Some Common-Sense Design Principles

Updated: Apr 25


Gary Griffin, President of Fast Interconnect


After having the privilege of managing PCB designers for over 30 years, I

have determined that common sense is not usually their strongest point.

I don't say this to insult anyone. It is merely an observation. Even the

most seasoned designer needs help now and then, if they are only willing

to ask for it.


So, with that said, I'd like to offer a few common-sense tips for PCB

designers. First, I would have to say that the most common mistake

made by designers is that they end up designing themselves into a

corner, much like a novice house painter. Then, it is usually suggested

that the designers delete most of their work and approach the design

from a different angle.


The best person to provide such advice is usually a non-designer who is

familiar with the project, because he can usually see the bigger picture

better than a designer who has been staring at the computer screen all

day long.


Problems like this often arise when designers attack the easiest routes

first. In fact, the most critical trace routes should be considered and

routed first. We all know that PCB layers get expensive on production

runs and the customer wants to save layers to cut costs, which end up

increasing the design complexity and density. Thus, the most critical

routes should be done first, and everything else is secondary.

Today's PCB designer has access to some very good design tools,

unlike back in the old days when we were forced to use architectural

design packages to lay out boards. Current tools allow the user to shut

off the rats (the little lines that look like string art and show what is

connected to what) that you don't want to focus on. This allows the

designer to focus on critical routes without a lot of unnecessary

clutter.


For complex designs, it is also a good idea to take the time up front to

select rat net groups and color them. This allows the user to easily view

groups of nets, thus helping the designer focus and increase efficiency.

For designs that require controlled impedance, always contact your PCB

fabrication house prior to routing the first trace. Ask the engineering

group or manager for the information necessary for setting up trace width

and spacing as well as the layer stack. I have seen projects fail because

this simple action was not taken, and plenty of time has been wasted

going back to adjust trace widths and spacing.


And it is best for the PCB designer to speak directly with the customer.

This way, the designer gets the info he needs quickly, plus it decreases

the chance that data and answers to specific questions will be

reinterpreted by a third party. Communication is paramount to a

successful, on-time design, and allowing the designer to speak directly

with the customer only makes common sense.


Some projects are over-engineered during the initial design cycle. It is

the responsibility of the customer and the design manager to recognize

this and come to an agreement before the endless cycle of design

enhancements take place, often do nothing to make the product work

better.


As the design nears completion, it's important that the designer, the

project manager and the customer all agree on the end results. I was

once told by a designer with many years’ experience that designs are

never completed, just abandoned. This means that, given the chance,

the end-customer (often an engineer) will see areas that can be tweaked,

re-done, removed or replaced. This over-engineering cycle must be

stopped at some point so the project can be completed.


The common-sense rule applies here: Good enough is indeed good

enough. A first-time project will require some tweaks or corrections

before it goes to production. However, if the prototype is never

completed no one will know what these tweaks should be. Engineering

reviews cannot possibly catch everything that may go wrong in a final

product. Actual testing must be conducted in order to determine the most

effective changes. More often than not, these changes end up being very

simple and very quick.


And lastly, do not fall for the old "The prettier it is, the better it will work"

scenario. This is never the case. The customer deserves a good-looking

product and is paying for that; however, aesthetics and functionality do

not necessarily go hand-in-hand.


An efficient design is a pretty design. A design that works is beautiful by

any standard.


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