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Inkjet Imaging of Etch Resist: A Real World Solution
Tuesday, September 13, 2011 | Stuart Hayton

Editor's Note: This column originally appeared in the July issue of The PCB Magazine. 

Printed circuits boards have essentially been made the same way since the 1960s. A copper-clad substrate is prepared before being fully covered in a photosensitive resist. A phototool of the circuit image is then used to mask parts of the board from light in an exposure unit. The unexposed areas of the resist are then developed before the panel moves on to etch and strip.

There have been several changes in imaging techniques over this timeline. Taped artworks were still being used throughout the industry into the 1980s, and a significant portion of revenues came from photographic reduction of these hand-taped artworks. Once reduced to size, other photographic techniques were used to merge positive and negative films into power planes, and then step and repeat the images at required spaces. This was a laborious process and prone to error--hence the use of photoplotters.

These early plotters were vector based and driven by Gerber data, a simple code containing X-Y co-ordinates and size, flash or draw instructions. Photoplotted artworks were of the highest quality, but the process was slow and it still often required photographic techniques for pos/neg merges for power planes. In the mid-1980s things changed quite radically with the advent of laser plotters. This new technology tackled all of the historical problems of time and cost and was able to produce pos/neg merges as a single plotted image; the merge having taken place in CAM beforehand. As with any new technology, laser plotters were, for a time, the subject of scorn from traditionalists who argued that laser films would not be accepted by manufacturers because the image, particularly of angled tracks, was not a smooth line.

Fortunately, pragmatism prevailed and laser plotters are now the imaging workhorse of the PCB industry and have been for the best part of 25 years. Most statistics suggest around 95% of printed circuits are produced using laser plotted artworks. The remainder are being imaged by the last of the technology shifts--Laser Direct Imaging, or LDI.

LDI brings with it a couple of clear benefits over artwork, namely accuracy and registration as the image is not subjected to film movement. Consequently, the technology has found its niche at the very high end of PCB manufacture where track of 3mil and below are used to produce boards for smart phones and the like. Though often considered a digital technology, one school of thought argues that LDI is an extremely accurate and very expensive exposure unit in that, with the exception of artwork production, all other aspects of process remain intact.

There we have it: A plotted history of imaging techniques adopted by the industry since the 1960s. Perhaps the most striking aspect of this story is that while there have been dramatic improvements, there have been few step changes. Intelligent imbedded data forms like ODB++ are gaining prominence, but the large majority of PCBs are still manufactured from simple Gerber data. High accuracy LDI technology is available, yet most PCBs are produced using films imaged on a laser plotter--technology that has been around since the mid-1980s.

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