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PCB Design Layout Rules Recommendation

In the PCB design of electronics circuit, it is important that one plan and
has a checklist of the do's and don'ts before proceeding to do the printed
circuit board layout. The understanding of the circuit is critical to the
design, for example one needs to understand the maximum current and voltage that
are carried by each conductor in order to determine the track width of the
conductor and the type of PCB that will be used.

The voltage difference between each track will determine the clearance
between each conductor. If the clearance is not enough, chances are that the
electrical potential between each track will cause spark over and short circuit
the PCB.

This will cause functional failure to the product and the safety of the users
that are using the product will be compromised. It is therefore critical for one
to understand some of this basics requirements before one proceed to design the
PCB.

Tracks
Restricted Area

Tracks should not be located on the areas that can caused them to be peeled
off easily. One of the restricted area is holes on the PCB which are used to
mount screws or PCB spacers. These holes are usually used to secure the PCB to a
casing or to secure it in a fixed place.

The edges of the PCB should not have any tracks as these areas are usually
used to transport the PCB from one process to another process by using a
conveyor belt. These edges are places where the possibility of scratches and
cracking of the PCB happens. The recommended areas that should not have any
track is as shown in the diagram below assuming a hole diameter of 4 mm which is
used to mount a PCB spacer.

Conductor Thickness
and Width

The PCB conductor thickness and width will determine the current carrying
capacity of the track. The IPC standard for the conductor thickness and width of
the common 1 oz/square-feet PCB is as shown below. However, it is always
advisable to use a bigger value due to the tolerance and variation of the PCB
processes. If higher current carrying capacity is required, a 2 oz/square-feet
or 3 oz/square-feet type of PCB is preferred. Many electronics hobbyist prefer
to solder a thick cooper conductor on the PCB track to increase the current
carrying capacity of the track.

PCB Design
Electrical Clearance

Many safety standards call for a minimum of 8mm clearance between 40V mains
and other isolated signal tracks. These safety standards are to ensure that the
users that are using the products will be protected from any electrical
hazards.

For non main voltages, IPC recommend the electrical clearance between
adjacent tracks. It is important to know the maximum difference in voltage that
are applied on the adjacent tracks of a PCB. The electrical clearance specs of
IPC standard is as shown below for various condition of the PCB.

Coating the PCB will help to reduce the requirements of the track clearance.
However, the quality of the coating as well as the material used are critical to
ensure that these requirements are met. Again, it is always advisable to
increase the clearance to cater for the variations of the PCB processes.

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