Prototyping 4

Antennas and Microwaves

DESKTOP SIGN MAKERS

Desktop sign makers or vinyl cutters as they are sometimes known, are basically plotters with a small knife blade in place of the pen. Normally a design is produced on a suitable CAD or drawing package which is then plotted, scoring the design onto a sheet of self adhesive vinyl. The unwanted areas are peeled off and the remaining design transferred from the carrier sheet to the shop window, car, T-shirt etc.

The knife blades used are very small (2mm), usually made from Tungsten Carbide and can easily cut copper. The blade sits in a miniature bearing so that it can turn to follow the direction of the cut. On arriving at corners the software drivers cause the knife to do a tiny “jig” to align it ready for the next cut, this is important, you will see why later.

The advantage of scoring the design from a CAD/CAM point of view is that the tool thickness is effectively zero. This means that there is no need to calculate complicated routing paths and tool changes that you do with conventional milling machines. All this helps reduce the complexity of the system and therefore cost.

That said, these are precision CNC machines that are not made in huge numbers, so are not in the same price bracket as inkjet printers. The only model I have used to date is a Roland CAMM-1, these currently retail for around 1500 Euros (June 2008). There are other makes and cheaper models (see links on page 5) but you need to read the specifications carefully and talk to the supplier.

For comparison, a CNC micro-milling system , although capable of much more is likely to set you back nearer 15000 Euros !

SCORING THE COPPER (Automated Method)

Assuming you have your CAMM-1 or similar, you will need a suitable CAD or drawing package, the CAMM-1 driver installs just like any other printer/plotter in Windows. The most important things to master are :

Scale - Make sure you can plot accurately at 1:1 (not fit to page), the CAMM-1 knife is interchangeable with a plotter pen, ideal for dry runs using paper.

Positioning - Within a standard sheet e.g. A4/A3 you need to be able to plot your design and only your design. The required area of RO4350 material will be attached to a carrier for cutting (e.g. 0.5mm FR4 board), this saves material but it means there will be a step (RO4350 thickness) in your potential cutting area. The blade won’t want to cross this, so knowing exactly where the cutter will go is important ! Use a plotter pen and paper to practice with.

Drawing Order - Ideally you need to construct your design from polygons, not individual lines. This is so that the knife makes a continuous cut round each shape, remember the “jig”. This also increases accuracy because the cutter is not having to move around so much between cuts.

Cutting Force - The CAMM-1 has a variable cutting pressure for the blade. As with the manual method it is best to cut in 2 passes using a reduced force. To do this, simply plot twice, without removing the material from the machine. A good starting point is probably around 1/3 of the maximum cutting force.

PCB Prototyping

Layout for a 2-stage PHEMT LNA using FastCAD

RO4350 fixed with masking tape to an A4 0.5mm FR4 carrier board.

Page 4 of 5

PCB Prototyping

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