution [Allen, 2016] and is playing an increasingly prominent part in the commercial world of miniaturisation in the twenty-first century [Micrometal]. Wikipedia (accessed 2017) states: “Photochemical machining (PCM), also known as photochemical or photo , is a process used to fabricate sheet components using a photoresist and etchants to corrosively machine away selected areas. This process emerged in the 1960s as an offshoot of the industry.” However, I believe this statement is questionable and historical evidence suggests that it is incorrect. This paper aims to show that PCM is derived from processes and not printed circuit board (PCB) production. It also explains why the PCM process is receiving considerable success as a rapid turnaround manufacturing process, especially in the fast-expanding field of microengineering, and looks forward to the implementation of the latest process developments that will extend the PCM process capability further for future products. History In the fifteenth century, a vinegar-based etchant was used in conjunction with a linseed oil paint acting as a maskant to decorate plate armour. Other maskants, or resists, were developed from waxes, resins and other natural products such that, a century later, the technique was being used for intaglio print-making by etching into iron or plates through a wax ‘ground’ cut with a needle-point. Intaglio printmaking was derived from observations that the contrast of an etched or engraved pattern on metal could be increased by filling the pattern with an ink or dye, enabling the craftsman to view the progress of the work in hand more readily. This concept was then exploited commercially as a method of printing by transferring the ink onto paper to form a reversed image. The entire world thirsts for knowledge and the expression “Knowledge is power” has been attributed to Sir Francis Bacon (1561-1626). The spread of knowledge is often tightly controlled for political purposes and whilst illuminated manuscripts hand-written by monks were an excellent source of knowledge such manuscripts were unique and took a very long time to produce. Woodblock printing has been used as a faster method to produce illustrated books since 200 AD. Its main principles include carving into a wooden block to create recesses, the spreading of ink onto the flat (uncarved) wooden surface and transferring the ink onto paper by the application of pressure (letterpress). The next major development was to use metal as the ink transfer medium instead of wood. The Gutenberg Bible was the first major book printed using mass-produced movable metal type in Europe. Written in Latin, the Catholic Gutenberg Bible is an edition of the Vulgate, and was printed by Johannes Gutenberg, in Mainz, in present-day Germany, in the 1450s. The nineteenth century saw the beginnings of as we know it today and with it came the development of a new type of photosensitive resist known as photoresist. J.N. Niépce has been credited with the first photoetching, having succeeded in 1826 in etching pewter (an alloy of and lead) through a photoresist stencil formed from bitumen of Judea asphalt that had been developed in a mixture of lavender oil and mineral spirit [Hepher]. Niépce’s pewter plate was the source of the first from nature, made in a camera obscura with an eight-hour exposure to sunlight! Niépce experimented further by reproducing an by Isaac Briot (1585-1670) as shown in Figure 1.

Figure 1. (Left) Engraving of Cardinal d’Amboise (Right) Photoetched plate of the engraving made in 1826 or 1827. 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 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 were also photoengraved: was introduced in the 1930s in Germany where they called it Elektron metal and a patent (US Patent 2,180,293) was granted to W.H. Finkeldey in 1939. 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 , carbon steel, iron- alloy and 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 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. In 2001, Orbel purchased the assets of Industrial Engraving Co., the oldest photochemical milling operation in America, formed in 1902 to make printing plates for the Easton Express newspaper (see Figure 3 showing one of the processes used in the manufacture of a magnesium printing cylinder). Commercial Developments in Europe Established in 1907, probably the oldest of its type in Europe, V Siviter Smith set up his engraving company in Moseley Street, Birmingham, UK, and over the next 50 years grew to be one of the most respected engraving companies in England. At the forefront of technological advances, Siviter Smith was introduced to the DOW Chemical engraving process for magnesium and zinc in the 1950s, which was adopted by the company to produce letterpress plates for the printing industry. Kodak was supplying the photoresists used to etch the printing blocks and printed circuits and was pioneering the PCM process for etching metal parts.

