From: Doug Brooks ([email protected])
Date: Thu Oct 28 1999 - 09:59:12 PDT
>But a comment on our industry in general,
>
> I went to several courses at the PCB Design East, and each course
>instructor had their own opinion on what they believe is the correct way of
>doing things.
>It is sad that our industry cannot take a concensus and come up with the
>CORRECT way of doing things. Instead of using testing and empirical data to
>determine what is accurate, they bicker about why ones methods will or won't
>work.
>
As a seminar presenter at PCB East, and one who is also concerned about the
fact that students hear different things in different courses, I'd like to
offer a few random comments here.
First, people in our industry need a better understanding about fundamental
electrical engineering!! And I am not just talking about those without an
engineering degree, but also those with an engineering degree who (1)
didn't take certain kinds of classes related to such high speed issues as
crosstalk, transmission lines, and stray trace/lead inductance, etc. (2)
took them and didn't understand them, or (3) took them and forgot them!!
And I am not criticizing them --- in my second job out of college my
company was designing state-of-the-art components for the state-of-the-art
Illiac IV computer that were water cooled ECL devices running at the
remarkable speed of 3 MHZ! Things DO change.
Second, it's nice to have rules of thumb, but it is better to understand
where those rules of thumb came from and when they might (and might not)
apply. I often get comments like "In so-and-so's class HE said ...". My
response is to try to make the issue UNDERSTANDABLE for the student so
he/she can make up his/her OWN mind about what position seems more
reasonable. But that can be a challenge when the student has very little
technical understanding.
Thirdly, as has been pointed out, there aren't a lot of absolutes in our
industry. If there were, we'd all understand and be teaching the same
(absolute) rules of thumb. While I am a strong supporter of studies (and
have contributed to two of them --- the effects of vias on traces and the
effects of 90 degree corners) this is not always the answer. Because ...
each design has a unique environment. So, what works in one environment
might not apply to another. Once again, my approach is usually to try to
present to the student the ISSUES and the alternative opinions, so they can
recognize problems and (hopefully) potential solutions when they arise. As
before, it is improved understanding that helps the designer (and the
engineer) solve problems, not rules of thumb or others' studies.
Finally one last observation about studies. We lead a study on right angle
corners where the measurements were taken by the respected people at the
University of Missouri (Rolla). The results of that study were
independently confirmed by Mark Montrose with (a) a board of his own design
and (b) another board from our study. These results have appeared in at
least two publications. Nevertheless, take a position on right angle
corners in one of these e-mail forums and see how much discussion it
generates!! Some people's minds are made up, facts be darned!
Doug Brooks
.
****************************************************
Doug Brooks, President [email protected]
UltraCAD Design, Inc. http://www.ultracad.com
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