At 09:58 AM 7/30/98 -0400, you wrote:
>On 29 Jul 98, Doug Brooks wrote:
>
>> I would call your attention to a freeware program available through the
>> links at http://www.ultracad.com called pcbtemp
>>
>> It does what you asked!
>
>
>If only men would.......... but I digress.
Now, now!!
>
>Since you are the guru behind PCB Temp I downloaded it on your
>recommendations but I can just hear my boss's questions. The variable for
the
>temp change is change in which temperatures? Rise above ambient, ambient
>of the outside air or inside the unit (these are his questions, not mine)
what
>board temperature is it based on? What else would he ask........ never
mind.
>Can you maybe answer those vague questions? I'd like toshow him this but
>would really love to be able to answer his questions rather than give him
dumb
>looks and shrug my shoulders. Please?
>
>JO
>
>JoAnn L. Amerson
>Design Librarian
>Red Lion Controls, Inc.
>E-mail: [log in to unmask]
>Voice: (717) 767-6961 ext 6308
>
>
Mmmmmmmmm..........
You might be sorry you asked!
The researchers injected a current down a trace and measured the
temperature of the trace 30 seconds (I believe) after the current was
applied. The 'measured' temperature rise is the difference between that
trace temperature and what the temperature of the air adjacent to the trace
was just before the current was applied.
Q. Do the IPC charts accurately reflect the measured dtaa?
A. No. They have been redrawn, rescaled, smoothed and prettied-up so many
times they do not absolutely reflect the test results. You can sort of get
an understanding of this by noticing that the charts imply you can push up
to 250 ma through a zero width trace! The original data was taken over 30
years ago, the researchers have retired, and as far as I know, the original
data is lost.
Q. Is this what I will get with my board?
A. No. (see below)
Q. What is the difference between PCBAMP and PCBTEMP?
A. PCBAMP was designed with a table 'look-up' strategy to fit the IPC
curves as closely as possible. It does this well. PCBTEMP was designed
using a complicated analytical approach in an attempt to define the
underlying equations (what it did well was define the shape of the curve).
PCBTEMP is based on two independent sets of reported data, PCBAMP focuses
solely on the IPC charts, as drawn.
Q. Is one better than the other?
A. Depends on who you ask:) Also depends on what you want to do. The short
answer is No. But because the two calculators are based on different
strategies, you will get different results from them.
Q. Why won't my results be the same as the IPC data?
A. In the two sets of reported data I looked at, the results differed by as
much as 40%! An explanation of why this is so is included in the background
information in the help file. (I encourage you to read it.) The test boards
do not reflect real life:
1. The proximity of other traces will be different
2. The flow of air will be different
3. The thermal characteristics of the underlying board material will be
different.
4. etc.
5. etc.
6. etc.
Q. So what is all this worth, then?
A. It is an approximate starting point. The curves seem to work acceptably
well because people have been using them for a long time and there is
little to indicate that people have gotten into trouble doing so. In that
sense they have stood the test of time. They also are the best that is
available, despite their shortcomings. But the industry really needs some
new, original work done in this area.
Doug Brooks
.
****************************************************
Doug Brooks, President [log in to unmask]
UltraCAD Design, Inc. http://www.ultracad.com
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