Convective cooling may prove difficult so you may want to consider a type
of conductive cooling.  A couple of years ago I came across liquid heat
sinks being used in a design such as yours.

These were bags of a special liquid - (Flourinert I think) - from 3M Corp.
I don't have any product info. so I am not sure about its thermal
properties.  The bags of liquid are sandwiched between boards and conduct
the heat away from any hot-spots.   

This may seem a little off the wall, but it may be a solution depending on
your particular situation.

Hope this helps

Brendan Mc Cormack
Senior Interconnect Engineer
AMT Ireland
University of Limerick.

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On Tue, 10 Oct 1995 [log in to unmask] wrote:

> I need some help for a colleague on thermal dissapation.
> A design with one mother board and three daughter boards, ALL 
> with 10 lbs in a 5 lb container. The daughter boards are all 
> within a half inch of motherboard. Needless to say when 
> running inside the box after awhile things tend to get
> somewhat hot approx 125 degrees C. If outside in open air the
> temp drop is only about 15 degrees C. They do not want to use
> fans and conventional heatsinks seem out. A thermal plane on both
> outside layers is being considered realizing that although you 
> may be able to solder the board once rework is out. Is there
> any suggestions out there in the board material. I suggested 
> liquid cooling ha ha. I don,t now of any other means of 
> bringing the temp down that much without fans. Any help 
> would be greatly appreciated. The mechanical designer asking
> me this is supposed to do something with the enclosure, but no 
> fans are being considered. He has no other ideas either. 
> Thanks up front. I could not believe anyone would try and cram
> that much electronics in such a small area with no thought for 
> heat untill after the design was done. The desity is double sided
> surface mount with thru hole jammed on all four boards.
> P.S. I could really use this help ASAP so E-Mail or Phone would be great.
> 
> Robert M. Wolfe
> 203-882-6405
>