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The Rainbow Process: Up Close and Personal

I have followed the evolution of the Rainbow Technology Systems primary imaging process since a conversation with CEO Jonathan Kennett at productronica 2009. At the time, he took me aside and described to me confidentially the principles of his current development project although he was not in a position to disclose substantial details.

The project continued behind closed doors until Rainbow unveiled their process line at productronica 2011, although only part of the hardware was exhibited and certain key elements of the system appeared to exist predominantly in the form of elegant computer animations. Despite my scepticism, Kennett, together with Chief Chemist John Cunningham and Chief Engineer Robert Gibson, went to a lot of trouble to answer my questions about the workings of the process and I continued to follow the development over the next 18 months through their presentations at exhibitions and conferences.

But I wanted to see the system in real life and an impromptu meeting with Sales and Marketing Manager Chris O'Brien at an ICT seminar late last year resulted in an invitation to visit Rainbow’s facility in Glasgow, Scotland, to see the complete process line in full operation.   The Rainbow process is built around a solvent-free 100% polymerisable negative-working liquid photoresist with a very fast photoinitiator, which is extremely resistant to oxygen inhibition, and a unique feature of the concept is that the resist is contact-exposed without drying, phototools being placed directly on to the wet material. A protective coating on the phototools prevents the resist from sticking to them so they can be peeled off clean after UV exposure, leaving the resist selectively polymerised in the areas which have been exposed, but still liquid in the areas corresponding to the opaque features of the phototool, and therefore very readily removed in the developing process. So what of the reality? For reasons of accessibility and visibility the demonstration line stood in an open shop environment (the production version will be contained in a clean air enclosure) and consisted of four modules connected by an overhead transport mechanism, in the process sequence: Coat, expose, develop, rinse. And the whole line stood within a very compact 12m² footprint.

Entry and exit stations were horizontal conveyors, but each active process step was carried out with the panel suspended vertically from its upper edge. Entering the coating unit horizontally, the panel was turned through 90° and fed upward through a double-sided grooved-roller coater, each roller being independently driven and fed from its own resist reservoir. Because the resist was solvent-free, there was no worry about rollers drying out. The groove geometry was calculated to give a wet coating thickness of 5 microns.

The rollers closed on the panel just after the leading edge emerged and a gripper first matched its speed to the panel then closed, supported the panel at it exited the coating rollers and lifted it clear before presenting it to a traverse mechanism which carried it to the next process station--the exposure unit.

This was the really clever bit: The operation I had waited a long time to see. The wet panel was lowered between the open jaws of the exposure module and, at the bottom of its travel, a pair of tool bars closed above it and locked on to the carrier in exact side-to-side register. Pre-aligned phototools were positively edge-located on these tool bars.

Then a lot happened in quick succession: Nip rollers closed to bring the phototools into hard contact with the wet resist, below a pair of narrow horizontal arrays of UV-LEDs, and there was a brief glow of purple light as the carrier and tool-bar combination began its travel upward, drawing the sandwich of panel and phototools smoothly past the light sources. Once the panel had moved a few inches, a pair of guide-rollers closed on the sandwich and the tool bars parted from the carrier and literally peeled-off the phototools, which detached cleanly from the panel as its withdrawal progressed.

It really was poetry in motion as the panel continued to move upward and the photo tools continued to peel away. The panel still had a complete covering of blue resist although now a latent image was clearly visible. There was no resist on the phototools! As the bottom edge of the sandwich cleared the upper rollers and the phototools had completely detached, the rollers retracted, the phototools were lowered back into the body of the exposure module ready for the next cycle, and the panel proceeded to the next station to be developed.

Developing was an almost instantaneous operation, for two reasons: The resist layer was only 5 microns thick, and the material to be removed was still in its original liquid state. The panel was lowered into a spray chamber with a dilute carbonate developer and withdrawn within a couple of seconds, a pair of squeegee rollers closing on it to remove dragout. The final process station was a water rinse, in a module similar to the developer, after which the panel was carried to an exit conveyor that rotated it back to horizontal ready for etching.

Mesmerizing to watch in continuous operation with panels at every station, the line is designed to process 120 double-sided 21” x 24” panels per hour. Changeover between designs is quick and easy - a simple matter of unlocking and wheeling-out a self-locating cart and wheeling-in a second cart with the next job pre-aligned on its tool bars ready for immediate use, a two-minute operation.

I spent a long time discussing and debating process details with Cunningham. Suffice it to say that the system has extremely high-resolution capability as a consequence of the thinness of the coating and the intimate contact with the phototool. Twenty-micron lines are routine, and 10-micron lines are well within the capability of the system. And at remarkably high yield, even though the demonstrator is presently operating in an open environment--a major area of customer interest in the Rainbow system is for yield improvement. The operating principles: Wet coating and automated vertical operation, eliminate problems associated with film adhesion, and physical damage from handling and conveyor systems. The compact footprint and very low power consumption--no drying ovens or high-intensity UV sources are required--are additional advantages.

The best word to describe the design of the system is “elegant.” The concept is simple, but a lot of creative engineering and precision mechatronics has gone into bringing the concept to physical realisation. Construction is robust, all the working parts are easily accessible and there appears to be nothing that would be beyond the scope of a competent maintenance engineer to keep in good order.

I admit to being a bit cynical about “revolutionary” technologies--over the years I’ve seen too many bright ideas over-hyped, never achieving their claimed performance and quietly fading out of sight.

But this one achieves what it claims. It’s a remarkable process at a very mature stage of development and, as Kennett remarked, “The principles we have established give lots of possibilities for alternative applications such as additive circuitry, flat panel displays, touch screens, and chemical milling, but our initial objective is to establish our presence in inner-layer production.”

For more information, visit http://rainbow-technology.com.