Figure 2. Buckbee-Mears leaflet of 1979 showing emphasis on PCM and PEF technologies. Figure 3. Photograph showing an Industrial Engraving Co. operator, circa 1950, heating a Magnasleeve™ over a gas element prior to removing it from the press cylinder. (Courtesy of Ken Marino from the historical archive of Orbel Co., Easton, PA, USA) A subdivision of V Siviter Smith, Microponent Development Ltd was set-up in Livery Street, Birmingham in the early 1960s (Figure 4) to develop PCM into a viable business. Microponent Development Ltd enjoyed success and moved once more to nearby Belmont Row, using its photomechanical expertise to manufacture plated through hole printed circuits for a growing electronics industry. In the late 1960s, Siviter Smith Group was placed into receivership. Norman Marrett, a former employee who had formed his own company, Fotomechanix Ltd., manufacturing and selling pre-sensitised PCB material and other associated products, acquired Microponent Development Ltd. The company merged with Micrometallic in 2004 to form Precision Micro. Electrolytic etching, probably using a solution of common salt as an electrolyte, was used as a production etching process over 60 years ago. Salfret (formerly of London, UK) was one of the first job-shops in the world to make commercial metal parts by etching. Surprisingly, this company made parts by electrolytic etching rather than spray or immersion etching. Figure 5 shows an historical item from my collection of PCM artefacts. The 8-page brochure is back-stamped ‘Received 14 December 1957’ and attached samples included part of a 0.15mm thick microfilter containing rows of 0.3mm diameter holes.

Figure 4. Siviter Smith personnel in the early 1960s. (Photograph courtesy of Precision Micro) Years of rapid expansion PCM grew strongly from its bases in USA and Europe and many other extant PCM companies have a long history. For example:

• Veco, located in The Netherlands, acquired the UK company Tecan Components Ltd. in 2009. Tecan had previously incorporated Checkmate Devices Ltd., a UK company that pioneered PCM in the 1950s. • Tech-Etch, USA, began as a small engraving company in 1964 • Conard Corporation, USA was founded in 1965 • Fotofabrication Corporation, USA was formed in 1967 • Metaq, Germany, began making parts by PCM in 1969 • Newcut, USA, was founded in 1970 • Lasertech Chemical Machining Group, Italy (formerly Chemical Machining) began etching in 1970 • Photofabrication (Services) Ltd., England, was incorporated in 1971 • Ätztechnik Herz, Germany, was formed in 1974 • Precision Photofabrication Developments Ltd., Scotland, was established in 1977 • Northwest Etch Technology, USA was established as Northwest Chemical Milling Company in 1978 and • CMT Rickenbach, Switzerland, was formed in 1982.

Figure 5. Salfret brochure with samples of parts (left) made by electrolytic etching [Allen, 2016] Coming together for mutual benefit The Photo Chemical Machining Institute (PCMI) was formed in Chicago, IL, USA in 1967 to act as a trade organisation and focal point for PCM activities. The five founder members of PCMI were all USA companies, namely:

• Eastman-Kodak Co. (photographic plate and photoresist manufacturers, founded on 4th September 1888 by George Eastman and Henry A. Strong) • Buckbee-Mears (photoengravers, formed by cousins Charles E. Buckbee and Norman T. Mears in 1907) • Philip A. Hunt Corporation (photoresist manufacturers, formed in 1909) • Chemcut Corporation (etching machine and equipment suppliers, formed in 1956) • Norland Products Inc. (fish gelatin photoresist manufacturers, formed in 1960) PCMI therefore celebrated its 50th Anniversary in 2017 and meetings were held in Chicago (Spring) and Como, Italy (Fall) to commemorate this landmark event. PCMI attracted the attention of Japanese PCM companies and following a visit from Yasuhiro Ueda, Hirai Seimitsu Corporation became the first Asian member of PCMI in 1979. That contact also led to the formation of the Japanese Photo Fabrication Association (JPFA) that held its first meeting in Tokyo on May 8th, 1985 [PCMI]. The first European member of PCMI was Photofabrication (Services) Ltd, England, represented by Hugh McCallion, that joined in 1980. Getting up to speed: PCM Research and Development As sub-contract companies have little spare capacity for detailed research, PCM technical literature is dominated by research publications from universities and technical institutes including:

• Cranfield University, UK • University of Bremen, Germany • Hokkaido University, Japan • Academy of Sciences, Bulgaria • Sandia National Laboratories, Albuquerque, NM, USA • International Prepress Association, South Holland, IL, USA • University of Applied Sciences, Kaiserslautern, Germany • Karlsruhe Institute of Technology (formerly Kernforschungszentrum Karlsruhe), Germany • Nanjing University of Aeronautics and Astronautics, China • College of Engineering (Polytechnic), Pandharpur, India And large companies with in-house production facilities such as:

• Philips (The Netherlands) • Buckbee-Mears (USA) and • RCA (USA) All of which were prominent in the manufacturing of colour TV shadow masks by PCM, together with smaller companies supplying materials (especially photoresists) and etching equipment. Potential PCM technology of the future: The Future for Etching and Resists As PCM is developing rapidly in many fields such as device miniaturisation, medical, automotive, aerospace and electro-optical applications, it is important to understand that the process has a viable future and that further technological advances will extend the process capability for applications in new markets including high-tech products. Questions need to be asked as to how such advances might be made, such as

• Will the metal cleaning processes remain the same? Perhaps not! Is it essential for the metal cleaning process to be carried out in PCM companies? PCM companies need closer relationships with metal suppliers in my opinion. Why can’t the metal suppliers provide pre-cleaned metals? They are responsible for putting the oils and greases on the metals in the first place! I also believe that there will be an increase in the use of very thin materials. Unfortunately, this can present problems through damage by manual handling and automated transportation in exposure and etching machines. Reel-to-reel PCM could be of immense benefit in damage limitation but the volumes etched need to be substantial for economic success.

