On Sat, Sep 10, 2005 at 02:24:58AM -0400, Tim Schmidt wrote:
> > Our intention from the beginning has been to find something
> > off-the-shelf, preferably something we can get volume discounts out of
> > a catalog. Digikey, Mouser and the like are places we'd expect to
> > find what we're after.
> >
> > The time-consuming part is going through the available options,
> > comparing heat dissipation properties, price, and other factors, and
> > deciding on one. That's the week we need to save.
>
> Fair enough... So, exactly what heat dissipation properties, and
> price are you looking for? Even if that's not quite enough to make
> the decision, someone can be narrowing the list right now, and when
> you decide what else you need it will be that much shorter to look
> through.
Unfortunately, even picking something off the shelf is going to
involve some serious engineering and a lot of detail work.
The place to start is with a written statement of the requirements.
Let's get into the numbers. What will be the maximum power dissipation, the
required IC case temperature, the maximum air temperature at the heat sink
fins, the IC contact area, the thermal interface material and its thermal
resistivity, and the space envelope available for the heat sink, its
mountings, and the airflow paths in and out?
With that information, it will be possible to calculate the general
shape and required thermal resistance of the heat sink, look at what's
available in catalogs, and determine whether a natural convection solution
is even possible.
If calculations show that natural convection won't work for the
worst-case combination of power dissipation at maximum clock rate and
maximum specified air temperature, then we can move on to selecting a
forced-air heat sink that can do the job, a blower that can deliver the
required air flow rate against the flow resistance of the fin assembly, and
the necessary ducting and mounting features to hold it all together.
Only after we know what we're mounting and how we're mounting it can
we place the mounting holes in the board. This must be done at the same
time as IC parts placement and before placing anything else, otherwise
layout work will have to be re-done. That would waste time.
The OGD development board offers a little more freedom to select
heat sinks and blowers than the OGC ASIC-based graphics board does. Its
production life will be short. We would be safe using no-name imported
parts that might not be available in a year. For the OGC we should select
established domestic parts manufacturers who can be depended on to continue
production for a number of years; otherwise see if there are
industry-standard mounting hole patterns that multiple suppliers conform to.
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