Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

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.

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. 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. 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)

2) Slow down the rate of head movement

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

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

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?

Thanks,

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