PCB - Design Rules

By froa0112 Mar 14th, 2009

Pcb
Design

In the world of integrated electronics there are many competing priorities that need to be balance out in order for the designer to complete her task in time, under budget and meeting the requirements set from the beginning. Many are related to performance and specs (number of layers for power distribution, fan out to avoid I/O crosstalk, electromagnetic coupling, etc) Some are related to size constraints (form factor specific, real estate distribution, thickness, etc), others are related to manufacturing and cost directives (drill size, cheaper materials with different properties, etc). This means that if each design were to begin as a separate, completely new project, the challenge to incorporate all of those different dimensions could be insurmountable.

Fortunately, thanks to the accumulation of knowledge through years of designs, simulations, trial and error, etc., has enabled to compilate a very complete list of rules that assist the designer in her task. These rules collect the information about available technology, threshold limits for electrical, mechanical and material factor that impacts the PCB project.

Rules guide every aspect of the design, from thickness of each layer to how close a conductive trace can be placed to a neighbor trace. This also takes into consideration the limitations of the available manufacturing technology; for example limiting the allocation of through hole vias to a given diameter and density. For very size constrained environments the rules can become more aggressive but also the manufacturing cost will rise accordingly.

Take a look at the component below: it can be seen how the designer decided to take the individual metal traces from the I/O outlet through the PCB ( this process is called fan out). Initially the traces go paralell to each other until space is sufficient that they can be diverged. There has to be a minimum distance between trace and trace to minimize electrical and magnetic coupling that could degenerate the signal. That distance is an example of a Design Rule.

Evidently, design rules have to capture current process capabilities, have to be aligned to product application (insure signal integrity, survive different stress conditions), have to enable cheap and reliable manufacture. So the basic idea of DRs is to prevent costly mistakes, formalize the existing tribal knowledge and allow for standarization of the design task.