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'[OT] Bullet Proof Circuit...'
1999\10\22@124843 by Lawrence Lile

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Here are some additional rules for "Wagners 10 golden rules"  (Excellent,
Wagner)

Often I design low cost  PC boards with 120Vac and 10 amps on the board.
These wierd conditions require some extra care:

1.  Traces:  Try to use the fattest trace with the widest spacing that you
can.  Just because your board house can print 8 mil traces with 8 mil
spacing doesn't mean you have to use them.

2.  UL requires 0.064"/1.6mm spacing between 120Vac traces.  Use 0.080"/2mm
where possible.

3.  Watch for ways that high power sections of the board can short out.
This is especially a problem on vertical PC boards.  Overheated solder
joints drip!

4.  Add a thin trace going into the power section of a high power board.
Hourglass it down to 0.013"/0.3mm.  If you get a short on the board, this
will act as a fuse.  Only use such a crude technique if you can't afford a
real fuse.  Make sure that you don't put much current through it!

5. Power resistors heat the board less if they are standing on end, rather
than parallel to the board.

6.  Never overstress a capacitor more than 80% of it's votlage rating.  50%
is better.

7. Wattage calculations are about as good as weather calculations or
predictions of the end of the world.  If you have the chance, glue a
thermocouple on a component and measure the temperature in the actual
conditions.  Then go ask your electronics teacher why wattage calculations
are never right.

8, Poor connections make lots of heat, when high currents are flowing.
Often crimped or riveted connections will fail, due to creep or poor
technique.  You'll see a big burnt spot around the connection on the PC
board.  Spade terminals are a common culprit.  A loosely crimped spade
terminal can get so hot as to melt solder!

9.  There are charts out there purporting to show "maximum" current ratings
for specified trace width - always too conservative.  There are other charts
showing TEMPERATURE RISE for a certain current and trace width.  These are
the better ones to use.  Here are two good ones:

http://www.geocities.com/CapeCanaveral/Lab/9643/TraceWidth.htm   Nice online
calculator


http://www.aracnet.com/~gpatrick/  useful charts for trace width and a lot
of other PCB design rules



-----Original Message-----
From: Wagner Lipnharski <spam_OUTwagnerlTakeThisOuTspamEARTHLINK.NET>
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU <PICLISTspamKILLspamMITVMA.MIT.EDU>
Date: Thursday, October 21, 1999 8:09 PM
Subject: Re: [OT] Bullet Proof Circuit... was: Survey... What is an
acceptable failure rate?


>No books about it AFIK, but some tips perhaps,
>here goes "Wagner's 10 golden rules":
>

1999\10\22@205513 by William K. Borsum

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While on the topic of protecting boards......

Does anyone know of a good, cheap, flexible, repairable coating and potting
material--preferably transparent??

I have used a variety of urethanes...most stick pretty tightly, but will
come off with a hot knife, the right solvent (which also dissolves the
PCB), and a lot of elbow grease.  Not what I would call repairable--usually
end up doing more damage than gets fixed.  Potted units are in effect
potted for life--and that's the problem.  Great if you want a throw-away
device--but not if you've got a very expensive board you can't get to to
repair.

Also tried silicone rubber--nice coating, transparent, cuts with an exacto
knife, and peels right off.  Not good for thin coatings since it will peel
off with the slightest touch around the edges, but great for potting the
whole shebang.  Also, NOT cheap.  about $30 for a 1-Kilo kit.  Not real
sure it will completely protect the circuit from water creeping in around
leads, etc.

HINT & Kink for the day:  you can de-bubble mixed silicone rubber by
putting the mixed material in the freezer overnite.  Stuff stays liquid,
and does not cure, so the bubbles have plenty of time to make their way to
the top and pop.  Let it come up to room temperature away from moisture
however--in a baggie works well on those really humid days.  Stuff will
then go through its normal pot life of several hours, then cure overnite at
room temperature, or a few hours at 50 degC.

Kelly

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<.....borsumKILLspamspam.....dascor.com> & <http://www.dascor.com>San Diego, California, USA

1999\10\22@210724 by Reginald Neale

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>HINT & Kink for the day:  you can de-bubble mixed silicone rubber by
>putting the mixed material in the freezer overnite.  Stuff stays liquid,
>and does not cure, so the bubbles have plenty of time to make their way to
>the top and pop.  Let it come up to room temperature away from moisture
>however--in a baggie works well on those really humid days.  Stuff will
>then go through its normal pot life of several hours, then cure overnite at
>room temperature, or a few hours at 50 degC.
>


 Clever! The recommended vacuum method is a huge pain in the butt.

 Reg Neale

1999\10\23@002658 by William K. Borsum

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At 09:08 PM 10/22/99 -0400, you wrote:
>>HINT & Kink for the day:  you can de-bubble mixed silicone rubber by
>>putting the mixed material in the freezer overnite.  Stuff stays liquid,
>>and does not cure, so the bubbles have plenty of time to make their way to
>>the top and pop.  Let it come up to room temperature away from moisture
>>however--in a baggie works well on those really humid days.  Stuff will
>>then go through its normal pot life of several hours, then cure overnite at
>>room temperature, or a few hours at 50 degC.
>>
>
>
>  Clever! The recommended vacuum method is a huge pain in the butt.

I sure had my doubts when the Dow Chemical guru suggested it--but it does
work--and works well.
He also suggested storing the unmixed chemicals in the freezer (isolated
from food!--and preferably in a freezer of their own) for an almost
unlimited storage life.  Just watch out for water condensing when you bring
the stuff out to use.  Let it come up to room temperature before opening
the containers.

