I think Micrel has an Apnote on PCB heatsinks, with some diagrams in it. I
don't know if there are any formulas.....

Check out the Micrel web page.

Regards,

Peter Marek
General Director
MarekMicro GmbH
Kropfersrichter Str. 6-8
D-92237 Sulzbach-Rosenberg
Germany
Phone: 049 - 9661 - 908 - 210
Fax:      049 - 9661 - 908 - 100
----- Original Message -----
From: Greg Bordash <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, November 17, 1999 4:06 AM
Subject: Re: [DC] Calculating thermal dissipation of external plane heat s
ink area


> Thanks Abd ul-Rahman & Bob your viewpoints on heat transfer but the
question
> I asked in the being has not been answered yet.  You've talked about vias
in
> the plane and an informative debate on the types of heat transfer and
their
> effects but what I'm looking for is a formula that one can use to
determine
> the area required for a heatsink using the PCB.   Therefore I ask again.
>
> Original question;
>
> "Does anyone know of or have a formula for calculating the "natural"
thermal
> (heat) dissipation of an external plane area for 1 oz FR4 based material.
> We are in the process of using an external (Primary or Secondary) layer
heat
> sink plane for a regulator type device on a multi layer design and require
a
> means of calculating the required area based on its power requirements.
> This design is for a general / commercial Class 1 or 2 application using
> convection type heat dissipation supplemented with a forced (fan)
> convection."
>
> Any help would be greatly appreciated.
>
> Regards, Greg.
>
> Gregory E. Bordash,
> Team Leader, PCB CAD Group
> ATI Technologies Inc.,
> 33 Commerce Valley Drive East,
> Thornhill, Ontario, Canada, L3T 7N6
> Phone:(905) 882-2600 ext: 8370,   Fax: (905) 882-9339
> Email: [log in to unmask]
>
>
>
> -----Original Message-----
> From: Abd ul-Rahman Lomax [mailto:[log in to unmask]]
> Sent: Tuesday, November 16, 1999 5:23 PM
> To: [log in to unmask]
> Subject: Re: [DC] Calculating thermal dissipation of external plane heat
> sink area
>
>
> At 04:19 PM 11/16/99 -0500, Bob Landman wrote:
> >Well...
> >
> >If you want to get into the nitty-gritty, there are but TWO forms of
> >heat transfer I am aware of to either heat or cool, radiation and
> >conduction.  Convection is not a means of heat transfer.
>
> Mr. Landman is not speaking precisely:
>
> [...]
> >I should have said "natural or free convection" as none of the
> >equipment I design uses fans but instead relies upon natural
> >convection to remove heat from components to the ambient environment
> >(which can be up to 85C).
>
> Convection moves heat. It is distinct from conduction; with conduction the
> heat is transferred between stationary parts of a system at different
> temperatures; with convection, the heat is transferred by the physical
> movement of a gas or liquid.
>
> Air-cooling is both conductive and convective, but the latter is generally
> more efficient. True, the transfer of heat between the tin/lead surface
and
> the air is conductive, but if not for convection, the overall rate of
> transfer would be very low; it is only the constant replacement of heated
> air with cooler air that makes air-cooling effective.
>
> >Obviously, a fan does increase the exchange rate of the air so that
> >new cooler air replaces it and thus conduction again heats the new
> >air.  Of course if this fan system is in a sealed room you eventually
> >reach an equilibrium temperature which then depends on the radiation
> >out of the room and that depends on the temperature of the environment
> >outside the room.
>
> Yes, of course.
>
> [...]
>
> >Yes, of course, two surfaces that radiate (both sides of the PCB) are
> >better than one as is twice the surface area.  And the fact is that a
> >tin-lead plated copper plate is a lousy radiator - blacken it and it's
> >much better.  But then what is it radiating TO?  Blacken the walls of
> >the container (3M used to make a lampblack type paint that was ~90%
> >absorbing) and it can suck up a lot of radiated heat.  Of course then
> >it has to radiate that heat to the outside environment.
>
> Of course. But anything which lowers the thermal resistance will help.
>
> >I find it hard to believe the general statement that holes in a PCB
> >will cool unless the air is flowing through the board and that depends
> >upon the position of the board doesn't it?  If you are going to all
> >that trouble, a clip-on heatsink sticking up in the breeze would be
> >far better than the PCB.
>
> First of all, the holes are almost zero trouble; perhaps they add a penny
> to the cost of the board, if that.
>
> Secondly, the holes will increase the transfer of heat to the air even if
> the air is not moving. But the air will move. A clip-on heatsink may well
> be "better" but then we are talking about cost and, in one application
> where I used a holey board, available vertical space.
>
> >As for me, I'm not a big fan of "experimental" evidence.
>
> Especially when it does not confirm the theory!!!
>
> Obviously a sound theoretical knowledge confirmed by experiment is the way
> to go, if one can. Otherwise the theory can be worse than simple
> ignorance....
>
> >I prefer
> >good theory backed by some experiments on the particular design
> >because there are too many variables in each design that influence
> >cooling.  I'd still like to see the calculations.  I've not got the
> >time to dust off my old physics or ME books but it's in there under
> >either radiation or heat transfer topics, I'm sure.
>
> We use theory and simulation and calculation in an attempt to produce good
> design in advance of an actual physical prototype. But the proof is in the
> prototype, in the experiment. In a real system in an atmosphere,
> conductive/convective cooling may be a larger factor, I would expect, than
> radiative cooling. It would be odd to ignore air cooling simply because it
> is easier to calculate radiative....
>
> Anyway, perhaps someone has the equations Mr. Landman is looking for....
>
> [log in to unmask]
> Abdulrahman Lomax
> P.O. Box 690
> El Verano, CA 95433