John Cunningham, Chief Chemist at Rainbow Technology Systems ­discusses a new PCB production system that is proving a pot of gold at the end of the production chain for Rainbow by improving speed and efficiency.

PCBs have pretty much been manufactured using the same techniques for the past 50 years.  Small refinements have been made over the years but essentially there has been no real change in the technology used.

Having looked closely at the traditional methods of PCB manufacture Rainbow felt there had to be a more efficient method of production.  The result after six years of intensive research and development is the company’s Processing Unit.

Coating, imaging and development of the board is carried out within the one unit and the process is fully automated. The process uses solvent free resist and lithographic film (standard phototools) in the imaging process.

It is capable of high volume production producing up to 200 double-sided panels per hour compared to 80 single sided panels using a Laser Direct Imaging system. The total elapsed cycle time is only one minute from beginning to end for producing a double-sided panel.

As the unit uses low-power LEDs to expose the film and does not require a drying oven, the power consumption is very low averaging around 3kW. In addition the unit is housed in its own clean controlled enclosure so does not require a clean room.

The unit is particularly suited to printing fine line detail as it can produce tracks under 20 microns. As designers strive for ever smaller features in devices such as smartphones the company believes this facility will become increasingly important in the future of the manufacturing industry.

One of the key elements in the process is a proprietary solvent-free resist developed by the company. The coating is only 5-microns thick and is exposed in its wet state (no need for pre-drying) and is developed using industry standard chemicals (potassium carbonate or sodium carbornate).

The use of wet resist has several advantages over dry film. Firstly, adherence of the resist to the copper is improved as it is possible to squeeze the resist into all the contours of the substrate. This level of closeness to the substrate cannot be achieved with dry film due to the surface characteristics of copper.  Secondly, much finer detail can be imaged as the optical path is shorter at eight microns compared to over 50 microns with dry film with its Mylar protection. 

Another issue is collimation. With laser systems there can be a high degree of collimation which highlights any trapped particles which could mean the board having to be scrapped. 

With this process, collimation is only six degrees and any particles are trapped in the cured resist. Moreover, as there is only a nine second gap between the coating and imaging stages and as this operation is carried out in a vertical position there is very little opportunity for debris to attach itself to the wet ink. 

By contrast in a conventional system the board spends several minutes in a horizontal position in a standard hot air oven drying system. This feature and the fact that the process is almost entirely automated, should mean higher yields.

Standard phototools

One of the aims in developing the system is to give a new lease of life to standard phototools without the costly maintenance connected with LDI.  Very little heat is generated in the process so that phototool thermal management is very carefully controlled.

The process uses silver halide phototools and a special protective coating (3 microns thick) is applied using the company’s Panda Coater which protects the phototool from moisture and chemical attack from the resist. 

Changeover of phototools is very quick taking under two minutes to change part numbers. A further benefit of this process is that the panels come out ready for etching, there is no need to store or transport them under yellow light.

Touchscreen display manufacture

Apart from PCB production, another application for the process is in touchscreen display manufacture. Cond­uctive tracks, invisible to the human eye, can be printed on to a clear ­substrate. Grid patterns of five micron by 300 micron pitch offer more conductivity than ITO or conductive polymers. 

The process can be done additively using an imaged seed layer to create electroless plated nickel or copper with an imaged plating resist to create tracks. Alternatively the process can be done subtractively using sputtered metal on plastic using an imaged etch resist to create tracks.

A key benefit of using the process to produce touchscreens is that closer tracks and gaps (down to 10 micron spacing) means that the screen will be more responsive and designers will be able to introduce more features and functionality to the screen.

Rainbow Technology Systems