Re: From Member: RE: [CAD_CAM_EDM_DRO] Keeping heat sinks on chips???

What I need to protect is some ICs nothing else.I was concerned about two part epoxy generating too much
heat while curing. I am looking at a number of things I can do.
Thanks for you input Danny.
Are you in Austin Texas ? I am in Grapevine up by DFW airport.
Jim




________________________________
From: "dannym@..." <dannym@...>
To: CAD_CAM_EDM_DRO@yahoogroups.com
Cc: JCullins <james_cullins@...>
Sent: Tuesday, April 21, 2009 3:35:37 PM
Subject: From Member: RE: [CAD_CAM_EDM_DRO] Keeping heat sinks on chips???

I wouldn't, not if we're talking power driver. AFAIK no potting epoxy provides a decent thermal conduction coefficient for a power driver. Then there's all sorts of issues. Electrolytic caps swell over time for example, but the epoxy provides no room for expansion.

If it's some kind of controller, the "meat" is all in the microcontroller/FPGA anyways. Microcontrollers provide fairly robust methods for protecting the code. Actually most mfgs simply brush off the labels on the chips which makes it very difficult to reverse engineer. Small SMD packages are unlabeled by nature and while someone can measure the capacitance on a cap meter, the dielectric type and breakdown voltage are harder to discern.

Code protection can be defeated, and there's actually ways to dissolve the potting epoxy leaving the components unharmed. They're pretty "extreme" though. No offense but typically I see people asking about them have circuits which could be designed independently for less trouble than trying to crack code protection. The guy who's trying to figure out what the cap voltage was could do it a lot faster by just looking at what the driver does and rating it for a new design.

Some people have actually used techniques of adding fake components and traces. In fact, IIRC there's a company making dummy components somewhere. This is really funny, adding a fake diode in yours that is actually an "open" but will short out someone else's copy when they put in a real one.

Danny

---- JCullins <james_cullins@...> wrote:

> Danny,
> What would you recommend to epoxy some circuitry for security reasons. I
> would need to be non-conductive.
> Thanks
>
> www.soundlogicus.com
> Plasma THC-300
> Serial Spindle Speed control
>
>
> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Danny Miller
> Sent: Tuesday, April 21, 2009 1:42 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] Keeping heat sinks on chips???
>
> There are many thermal epoxies out there, Arctic Silver is the best,
> Arctic Alumina is cheaper and just fine for this app. Clean the
> surfaces and measure and mix carefully.
>
> Be aware that adding heatsinks to the TOPS of chips, where there's no
> thermal pad, has very limited effectiveness. The die inside is what
> needs to be kept cool but the chip's case- probably plastic- strongly
> insulates the die from the cooling heatsink. In this case reducing the
> thermal resistance of the bond layer between the sink and the case is
> beside the point when the die-to-case resistance is 100x bigger anyways.
>
> I know you can't change the board, but any chip mfg who intended their
> chip to be run at power levels exceeding the normal case/lead
> dissipation either includes leads with a direct thermal connection to
> the die (instead of a thin gold bondwire) or, far better, a thermal pad
> on the bottom of the chip that can be soldered to a big copper pad on
> the PCB to spread out the heat.
>
> This weird stuff of gluing on heatsinks tends to mean the board designer
> wants to run the chip outside its mfg specified ratings. And it often
> won't end up getting the benefit expected.
>
> Danny
>
> Jon Elson wrote:
> > Jack Mc Kie wrote:
> >
> >> My Mechatronics controller has heat sinks attached to the chips with
> double sided tape. Sadly this doesn't want to stick and they keep falling
> off. Any suggestions???
> >>
> >>
> >>
> > Either Thermalloy or Wakefield (heat sink manufacturers) used to make a
> > thermally-conductive epoxy called Output.
> > It was the only TC epoxy that didn't cost an arm and a leg. I don't
> > know if this is still available, but there has to be something that will
> > do the same job. Double-sided tape is worthless, unless it is designed
> > for the purpose. Bergquist makes thermally conductive electrical
> > insulating materials, and some of them come with adhesive coatings, but
> > mostly those are one-side only. Check the Digi-Key catalog, they have
> > an entire page of Bergquist items.
> >
> >
> > Jon
> >
> >
>
>
>
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