On 09:37 AM 27/06/2001 -0700, Brad Velander said:
>Rob,
> please understand as well that your hole must be routed to make the
>oval slot. Therefore you must talk with your fabricator to assure they have
>a routing bit available in the correct width. Common sizes to stick to would
>be 0.03125", 0.0625", 0.0935" because these are the most common routing bits
>the fabricators typically (but not always) have.
Good advice, Brad. Speak to your PCB maker and use standard tools of
practical size.
Several of the board makers I have dealt with prefer to have the rout width
slightly larger than a standard tool - at least for plated slots. This
allows them to run the router along both edges of the route. A router
apparently cuts cleaner in one direction than the other (presumably such
the the rotation is against the feed direction). This requirement (for a
clean edge) is not so critical with a unplated edge but is important when
you wish to plate the slot. Large amounts of narrow slots (below what can
be done with a 2.4mm dia router) can start to impact on board cost as the
feed rate for narrow routers is lower and the number of boards that can be
stacked is less- at least this is advice given to me a few years ago by a
number of PCB makers.
I have done quite a number of plated slots. All successful, all using a pad
at each end of the slot (or just one in the centre if the slot is short)
and tracks and arcs on a mechanical layer to show the slot. Oh, and,
copious notes on the PCB mech layer and in the manuf notes. I have done a
plated slot with 1.2 mm width - to accept a DC power connector with flat
broad pins, that had to be located accurately (I offset the various slots
to ensure alignment even though the flat pins where much narrower that the
slots). I was told at the time that I could have 0.8mm routs but I did not
want to pay the extra; so I simply offset the routs.
Now when laser drilling/cutting etc becomes common we should be able to
have virtually anything we want. And we will be hassling Protel for the
ability to be able to define non-round and complex holes. Maybe Protel
should be thinking about this now...
If there are to be significant changes to the pad stack definitions (which
would be nice) then some though should be given to support for slots and
possibly arbitrary shaped holes. I do not see it necessary now but in the
future who knows.
Ian Wilson
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