Environmentally friendly PCBs

De Montfort University researchers are developing a novel, environmentally friendly laser- printing process to produce small runs of high-value printed circuit boards.

De Montfort University (DMU) researchers are developing a novel, environmentally friendly laser- printing process to produce small runs of high-value printed circuit boards (PCBs).

Inspired by recent success developing a rapid manufacturing powder printer, DMU’s Rapid Prototyping & Manufacturing Group decided to explore using a similar process to develop small batches of PCBs.

Prof David Wimpenny, director of the group, said: ‘Our laser-printing process only puts down thin layers of material, and PCBs have the advantage of only needing one or two layers so it would be much quicker.’

A process already exists for depositing conductive elements onto glass using laser printing, but the applications are limited. Most PCBs are printed on a rigid glass-fibre epoxy substrate, or flexible polymers such as polyamide film.

Current PCB manufacturing processes involve a dozen stages, use aggressive chemicals and can prove wasteful. A specially printed piece of artwork [conductive pattern] is designed and a partly-cured epoxy resin known as ‘green resist’ is applied to the PCB, which has a copper layer on it. This is illuminated to cure the epoxy to the shape of the artwork. The whole PCB is placed in an etching vat, which removes copper not protected by the resist.

Wimpenny said: ‘We want to remove these stages and go to a single-stage process where you print what you want, where you want on the board. With the current route, you throw away 90 per cent of the copper. The economics and environmental issues with the current process are fairly horrendous.’

DMU’s technique is to put down conductive material, then fuse it using heat — a laser would be used in the case of copper. It could also add the legends (component labels), the resistive dielectric layer and a coating used to protect PCBs against harsh environments.

‘Currently all this artwork is fixed,’ added Wimpenny. ‘If someone wants to make a small number of boards it is very expensive, and if they want to modify a board they have to throw it away and start again. We want to print materials exactly where they are needed, with no waste, no chemicals, and using one machine to do everything. In principle, you could replace the factory with a single unit that sits on a desk.’

DMU’s laser-printed PCB technique would not compete with the existing successful mass- production process, but would be ideal for high-value PCBs produced in runs of around 100 a year. These are used for demanding and safety-critical markets such as defence, rail, aerospace, medical applications, special-purpose instruments, navigation devices and equipment control boards.

To carry out all the steps in a single machine, the group will have to overcome different temperature regimes required for the materials during fabrication. The legend inks and dielectric inks could be processed using a low-powered infrared radiant heater like those used in process engineering to dry paints. The conductive track requires high-powered and directed heat to melt the copper onto plastic surfaces.

Competing processes are currently being developed, including inkjet printing, but Wimpenny claims DMU’s method would be the only one where a blank board is put in one end and a finished PCB comes out of the other.

Berenice Baker