For those of you who don't know--the normal method to get the bubbles out
is to put the mix in a bell jar, and pull a vacuum--with a real vacuum pump
capable of going to at least 28" of Hg.  Problem is, the mix froths up to
MANY times its original volume--real high surface tension keeps the bubbles
from popping until they get huge.  Usually end up with silicone all over
everything.  Much worse than putting a double recipe in the bread machine. :-p

Haven't tried it with epoxies or urethanes, but don't see why it wouldn't
work.  Used to buy pre-mixed years ago that came in dry-ice, and had to be
stored well below freezing. Let me know if any of you try this, and the
results.

kelly

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<EraseMEborsumspam_OUTspamTakeThisOuTdascor.com> & <http://www.dascor.com>San Diego, California, USA

1999\10\23@022826 by William Chops Westfield

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   >>HINT & Kink for the day:  you can de-bubble mixed silicone rubber by
   >>putting the mixed material in the freezer overnite.

   Haven't tried it with epoxies or urethanes, but don't see why it wouldn't
   work.

There are two reasons that it probably wouldn't work:

1) epoxys and urethanes are much more temperature sensitive than silicones.
  Heat them up, they don't do so well, cool the resins down and they get
  VERY viscous or even solid.  (silicone's large temp ranges is a big
  selling point!)

2) curing agents for urethanes (especially) tend to react with water to
  form additional gasses.  Putting them in a (damp) freezer is probably
  not a good idea, taking them out again so that humidity could condense
  on/in them is an even worse idea.

But I haven't tried it, so I couldn't say for sure.

BillW

1999\10\23@023908 by Anne Ogborn

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>HINT & Kink for the day:  you can de-bubble mixed silicone rubber by
>putting the mixed material in the freezer overnite.  Stuff stays liquid,
>and does not cure, so the bubbles have plenty of time to make their way to
>the top and pop.  Let it come up to room temperature away from moisture
>however--in a baggie works well on those really humid days.  Stuff will
>then go through its normal pot life of several hours, then cure overnite at
>room temperature, or a few hours at 50 degC.


I take it you have to put it in a baggie??? - as I recall, moisture in the
mold is the root of all evil.

1999\10\23@120745 by Brian Kraut
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Your mention of epoxies brings up a tip I heard a long time ago that works great
.
You can heat the parts before mixing in the microwave for a few seconds or put
them in hot water.  This thins the epoxy for applications where you need it thin
and watery.  It does speed up curing considerably though, Five munute epoxy heat
d
is good for about 1 minute.

Another way to remove bubbles is to vibrate the mixture.  Dentists have vibrator
tables for their casting materials.  Probably won't work very god for something
thick like silicone, but it works great for thinner materials like heated epoxy.

William K. Borsum wrote:

{Quote hidden}

1999\10\23@122241 by PIC development

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> This thins the epoxy for applications where you need it thin
> and watery.  It does speed up curing considerably though, Five munute
> epoxy heatd is good for about 1 minute.

Be aware that the chemical reaction that cures epoxies is exothemic
(gets hotter) so don't mix too much at once - or if you need a lot,
spread it thin. If it sets when it's too hot it'll crack & you won't
get the strength or integrity from it.

regards
Pete
..............................................................................
. Never trust a man who, when left alone in ....... Pete Lynch               .
. a room with a tea cosy, doesn't try it on ....... Marlow, England          .
..........Billy Connolly. ......................... @spam@picKILLspamspambeowulf.demon.co.uk ..

1999\10\23@174421 by paulb

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Brian Kraut wrote:

> Another way to remove bubbles is to vibrate the mixture.  Dentists
> have vibrator tables for their casting materials.

 Dentists appear to be masters of this technology.  At a considerable
premium of course... ;-)
--
 Cheers,
       Paul B.

1999\10\25@162328 by l.allen

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Kelly Wrote
>
> For those of you who don't know--the normal method to get the bubbles out
> is to put the mix in a bell jar, and pull a vacuum--with a real vacuum pump
> capable of going to at least 28" of Hg.  Problem is, the mix froths up to
> MANY times its original volume--real high surface tension keeps the bubbles
> from popping until they get huge.  Usually end up with silicone all over
> everything.  Much worse than putting a double recipe in the bread machine. :-p
>
> Haven't tried it with epoxies or urethanes, but don't see why it wouldn't
> work.  Used to buy pre-mixed years ago that came in dry-ice, and had to be
> stored well below freezing. Let me know if any of you try this, and the
> results.
>
Yes indeed that is what happens, with a vacuum on Ureathanes,
Epoxies, and Silicones.
We made around 50 Hydrophones a few years ago to find the best way of
making cheap..ish (as in not US$10,000each) Hydrophones for our whale
research.
It was a toss up between  Epoxy and Ureathane depending on the
application BUT  we were able to manage the expansion by
letting air IN while the pump was sucking it OUT. The point was we
had good control over the tap going in but the pump was all ' brute
force and ignorance'.Allowing the vacuum to increase only as the
mixture frothed to an acceptable height  each time.
We also found each substance was 'de-Gassed' best (as in no unwanted
bubbles on setting) at certain temperatures.. e.g. the epoxy we were
using was best at 40 deg C before two part mixing.
The chemical manufacturers are very helpful with such info.
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

http://www.psych.auckland.ac.nz

_____________________________

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