On Fri, 17 Aug 2007 00:56:39 GMT, Rich Grise <rich@example.net> wrote:

>On Thu, 16 Aug 2007 08:52:19 -0700, John Larkin wrote:
>
>> On Wed, 15 Aug 2007 23:57:58 -0400, John
>> <jmuchow@SPAMMENOTcamlight.com> wrote:
>>
>>>>>>I don't trust the phase-change crayon stuff. It's likely to leave air
>>>>>>pockets, and I'm not sure if it will ever really flow thin. I've
>>>>>>measured the white Dow grease as squashing down to below 100
>>>>>>micro-inches under moderate pressure, about my resolution limit.
>>>>>>
>>>>>>The phase-change pads are definitely garbage.
>>>
>>>I totally agree.
>>>
>>>I've gotten the Aavid Ultrastick phase-change goop down to what I
>>>thought was about 1-2mils after clamping. It flowed like water when
>>>hot but never got any thinner than that. And that's with belleville
>>>washers, etc.
>>>
>>>The grease I'm using now starts at that thickness (if I put it on with
>>>care) and then just gets thinner with pressure. Their thermal
>>>conductivity specs are pretty close so I'm going with the grease.
>>>
>>>I'm using the Wakefield 126 Series stuff now but Dow TC-5022 looks
>>>great. Several times the conductivity of the 340 grease. I have a
>>>sample on order and will see if there's any difference. At these
>>>thicknesses though, I can probably use mayonnaise and still be OK.
>>>
>>
>> Yeah, any sort of flowable grease will probably be about the same. I
>> suspect that the thermal filler materials don't help much; what's
>> important is that it's not air and that it squashes down very thin.
>>
>> Vaseline maybe? Axle grease? Grey Poupon?
>>
>Nah, Grey Poupon has those little ground-up seeds.
>
>But what about RTV? It's silicone, and you wouldn't even need bolts!

Epoxy is good. If you load it with glass beads, you don't even need an
insulator.

John

>>>Get a dvm and a pair of sharp probes. Try to measure ohms as you poke
>>>the heatsink. If it's good anodize, you will have to apply real
>>>muscle, grinding the probes into the coating, before you see
>>>continuity.
[snip]
>>>It does usually have character, partly because of the surface finish
>>>before anodizing. Sometines we specify "heavy etch, 1.0 mil hard black
>>>anodize". The etch smooths the surface and removes any tiny burrs.
>>>
>>>But if your coating passes the probe test, it's probably fine.

OK, I'm feeling better about the coating. With a 10x loupe, the
hardcoating looks very much like a windshield after being
shattered...lots of tiny "platelets" sitting on the surface of the
metal, all adjoining each other. But, I grabbed a pair of pretty
sharp probes and went to town. I had to very visibly dig/scratch into
the coating to get any reading.

Would the heavy etch possibly have any impact on the case-to-sink
thermal impedance (by increasing surface roughness)?

We have several 6-32 holes for mounting the MOSFETs. I'm thinking it
can't etch away enough metal to affect the tightness of those, could
it? I guess it depends on how big a burr the etching could remove.

John

>>>Epoxy is good. If you load it with glass beads, you don't even need an
>>>insulator.

LOL, brilliant. The bead size could even be spec'd to get the desired
dielectric strength.

John

On Fri, 17 Aug 2007 05:11:05 -0400, John
<jmuchow@SPAMMENOTcamlight.com> wrote:

>>>>Epoxy is good. If you load it with glass beads, you don't even need an
>>>>insulator.
>
>LOL, brilliant. The bead size could even be spec'd to get the desired
>dielectric strength.
>
>John

Check out Cataphote.

John

>>>>LOL, brilliant. The bead size could even be spec'd to get the desired
>>>>dielectric strength.
>>>>John
>>>
>>>Check out Cataphote.
>>>John

I can't believe I can buy 2nm size nanospheres!
Cataphote doesn't seem to have a web site but I did find other
companies. Just an amazing selection of sizes and materials!

John

On Sat, 18 Aug 2007 06:45:45 -0400, John
<jmuchow@SPAMMENOTcamlight.com> wrote:

>>>>>LOL, brilliant. The bead size could even be spec'd to get the desired
>>>>>dielectric strength.
>>>>>John
>>>>
>>>>Check out Cataphote.
>>>>John
>
>I can't believe I can buy 2nm size nanospheres!
>Cataphote doesn't seem to have a web site but I did find other
>companies. Just an amazing selection of sizes and materials!
>
>John

http://www.cataphote.com/


They make the bulk glass beads used for retroflectors. Here in
California, at every stop sign they also lay down a STOP stencil in
the street, spray on white paint, the toss on handsfull of Cataphote
beads to make the lettering reflective. Some blow away and leave
little sparkles in the neighborhood here and there, until the rain
washes them away.

