There are a few hard and fast rules when it comes to designing printed circuit boards. There are four main areas of concern: Pads, Obstacles, Text, and Origins. Even with the best auto layout software you need to have a basic understanding of these four areas, in order to design a good PCB.
Thickness and Width
However thick the copper layer is over the printed circuit board effects the way the circuit behaves. PCB copper is usually measured in ounces per square foot, occasionally it will be stated in inches, mils or even micrometers. If the copper is very thin you will experience less resistance, resistance is the force which opposes the flow of an electric current around a circuit. As a result of this more energy is required to push the charged particles around the circuit. When you have the thinner copper, the circuit itself can resist the flow of particles.
So the thicker the copper the more you encounter:
•Increased resistance to thermal strains.
•Increased ability to carry stronger currents.
•Increased mechanical strength at points of weakness like connector sites and PTH holes.
The formula used to calculate the thickness of the copper layer is:
Thickness = mass / Area X density
Capacitors And Batteries
The outline for the capacitors and batteries should always include polarity indicators. A plus sign (+) polarity marking on the silk screen or schematics should be indicated next to the positive pad. The indicator stripe labeling the positive end for the capacitor itself should be located closest the plus sign.
Restricted Areas:
When designing the PCB, tracks or the copper path of the circuit should never be located on or close to holes where screws will be mounted or other spacers. Care should be made to ensure that the tracks cannot be peeled off easily by objects. When designing the chips edges should always be avoided as going from conveyer belt and transport can cause serious scratched or cracks in the track if it runs to close to the edge. Best practice dictates that around the edge of a PCB should have 3mm clearance from any tracks.
Electrical Clearance:
When talking about electrical clearance and PCB design, the first rule always is that when you calculate electrical clearance it is important that you calculate it as the shortest distance between two conductive tracks or pads. This has to be done using the measured through air method, as currents jump. So despite there being a space in between to the copper lines, the voltage can cause the current to jump or go through the air. There needs to be enough space to account for this phenomenon. Not taking this into account can lead to dielectric breakdown.
Creepage Distance:
Lastly, you need to calculate something called a creepage distance this is somewhat similar to electrical clearance, except in this case you calculate the shortest distance between to conductive tracks or pads. The method of measurement also differs as you are measuring along the insulation instead of a through the airaas mentioned above. There are specific guidelines for an exact measurement for this part of the design. The aim is to protect your PCB from a problem called tracking. When you encounter tracking it is as a result of the electric discharges on or close to an insulation surface. This produces a partially conducting path of localized deterioration on the surface of an insulating material.
