As a printed circuit board fabricator we’re frequently asked, “What are your basic design rules for pcb?” Typically what designers are looking for are some guidelines to follow when designing a PCB layout, though this could mean a variety of things. Over the years there’s been a movement towards designing smaller and faster electronics but smaller and faster can also mean more difficult to fabricate resulting in higher costs. While some may put more emphasis on performance related attributes others may be more flexible with their designs and be more concerned with keeping costs down. Understanding design guidelines will help ensure that your design is efficient; the right mix of performance and cost for your specific requirements.
Trace and Space
One of the primary areas for consideration is the size and spacing of copper features; often referred to as the trace and space on the board. The copper trace is the line that connects components to other components or to vias (holes plated with copper). It essentially acts as the wire in your connections. A trace with a larger width is easier to manufacture and a trace with a smaller width is more difficult to manufacture. 0.005” is a good minimum trace width but 0.007” or 0.008” would help ensure that the manufacturing process produces a high quality board. While a trace width smaller than 0.005” is possible the extra care, processes and restrictions required can result in higher costs.
The same is true of spacing. A minimum of 0.005” of space between copper traces, or between copper traces and other features like holes and ground planes, will allow for efficient manufacturing. Spacing greater than 0.005” will help prevent instances of copper features “shorting out”.
Another important consideration are the holes on the board; primarily the hole size and how the holes will be connected to traces in the design. The ability to drill smaller holes has increased greatly in the last decade. Today, it is standard to be able to have a finished hole size of 0.010″ but of course, if you can use 0.015″ or 0.020″ the process and quality will be much easier to maintain.
It’s also important to consider the difference in the size of the hole that is drilled into a board versus the finished hole size after copper plating. For example, a 0.010” finished hole size may be drilled with a 0.012” drill bit.
In order for that hole to make a connection from a copper trace on top of the board to a copper trace on bottom of the board, that hole must be plated with copper. The more copper that is plated, the smaller the hole will become. Typical plating in a hole uses approximately 1 oz of copper or around 0.0014″ thick. This will ensure a solid electrical connection from the copper pad on top of the hole to the copper pad on the bottom of the hole.
Today, the smallest hole a mechanical drill can make is around 0.010″ which would create a finished hole size around 0.008″. Smaller holes require a laser drill which, as you might suspect, uses a laser to “drill” the hole.
When you are creating your design you should only be concerned with the finished hole size. You do not need to specify the exact drill size that needs to be used. A good PCB fabricator will make the required adjustments to the drill size in order to get as close as possible to the desired finished hole size. The exception would be a hole that does not require plating and in those cases the fabricator would use the drill size specified by the designer.
Related to the finished hole size another important consideration is the pad size for the hole. The pad can be defined as the copper that surrounds the hole at the top or bottom of the board. The purpose of the pad is to ensure that the copper plated in the hole makes a solid connection to the traces that connected to the pad. Ideally the pad size should be at least 0.015″ larger than the hole size. For instance if you have a finished hole size of 0.010″ then the final size of the pad for that hole would be 0.025″.
Another key rule is related to the spacing from copper to the edge of the board. Ideally any copper used in your design should be no closer than 0.010” to the edge of the board. Any closer than 0.010” will increase the risk of copper being exposed after the board has been routed.
In summary, here are the key rules to follow when designing a PCB layout:
● Trace/Space Minimum No Less than 0.005″
● Smallest Finished Hole No Less than 0.010″
● Pad Size 0.015″ Larger than the Hole Size
● Copper to Edge of Board At Least 0.010″
● Mask/Silk Smallest Feature Size 0.003″
● Following these simple design rules will save you time and money, making your next PCB board project a great success!