Juan:
Thank you for your comments.
The truly “high power” modules, such as transmitters
with power dissipating devices of many Watts will still need to be constructed
using the E05 01 (1100 alloy aluminum) Heat Sink, which will in turn be bolted
directly to the spaceframe panel (Y side).
The intermediate power modules, such as the URx with the small
internal heat sinks could be special variation of the shown milled baseplate.
With such a provision, however, you would not have the latitude to shift the
location of the heat sink as you have done already without changing the
mechanical dimensions of the machined part, thus calling for another special
part. It also raises issues of the precision of locating such heat sink pads. I
like the concept, however, if it can be worked out, but it will require some
pretty good dimensional control.
Your concept of having the entire PCB rest on a thermal plate
raises many issues. Among them is the fact that you very often have traces on
the bottom side, placing them on the aluminum would not be very “cool”
electrically speaking. This issue also raises other issues, such as flatness.
There are Juan’s flatness and then there is Dick’s thermal flatness
and these two worlds are probably very
much different. Any kind of protrusion, traces or solder lumps or vias will
take such a surface quickly out of the class of being a thermal surface into a
Juan surface. Any non-purely-flat-surface becomes non-acceptable as a thermal
surface. That is why the heat sinks for the URx are only in contact with
specific copper-faced areas on the bottom of the URx PCB.
Your thermal gap filler material offers also only limited help,
even if it is space-worthy. Space worthiness is a matter of the outgassing
characteristics of the material, and most of these kind of materials are not
space acceptable. The thermal characteristics of such materials may also be
pretty limited, when I compare them to some of the power-density heat flow
needs.
Compared to locally clamping a PCB to a bottom heat sink, your
bridge idea does not conduct much heat. We can very quickly get into a
discussion of the conductance values of such heat sinking methods and in those
discussions the bridge method runs out of steam rapidly. You certainly would
not need top as well as the bottom heat sinking of a locale on a PCB.
Thanks for the discussions.
’73,
From: [email protected] [mailto:[email protected]] On Behalf Of Juan
Rivera
Sent: Monday, 15 October, 2007 15.38
To: Dick Jansson-rr
Cc: Bob Davis; AMSAT Eagle
Subject: Re: [eagle] Revised Module Suggestion
Dick,
That looks nice! It appears to solve the issue of
getting that front panel at exactly 90 degrees to the baseplate and also
increases the stiffness of the baseplate. Increasing the useful front
panel space also eases the problem of working around the CAN-Do PCB with all of
the necessary I/O connectors.
Would it be possible to customize the baseplate for the few
modules that draw high power? It would be nice to machine the baseplate
and heat sinks as one chunk of metal instead of the existing method of having
several individual heat sink pieces. I would like to see the PCB laying
flat on top of the baseplate with milled cutouts to accommodate any devices
attached to the bottom side. In a perfect world there would be no
components on the bottom and the PCB would make contact with the baseplate
across the entire surface. Another possibility that might be worth
considering would be the ability to include "U" shaped heat sinks
that would bridge over the top side of hot components and attach to the
baseplate through holes cut into the PCB on either side of the component.
Thermal gap filler could allow room for CTE mismatches so that the device isn't
crushed.
73, Juan - WA6HTP