• What types of photoresists will be used in practice in the future Negative-working DFRs are now almost exclusively aqueous-processable to reduce environmental impact by eliminating VOCs from the atmosphere. The availability of both old and new formulations for liquid resists suggests that they still play important roles in PCM applications where very thin materials suffer from traditional stripping (when positive-working resists can be used to advantage) and very high resolution cannot be obtained with negative DFRs. I believe that corrosion-resistant materials such as will be etched in larger quantities in the future as they are crucial to advance bioengineering and aerospace applications. These metals and alloys can only be etched in aggressive etchants such as hydrofluoric acid (HF). Several new resists (both liquids and DFRs) have been developed based on epoxy chemistry that are particularly resistant to aggressive chemical etchants but, unfortunately, stripping the resist after processing can be challenging. I also believe special photoresists such as the acid-modified epoxy acrylate photoresists recently formulated in Japan [Mitsubishi Paper Mills] should be a valuable resist for etching titanium and its alloys. These resists withstand etchants based on HF that attack many of the standard photoresists currently available. Electrophoretic resists, applied by cathodic deposition, are still struggling to be accepted in PCM applications following adverse publicity a few decades ago, but they may reappear and play a useful role in achieving uniform thickness (wedge-free), thin coatings with excellent resolution. LDI photoresists seem to have an excellent future and developments are on-going. Older LDI systems used a 4W Argon ion laser with λ = 351-364nm. Newer systems use a diode-pumped 8W Nd:YVO4 solid state laser (λ = 355nm) allowing cheaper photoresist to be used. Diode-pumped, solid state lasers are also now being used, emitting at 405nm. This requires new photosensitizers. In this exposure system, the rotating polygon reflecting mirror is replaced with a Digital Micro-mirror Device (DMD).

• Will the phototool disappear in the next few decades? I believe that it will, because the traditional imaging system will be replaced by technical advances in Laser Direct Imaging (LDI) and Ink Jet Printing (IJP). As the pressures of environmental legislation increase, these greener processes have the advantages of less chemistry to handle and a reduced environmental impact. As “Time is Money”, there will be a need for photoresists to be designed specifically to absorb particular wavelengths of actinic laser light and thus reduce exposure time. This involves manufacturing R&D into new photosensitizers that increase photoresist production costs. The advent of new high-power lasers means more energy is available to be absorbed and the exposure time will therefore drop to an acceptable level. As laser power continues to rise, the amount of photosensitizer required will reduce and result in a cheaper photoresist. Mechanically, I believe there is still a need for improved front-to-back registration when imaging both sides of the resist-coated material. IJP can be regarded as the future equivalent of silk screen printing where the lower resolution suffices the (often large) application. Without doubt, higher resolution IJP is feasible with ink droplet sizes in the nanometre range possible. However, as droplet size decreases to sub-micrometre, the time required for imaging increases so that an acceptable balance must be achieved between resolution and print time. One very big advantage of IJP is the fact that one-off imaging is possible with significant cost benefits. I believe that IJP has a very bright future in the world of PCM. • Will the etching process remain the same? I believe that the spray etching process will remain fundamentally the same, but process control will need to improve as future tolerances are reduced. Recycling efficiency will need to improve progressively over the next decade to reduce costs of waste etchant disposal. The extraction of dissolved metals such as nickel and chromium may even become economically feasible as waste disposal costs continue to rise. Increasing use of computer hardware and software will also be utilised to improve uniformity of etching. I also think that the electrolytic etching process may be re-examined as a method for etching corrosion-resistant materials due to Health & Safety considerations in handling toxic etchants and increased environmental legislation pressures for environment-friendly disposal of waste etchants. However, in the future, better control of current density distribution will be required to obtain uniform etching across a sheet of metal in comparison to that obtained by Salfret in the 1950s. This can be made possible by sophisticated computer modelling of the current density distribution of each component design.

