Homebrew PCBs

At 02:34 PM 12/3/2005, you wrote:
>Hi, Group!  I teach at a university which recently bought a T-Tech
>Quick Circuit 5000 for student project fabrication.  I'm trying to
>mill a TQFP-100 footprint, which has 0.22 mm (nominal) wide pads
>(approx. 8.7 mils) on 0.5 mm centers.  This means removing a 0.28 mm
>(11 mil) wide space between pads, but IsoPRO tells me it needs to
>use a 6-mil tool  The reason seems to be that it cuts twice between
>pads, and I wonder if that's really necessary.

No' it's not necessary, but IsoPro has to mill each border of a trace. It 
not mill the isolation as a channel width,
but as isolation width.
If the isolation is, let say 11mil, on the first pass (let say the left 
side of a channel)will mill 6mil and on the second pass
(the right side of a channel) the remaining 5 mil (one mil is gone already).


>I've made one attempt so far, using a T1 pointed tool which has a 60-
>degree point and which T-Tech says will mill 8-12 mil paths.

Use 30degree for fine channels.

>I set
>it for a depth of about 7 mils, which should have made the cut width
>about 5.4 mils on a 1-oz copper board, if I'm not mistaken.  I set
>the depth by carefully lowering the tool until it just touches the
>copper surface, then counting clicks.

Is better to use the mechanical depth adjustment measuring the channel's 
width with a special microscope.
$200 or so for a 50x hand held one.

>The manual says each click
>lowers the tool by 0.4 mil, but it seems as it may actualy lower it
>a bit more than that.
>
>The results of my first cut were not acceptable.  Most of the pads
>appeared to be narrower than 8.7 mils, and pretty ragged.  some 10-
>mill traces also looked pretty chewed up.
>
>I'm going to try  the following:
>
>1)  run the spindle at top speed (24,000 rpm)

always use the maximum speed.


>2)  Slow down the rate of head movement

so and so


>3)  use 1/2 oz copper instead of 1 oz.

yes, but with 30 deg tool


>4) use a smaller pointed tool, the T-4 which is supposed to be good
>down to 4 mils.

You can also try end mills instead of V shaped one.They will cut the exact 
diameter, but are expensive.
Try to find a SW which will allow you to use fine mill on the fine portion 
of your work only and on the rest a coarse,
inexpensive one. Try PlatinCNC.


>What is your opinion of these three steps?  Am I going in the right
>direction?  Is there a way to keep the machine from cutting twice
>between pads, and would that be a good idea?

No way to cut once only.


>Thanks,
>
>Pete Goodmann
>Assistant Professor,
>Electrical and Computer Engineering Technology
>Indiana-Purdue University Fort Wayne (IPFW)
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