Printed Circuit Board Design-Selecting the
by Tim Mintzer EDC Inc.
Select the Right Technology:
Most PCB designs these days are required to
pack "10 pounds of stuff into a 2 pound bag". This leads to more
complex manufacturing and test techniques. By addressing the issues
presented below, one can help to alleviate these problems before they
hit the production floor.
Components-Surface Mount, Through Hole or
Even though most common components are
available in Surface Mount form, it is important to identify each
component and package to be used in the design at the beginning. Top
side components or Top and Bottom side components need to also be
defined. If at all possible, all components should go on the Top Side.
Manufacturing Technologies-Reflow or Wave
Although most OEMs and Contract
Manufacturers have SMT capability, many boards require mixed
technologies causing an additional wave-soldering step.
Higher density circuits sometimes require
the use of Ball Grid Array (BGA) packages. This provides for higher
levels of integration, but may require expensive X-ray inspection
equipment. Knowing the capabilities of the Production Floor is
Testing-Automated or Manual:
Volume usually dictates this decision,
however, a properly designed PCB will facilitate the migration from
manual to automated. Even though they may not be used, it is a good
idea if space permits, to add test points for In-Circuit Test (ICT).
The addition of test/probe points will also help the tester look at
critical signals in a manual test.
Many design these days use multi-layer PCBs.
These do not add that much cost and allow a higher routing density for
a given board area. In addition, properly designed power and ground
planes will help keep noise and electromagnetic radiation from causing
problems in the circuit.
High Speed and RF boards may require
controlled impedances and a controlled dielectric constant of the
material used. Identifying these signals up front will and designing
accordingly will help ensure a successful design.
Most PCB fabricators can make PC boards down
to 8 mil traces and 8 mil spaces or smaller. This is the industry norm.
However, if not needed why use it? Using 10 mil traces and 10 mil
spaces will increase the PCB houses' yield. Also, identify any high
current traces (nets) and design those to handle the current. Over
design those if possible, as the current goes up in a PCB trace, not
only does the voltage drop due to losses, it will generate more heat.
Oh yes, on that subject, Heat. Consideration
must be given to high heat dissipating sections of the circuit. Thermal
mass must be given to components generating heat, whether with an add
on heat sink or heat dispersing copper planes. This needs to be
properly designed to provide maximum lifetime and low mean time between
Although there are many different aspects to
designing a Printed Circuit Board, I have tried to hit the main points
in this article. Careful consideration given to the items mentioned
above at the beginning of the design can eliminate problems early
instead of during a production run which could cost lots of time and
money. Don't design the PCB in a vacuum, soliciting input from
manufacturing and test engineers will help achieve the desired results.
© 2005 - Tim Mintzer and EDC Inc. -
Reproduction without permission prohibited. All Rights Reserved