A multi-purpose technology

Today, printed circuit boards – PCBs - are part and parcel of our everyday lives. We find them everywhere: in our computers, mobile phones, food processors, cars, radios, remote controls, etc. Although the associated technology no longer holds any secrets for us, its uses don't end here. Olivier Pizzirusso and his team are using the techniques involved in PCB manufacture to build the particle detectors of the future.

 

For the past five years Olivier Pizzirusso has worked at CERN as a technician specialising in the design and production of new-generation gas detectors known as Micro Pattern Gas Detectors (MPGDs). "Our workshop is in Building 102, where the standard PCBs used in the Laboratory are made by CERN's industrial service contractors", explains Olivier. The workshop houses drills, UV exposure units, presses and various other equipment needed for the manufacture of PCBs (see box). Olivier and his four colleagues thus have access to all the equipment they need for creating their detectors.

Olivier, who is a specialist in electronics and PCBs, draws on his training and past experience to make the MPGDs. "Sometimes, depending on the requirements of the physicists and the engineers working in the experiments, my colleagues and I have to invent new solutions, adapt existing technologies, work with alternative materials or use different manufacturing techniques", he says.

"More specifically, I am working on the Micromega Bulk detector, one of the two types of MPGD", explains Olivier. The Micromega detector was invented at CEA/Saclay in 1996, but the Bulk technology was developed at CERN, by Olivier and his colleague Antonio Texeira. Today, 15 years after the first developments, the RD51 collaboration, comprising 73 universities from 25 countries all over the world, is carrying out R&D on MPGDs.

There is a chance that this type of detector will be used more and more in the experiments that are under study for future accelerators", explains Rui de Oliveira, who is in charge of the production unit in Building 102. "They are less costly at this scale and much more resistant to radiation, with an almost unlimited lifetime". This means that MPGDs have important advantages for the physicists, who have already had the chance to see what they can do in the COMPASS experiment. "The ATLAS and CMS collaborations are currently considering their use in a future phase of detector exploitation", says Rui. "Other collaborations, such as the International Linear Collider (ILC) and Compact Linear Collider (CLIC) are also interested in them."

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How is a simple PCB made?

A monolayer PCB (see photo) consists of a board, generally made of epoxy resin, on which electronic components connected together by conducting copper strips are mounted.

Various steps are involved in the process. The starting point is the board, which is coated with a thin layer of copper (around 35 micrometres), then a varnish sensitive to ultraviolet (UV) rays.
An image of the electric circuit drawing (made in the design office) is printed onto a transparent film using a UV-opaque ink. The film is then placed on the varnish, and the whole thing is placed in a UV exposure unit. The varnish becomes fragile as a result of exposure to the UV light and is then removed using a developing solution, so that the copper protection is confined to the areas representing the electric circuit.

A chemical product, often ferric chloride, is used to attack the unprotected copper layer until it is totally dissolved. The areas of copper that are left form the wires of the electric circuit.

After the through-holes have been drilled, the electronic components (diodes, capacitors, resistors, transistors, etc.) are brazed onto the board using the tin soldering technique.

 

by Alizée Dauvergne