PCB manufacturing
the etching line. When it comes to fine line detail (under 50 microns) the most common methods employed are Laser Direct Imaging (LDI) or units incorporating a Digital Mirror Device (DMD). Both of these methods use imaging units which require the copper panels to be coated with a dry film laminate or a wet solvent-based photoimageable resist.
Both methods require considerable space for the laminating or coating lines and consume a lot of power as there require drying ovens (from 20kW to 70kW), laminators (averaging 20kW) and collimated UV light sources for the imaging process (averaging 20kW).
Resist
The first area Rainbow looked at was the type of resist commonly used. Traditionally dry film is used however it has a number of drawbacks. To obtain fine detail the dry film is very thin in the order of 12- 15 microns, it is difficult to handle does not conform easily to the surface of the copper and has a tendency to delaminate particularly when printing fine detail. It is also applied laminated with heated rollers prior to imaging which can lead to stress forces building with in the copper panel.
We therefore looked to develop a special wet resist which could be applied wet to the panel surface and squeezed into all the contours of the copper substrate thereby greatly improving surface conformity and subsequent adhesion. As the coating is 100% solids and solvent free it does not need to be pre-dried. The resist is cured using UV LED light and any unexposed resist is simply washed off as it remains in its liquid state. We then addressed the efficiency of the process. At present Laser Direct Imaging (LDI) is used for fine line printing. This can produce up to 80 single-sided panels per hour.
By contrast the Rainbow Processing Unit can process up to 200 double- sided panels per hour (total elapsed time for producing a double-sided panel is only 1 minute from beginning to end).
By using standard LEDs for imaging and avoiding the need for pre-drying the Rainbow system has an overall power consumption (averaging under 3kW). The unit itself takes up only
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12sq m of floor space and is in its own self- contained enclosure where the air quality is kept at cleanroom conditions by HEPA filters. With production space at a premium this is now a very important consideration.
With the dry film process a layer of protective Mylar is applied to the substrate to protect the film. This Mylar has to be stripped off and disposed of in land fill sites. In traditional resist systems solvents used in the base coatings release considerable amounts of CO2 into the atmosphere when removed from the substrate and burned off in scrubbing units before venting to atmosphere. As no Mylar is used in the Rainbow process and the resist is solvent free the process is significantly greener.
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