• Would it be worth the cost to tailor-make appropriate quality (fit-for-purpose) water and benefit accordingly? Water quality is rarely constant as it varies from season to season and district to district as it is dependent on both local weather conditions and local geology influencing water hardness. Water is required as the main diluent for many chemical processing solutions including metal cleaners, photoresist developers and photoresist strippers. Process water is also required for post-clean rinsing, post-development rinsing and post-stripping rinsing. Therefore, the quality of the water used may be matched to its specific application dependent on its source. For instance, when cleaning metal [Bridges, 2008] recommended the following sequence: 1. alkaline chemical cleaner 2. vigorous tap water rinse 3. mild acid rinse 4. vigorous tap water rinse and 5. a final rinse in deionised (DI) water to prevent any contamination from the tap water rinse (4) drying on the metal surface during the final drying stage. Thus, DI or distilled water may be economically viable for some process stages even if it does cost more than tap water due to the savings made by reducing the number of defective parts produced. On the other hand, for post-DFR development rinsing, it has been recommended that hard tap water is much preferred to soft tap water or DI water to achieve straighter resist sidewalls [Bridges, 2007]. A calcium carbonate concentration of 150-300ppm has been suggested as the optimum hardness by [Wilson] and may be modified by additions of Epsom salt. It can be seen from the above that to make small, but important improvements in imaging quality the use of bespoke water can be a critical factor. Conclusions PCM is a manufacturing technique that has developed from the printing industry and not the PCB industry. However, it should be acknowledged that the technique has benefitted from technology applied to the manufacture of PCBs especially with regards to applications of photoresists and the provision of processing equipment such as etching machines. In summary, PCM has had a very interesting history derived from photoengraving and the next few decades of progress should yield an even more cost-effective process for new technology part production and applications that require high resolution at low cost. Acknowledgements I wish to acknowledge useful discussions on PCM history with Bob Crutchley (ex-Precision Micro, UK), Peter Engel (Newcut Inc., USA), Bill Fox (Conard Corporation, USA), Alan Gosling (Attewell Ltd., UK), Marcus Heather (Precision Micro, UK), Kirk Lauver (Chemcut Corporation, USA), Ken Marino (Orbel Corporation, USA), Hugh McCallion (ex-Photofabrication (Services) Ltd., UK), Chester Poplaski (ex-Newcut Inc., USA) and Yasuhiro Ueda (ex-Hirai Seimitsu Kogyo Co. Ltd., Japan). References Allen D.M., Photochemical Machining and Photoelectroforming, ISBN 978-1-5262-0188-1, 2016, Frontispiece. Allen D.M., Photochemical Machining for Micro Parts, Chapter 3.21 in Design for Advanced Manufacturing: Technologies and Processes, (Ed. LaRoux Gillespie), McGraw-Hill Education, ISBN 978- 1-259-58745-0, 2017, 353-9. Borth P., Photoengraving, on-line Encyclopædia Britannica, accessed 2017. Bridges K., Optimising the Dry Film Process, PCMI Journal, 109, June 2008, 15-18. Bridges K., “Imaging of Photoresists” lecture in “Understanding Photochemical Machining: Part 1”, a Cranfield University Short Course directed by Prof D.M. Allen, July 10th-12th, 2007. DeForest W.S., Photoresist Materials and Processes, McGraw-Hill, ISBN 0-07-016230-1, 1975.

Hepher M., The Photo-Resist Story – from Niépce to the Modern Polymer Chemist, The Journal of Photographic Science, 12, 1964, 181-190. Loening E.E., Note on Cold Enamel, Process Engravers’ Monthly, 1948, 209-210 (July) and 241-2 (August).

Loening E.E., A Process for making Acid Resists for Graticules, Process Engravers’ Monthly, 1950, 266, 269-270 (September) and 297-8, 301 (October).

Micrometal GmbH, Redefining What’s Possible with Photochemical Etching, Commercial Micro Manufacturing International, Volume 10, No.5, September 2017, 7-9.

Mitsubishi Paper Mills, Photosensitive Resin Composition and Etching Process, US Patent Application 20180046079, filed March 16, 2016 and published February 15, 2018.

PCMI, Japan Photo Fabrication Association Established, PCMI Journal, 21, Summer 1985, 9.

Tsang A., The Design Engineer’s Guide to Photochemical Etching, Commercial Micro Manufacturing International, Volume 10, No.5, September 2017, 20-25.

Schaffert R.M., Winkler J.H., Vaaler L.E. and Deubner R.L., The Ferric Chloride Etching of Copper for Photoengravings, Photo-Engravers Research Inc., Columbus, Ohio, USA, 1949.

Smethurst P.C., The Cold Enamel Process – Some Underlying Factors, Process Engravers’ Monthly, 1942, 5-7 (January) and 37-39 (February).

Wallis F.G. and Cannon R.V., Letterpress Platemaking, Pergamon Press, Oxford, England, 1969.

Wilson W., Dry film imaging considerations for photochemical machining, PCMI Journal 112, March 2009, 20-21.