Flight Computer Mounting

[Dave Camarillo]

Hello Airframe Team, attached are a few snapshots of the first
proposed mounting technique for the new PC-104 flight computer
stack... Details are as follows:

Constraints:
-need to be able to slide whole FC stack in and out of module
-the heat sink needs to be close to the C-channel for thermal
interfacing and heat sinking
-needs to handle the various forces experienced by the rocket,
including 10G liftoff
-PCB connectors accessible

Features of design:
-the PCB edges sit on a rubber, foam or other material directly on top
of the aluminum plate. During liftoff, the forces are exerted over the
larger and stronger PCB edges instead of the normal metal standoffs.
-To insert or remove, there are (8) 4-40 cap screws that go into the
brackets attached to the flight computer. Remove these, and slide
whole FC stack out.

Possible issues:
-There is no support on the top edges of the PCB. We could add foam or
other material to help handle upward loads (i.e. during recovery and
hitting the ground. However, this also means that if the battery box
pushes downward, those forces will be passed thru the PCB's, not
desirable. Perhaps compressible foam. The (8) pieces of aluminum angle
holding the stack in place would normally take this load, but that may
or may not be enough. There's also the question of how much of a
moment can we apply to those 0.6" standoffs, noting that they are 4-40
threaded.

Let me know what you think....

thanks,
-dave

[Andrew Greenberg]

Post by Dave Camarillo
Hello Airframe Team, attached are a few snapshots of the first
proposed mounting technique for the new PC-104 flight computer
stack...

Sweet! I love it!
It doesn't look like there's room for a fourth PCB on that stack. Is
that true? If not, could we add that possibility? It might turn out to
be very, very convenient to have a FC "breakout" board as a fourth board.

Also, how far apart is the heatsink from the C channel? Can we arrange
for it to be exactly one thickness of thermal interface material (TIM)?

Post by Dave Camarillo
-There is no support on the top edges of the PCB. We could add foam
or other material to help handle upward loads (i.e. during recovery
and hitting the ground. However, this also means that if the battery
box pushes downward, those forces will be passed thru the PCB's, not
desirable.

.. which is why I wouldn't put anything up there. I bet we're OK, unless
we lawn dart, but we shouldn't design for that.

Post by Dave Camarillo
There's also the question of how much of a moment can we apply to
those 0.6" standoffs, noting that they are 4-40 threaded.

Can we directly answer this question using SolidWorks coolness?

Andrew

[Dave Camarillo]

Post by Andrew Greenberg

Post by Dave Camarillo
Hello Airframe Team, attached are a few snapshots of the first
proposed mounting technique for the new PC-104 flight computer
stack...

Sweet! I love it!
It doesn't look like there's room for a fourth PCB on that stack. Is
that true? If not, could we add that possibility? It might turn out to
be very, very convenient to have a FC "breakout" board as a fourth board.

There's barely enough room... it will be tight, but I think it's
doable... see attached images...

Post by Andrew Greenberg
Also, how far apart is the heatsink from the C channel? Can we arrange
for it to be exactly one thickness of thermal interface material (TIM)?

Maybe, depends on the thickness of the TIM... there are a few parts
that will interfere if we move the heat sink any closer to the
channel... It's currently 0.125" from the channel. How thick is the
TIM?

Post by Andrew Greenberg

Post by Dave Camarillo
-There is no support on the top edges of the PCB. We could add foam
or other material to help handle upward loads (i.e. during recovery
and hitting the ground. However, this also means that if the battery
box pushes downward, those forces will be passed thru the PCB's, not
 desirable.

.. which is why I wouldn't put anything up there. I bet we're OK, unless
we lawn dart, but we shouldn't design for that.

Post by Dave Camarillo
There's also the question of how much of a moment can we apply to
those 0.6" standoffs, noting that they are 4-40 threaded.

Can we directly answer this question using SolidWorks coolness?

Yes, we can FEA it....

Post by Andrew Greenberg
Andrew
--
-------------------------------------------------------
Andrew Greenberg
Portland State Aerospace Society (http://psas.pdx.edu/)
-------------------------------------------------------

[Doug Ausmus]

Use a 'tough' material for the standoffs - often pc-104 standoffs are
aluminum or plated brass or plated steel, but are available in nylon... I
suspect other materials can also be had. Some pc-104 systems use nylon edge
channels in tube enclosures (kinda like a rocket body? <g>) instead of the
standoffs. Not sure what is already planned, but using non-conductive
standoffs might be desirable in case of an early shear-off as it increases
survivability with no conductive particle loose and floating around in that
event. If you will be FEA'ing the standoffs anyway, suggest just running a
few additional materials to compare against.

Using *no* standoffs and, instead, clamping at connector points might also
work, as long as there are appropriately machined non-conductive spacers in
areas where the interboard connectors are not carried all the way through
the pc-104 stack. This might save weight (wishful) or not, but could be
cleverly designed to improve shear-force shedding/spreading.

-Doug

On Fri, Dec 23, 2011 at 10:36 AM, Dave Camarillo

Post by Dave Camarillo

Post by Andrew Greenberg

Post by Dave Camarillo
Hello Airframe Team, attached are a few snapshots of the first
proposed mounting technique for the new PC-104 flight computer
stack...

Sweet! I love it!
It doesn't look like there's room for a fourth PCB on that stack. Is
that true? If not, could we add that possibility? It might turn out to
be very, very convenient to have a FC "breakout" board as a fourth board.

There's barely enough room... it will be tight, but I think it's
doable... see attached images...

Post by Andrew Greenberg
Also, how far apart is the heatsink from the C channel? Can we arrange
for it to be exactly one thickness of thermal interface material (TIM)?

Maybe, depends on the thickness of the TIM... there are a few parts
that will interfere if we move the heat sink any closer to the
channel... It's currently 0.125" from the channel. How thick is the
TIM?

Post by Andrew Greenberg

Post by Dave Camarillo
-There is no support on the top edges of the PCB. We could add foam
or other material to help handle upward loads (i.e. during recovery
and hitting the ground. However, this also means that if the battery
box pushes downward, those forces will be passed thru the PCB's, not
desirable.

.. which is why I wouldn't put anything up there. I bet we're OK, unless
we lawn dart, but we shouldn't design for that.

Post by Dave Camarillo
There's also the question of how much of a moment can we apply to
those 0.6" standoffs, noting that they are 4-40 threaded.

Can we directly answer this question using SolidWorks coolness?

Yes, we can FEA it....

Post by Andrew Greenberg
Andrew
--
-------------------------------------------------------
Andrew Greenberg
Portland State Aerospace Society (http://psas.pdx.edu/)
-------------------------------------------------------

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http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe

[Andrew Greenberg]

Post by Dave Camarillo
There's barely enough room... it will be tight, but I think it's
doable... see attached images...

That's good enough for me. Could you send me a DXF of the board size,
including the keepout areas of the various hardware around that board?
I'll wait on doing anything until we've built this thing, but I'd like
it for planning over the next coupla weeks.

Post by Dave Camarillo
Maybe, depends on the thickness of the TIM... there are a few parts
that will interfere if we move the heat sink any closer to the
channel... It's currently 0.125" from the channel. How thick is the
TIM?

There are a huge amount of thicknesses we can get, all directly from
Digi-Key. In mils (sorry), they are:

1, 3, 4, 5, 10, 15, 19, 20, 40, 59, 60, 80, 98, 100, 125.

So the 0.125 will work, but I'd try and tighten it down to as little as
possible that still holds together, more like 40 - 80 mils.

Andrew