[s-cars] POS Source?

Thu Mar 11 23:25:10 EST 2004

I agree that the heatsink might be overkill and that the FET itself did not
fail from overheating.
One thing to note, if the case temperature is kept low, the opportunity for
thermal shock gone.

The potting inside doesn't do much for vibration since it is almost a
liquid.

Mike

----- Original Message ----- 
From: "Vincent Frégeac" <vfregeac at sympatico.ca>
To: "'Mike Sylvester'" <msylvester at verizon.net>
Cc: <s-car-list at audifans.com>
Sent: Thursday, March 11, 2004 11:09 PM
Subject: RE : [s-cars] POS Source?

Hi Mike,

In fact, the failure analysis you made also prove the failure mode is
not overheating. An overheating problem would have lead to an internal
failure in the FET, not a connection failure. The thermal paste may be
the kind of over engineering which allows some listers to go close to
400hp without modifying the internal components of the engine.

I would rather think about thermal shock as several listers have
experienced POS failure when washing the engine, me included. Vibrations
may also be a cause as solder joints do not like them. The paste you've
seen inside is commonly used on circuit boards to avoid vibration on the
heavy components like capacitors.

At the end, I think the thermal paste is a good safety measure. It may
be overkill but it cost almost nothing and just add a few seconds to the
POS swap. I was mainly reacting about the heatsink.

Vincent.

-----Message d'origine-----
De : Mike Sylvester [mailto:msylvester at verizon.net]
Envoyé : 11 mars 2004 22:39
À : Vincent Frégeac
Cc : s-car-list at audifans.com
Objet : Re: [s-cars] POS Source?

If those temp readings were accurate, why then did Audi even bother with
the
paste?
There must have been some concern.
A measurement taken with an infrared gun will be reading the plastic
case
which doesn't conduct very well.
A better test would be to unbolt one, run the car and measure the temp
of
the metal backing.

If this failure is not due to overheating, then these are really POS
because
too many of these have failed.
I had one fail at 96k miles.  Most people might think that a failure at
96k
is common, but not for electronic parts.
It probably is only about 3,000 hours.  The MTBF of this part is
probably
something like 200,000 hours.

I will admit, it doesn't look like a straight up overheating problem.
Like
I said in a previous post, it could be fatigue from repeated temp
extremes.
I really would like to open up a few more.

Mike

P.S.
I added the FA photos to my site.

http://mysite.verizon.net/vze6ngsr/S6POS.htm

----- Original Message ----- 
From: "Vincent Frégeac" <vfregeac at sympatico.ca>
To: "'Igor Kessel'" <igor at s-cars.org>; "'Mike Sylvester'"
<msylvester at verizon.net>
Cc: "'Miller, Casey'" <Casey.Miller at janus.com>;
<s-car-list at audifans.com>
Sent: Thursday, March 11, 2004 10:06 PM
Subject: RE : [s-cars] POS Source?

>From times to times, the list tends to go a bit crazy about "preventive
maintenance". So, a few facts to keep your wallet a bit thicker:
- Temperature on original POS without original crappy thermal paste:
107F
- Temperature on new POS with new thermal paste: 98F
This was in October, 70F ambient temperature after a 45min drive, engine
still running, measurement made with the calibrated infrared gun we have
at the plant. I've yet to see an electronic component with a recommended
temperature below 160F. So I think adding a heat sink behind the POS is
a bit overkill. Even the thermal paste seems a bit overkill as the low
power loss of the POS seems to be already very well dissipated with a
simple metal to metal contact. We're not talking high power class A
amplifier here, but just a switching device.

My 0.02$

-----Message d'origine-----
De : s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] De la part de Igor Kessel
Envoyé : 10 mars 2004 10:08
À : Mike Sylvester
Cc : Miller, Casey; s-car-list at audifans.com
Objet : Re: [s-cars] POS Source?

Mike Sylvester wrote:
> I just bought one yesterday from The Parts Connection.  It cost me
$199.
>
> When changing your bad POS I recommend applying new thermally
conductive
> paste to the other POS as well.
> Actually, I recommend this as a preventative maintenance on working
POS.
> I found that the factory past was dried up and it didn't appear that
there
> was good contact between the POS and the bracket.
>
> These parts are failing long before they should.  However they are not
> manufacturing defects since they would have failed much sooner if that
were
> true.
> It is quite possible that they are overheating.  When I replace mine
> tonight, I also installed a heat sink to the bracket behind each POS.
> http://mysite.verizon.net/vze6ngsr/S6POS.htm
>
> I should note that I sandblasted the bracket and when I repainted it I
> masked off the contact area of the POS and heatsink.
> The heatsink may be overkill, but it can't hurt.
>
> Mike Sylvester
> '95 S6 avant w/ POS running cool
> http://mysite.verizon.net/vze6ngsr/index.htm

Mike,
you are a damn GENIUS! Thank you so much for the idea! I have the paste
and I have a couple of old P-100 processor sinks. And I am just about to

start fusing the coils individually, having recently witnessed Stefan
flatbed his car only because of the stupid short in one of the coils.
I'll do everything in one go.

Thx again. The timing of your post couldn't be more perfect.

-- 
Igor Kessel
two turbo quattros
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