Obstacles

Special items as free tracks, copper areas and copper pour, details, etc, are usually called "obstacles" and are available on many PCB CAD systems. A copper pour, for instance, automatically fills in (or "floods") a desired area with copper, which "flows" around other pads and tracks. They are very useful for laying down ground planes. Make sure you perform copper pouring after you have placed all of your tacks and pads.

Copper pour areas can either be "solid" fills of copper, or "hatched" copper tracks in a crisscross fashion. Solid fills are preferred, hatched fills are basically a thing of the past.

 

Spacings and clearances

Electrical spacings/clearances are important requirements for all boards. Too tight a clearance between tracks and pads may lead to "hairline" shorts and other etching problems during the manufacturing process. These can be very hard to fault find once your board is assembled. Once again, don’t "push the limits" of your manufacturer unless you have to, stay above their recommended minimum spacing if at all possible.

At least 15 thou is a good clearance limit for basic through hole designs, with 10 thou or 8 thou being used for more dense surface mount layouts. If you go below this, it’s a good idea to consult with your PCB manufacturer first.

For 240V mains on PCB’s there are various legal requirements, and you’ll need to consult the relevant standards if you are doing this sort of work. As a rule of thumb, an absolute minimum of 8mm (315 thou) spacing should be allowed between 240V tracks and isolated signal tracks. Good design practice would dictate that you would have much larger clearances than this anyway.

For non-mains voltages, the IPC standard has a set of tables that define the clearance required for various voltages. A simplified table is shown here. The clearance will vary depending on whether the tracks are on an internal layers or the external surface. They also vary with the operational height of the board above sea level, due to the thinning of the atmosphere at high altitudes. Conformal coating also improves these figures for a given clearance, and this is often used on military spec PCBs.