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
|
