[eagle] Re: Revised Module Suggestion
John B. Stephensen
kd6ozh at comcast.net
Mon Oct 15 23:45:35 PDT 2007
We have good control over hole locations on the PCB. If the board edges are placed 10 mils from the inside surfaces of the module, all holes will be within 5 mils of the specified location.
Having the entire PCB rest on a plate might be useful for S2 and C band. The PCB dielectric should be 30 mils or less in thickness to minimize EM radiation losses and the dielectric material is much less rigid than the fiberglass used at lower frequencies. Microstrip circuits have a solid ground plane on the bottom.
----- Original Message -----
From: Dick Jansson-rr
To: 'Juan Rivera'
Cc: 'AMSAT Eagle'
Sent: Monday, October 15, 2007 17:40 UTC
Subject: [eagle] Re: Revised Module Suggestion
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.
Dick Jansson, KD1K
kd1k at amsat.org
kd1k at arrl.net
From: wa6htp at gmail.com [mailto:wa6htp at gmail.com] 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
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
Via the Eagle mailing list courtesy of AMSAT-NA
Eagle at amsat.org
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