Page 25 - PCMI Journal July 2018

David Allen | Emeritus Professor | Cranfield University | UK Photochemical Machining: Where has it come from, and where is it going?
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.
Issue 131 July 2018 PCMI Journal 24