John

On Fri, 17 Aug 2007 05:09:03 -0400, John wrote:

>>>>Get a dvm and a pair of sharp probes. Try to measure ohms as you poke
>>>>the heatsink. If it's good anodize, you will have to apply real
>>>>muscle, grinding the probes into the coating, before you see
>>>>continuity.
> [snip]
>>>>It does usually have character, partly because of the surface finish
>>>>before anodizing. Sometines we specify "heavy etch, 1.0 mil hard black
>>>>anodize". The etch smooths the surface and removes any tiny burrs.
>>>>
>>>>But if your coating passes the probe test, it's probably fine.
>
> OK, I'm feeling better about the coating. With a 10x loupe, the
> hardcoating looks very much like a windshield after being
> shattered...lots of tiny "platelets" sitting on the surface of the
> metal, all adjoining each other. But, I grabbed a pair of pretty
> sharp probes and went to town. I had to very visibly dig/scratch into
> the coating to get any reading.
>
> Would the heavy etch possibly have any impact on the case-to-sink
> thermal impedance (by increasing surface roughness)?
>
> We have several 6-32 holes for mounting the MOSFETs. I'm thinking it
> can't etch away enough metal to affect the tightness of those, could
> it? I guess it depends on how big a burr the etching could remove.
>

If you want it flatter, get some diamond grit and lap it.

Otherwise, I wouldn't worry if you've got good heatsink goo.

Cheers!
Rich

On Fri, 17 Aug 2007 05:09:03 -0400, John
<jmuchow@SPAMMENOTcamlight.com> wrote:

>>>>Get a dvm and a pair of sharp probes. Try to measure ohms as you poke
>>>>the heatsink. If it's good anodize, you will have to apply real
>>>>muscle, grinding the probes into the coating, before you see
>>>>continuity.
>[snip]
>>>>It does usually have character, partly because of the surface finish
>>>>before anodizing. Sometines we specify "heavy etch, 1.0 mil hard black
>>>>anodize". The etch smooths the surface and removes any tiny burrs.
>>>>
>>>>But if your coating passes the probe test, it's probably fine.
>
>OK, I'm feeling better about the coating. With a 10x loupe, the
>hardcoating looks very much like a windshield after being
>shattered...lots of tiny "platelets" sitting on the surface of the
>metal, all adjoining each other. But, I grabbed a pair of pretty
>sharp probes and went to town. I had to very visibly dig/scratch into
>the coating to get any reading.
>
>Would the heavy etch possibly have any impact on the case-to-sink
>thermal impedance (by increasing surface roughness)?
>
>We have several 6-32 holes for mounting the MOSFETs. I'm thinking it
>can't etch away enough metal to affect the tightness of those, could
>it? I guess it depends on how big a burr the etching could remove.


The etch does improve the burr situation. Even better is to
clamp-mount the transistors.

The hard anodize does tend to gunk up threads tapped into the
heatsink. If the hole is a bit countersunk before it's tapped, you're
less likely to have burrs, and you can chase the threads with a tap if
necessary, although that's real hard on taps. I think you can get
oversize taps, which allow for the buildup from anodizing and don't
need chasing.

We've done thousands of power amps with transistors bolted or clamped
to anodized heatsinks, up to 200 volts, and only had a few problems,
mostly from burrs or post-anodize dings as I recall.


John

>>>http://www.cataphote.com/

Thats' the site I tried. It's only an entrance to someone's search
portal.

John

>>>If you want it flatter, get some diamond grit and lap it.
>>>Otherwise, I wouldn't worry if you've got good heatsink goo.

The Wakefield stuff we have seems to be working out very well. We
also ordered a sample of the Dow TC5022 (I think) goop and they're
sending us a 1kg can!

I'm going to have to go out and give some of this stuff away. We'll
never use that much in 100 years.

John

>>>The etch does improve the burr situation. Even better is to
>>>clamp-mount the transistors.

You read our minds. If we make any more of these things, we're going
to switch over to clamping.


>>>The hard anodize does tend to gunk up threads tapped into the
>>>heatsink. If the hole is a bit countersunk before it's tapped, you're
>>>less likely to have burrs, and you can chase the threads with a tap if
>>>necessary, although that's real hard on taps. I think you can get
>>>oversize taps, which allow for the buildup from anodizing and don't
>>>need chasing.

Oversize taps...that sounds familiar. So far, with the couple of
sinks we've tested, it only took using a machine screw to chase the
threads.


>>>We've done thousands of power amps with transistors bolted or clamped
>>>to anodized heatsinks, up to 200 volts, and only had a few problems,
>>>mostly from burrs or post-anodize dings as I recall.

Currently, the holes are countersunk before tapping. So far, so good.
I hate having to grab the loupe and check each sink though. I guess
if we don't find any problems after checking a bunch, we'll just
assume all is OK until one of the sinks tests hot. Hmm...actually,
it'll take less time to 100% check the batch rather than swap out a
heat sink that has a burr.

John

On Sun, 19 Aug 2007 06:40:11 -0400, John
<jmuchow@SPAMMENOTcamlight.com> wrote:

>>>>http://www.cataphote.com/
>
>Thats' the site I tried. It's only an entrance to someone's search
>portal.
>

Hmmm, yes, it is strange. They *used* to have a real web site.

John