Tiny PCBs - score, route, or ?

I need to make some small PCBs (about 0.5" x 0.75" ea). I've compiled
some options, but I need some insight on the best way to cut these apart
(ideally, pre-scored / pre-cut and snap apart after assembly).

V-scoring seems good because it might require less PCB space than
routing, the parts are large enough to grip & snap apart, and the edge
would be straight, although rough (which is OK).

Tab routing would leave a clean edge except for the tabs, and it might
put less stress on the board to break apart. Drawback is the "jaggies"
left by the tabs.


Do-it-yourself alternatives...
The "string of holes" method using drill hits, which might prove
cheapest, but also the roughest edge. Any tips on how to space the
holes so they don't fall apart in assembly, but aren't too hard to break
later?

Cutting ourselves with a band saw or a shop router on a table stand w/
guide. This seems a bit hazardous with small parts, and may cost more
in the long run because the cuts may be wider (more waste). (Band saw
isn't a bad idea...)

Using a shear, which I've seen suggested, but not for an assembled
board. Pop! Crunch!

Any thoughts on these other other techniques?


(I've found a few board shops that will do tab routing and/or scoring
for protos, but they're pretty pricey so any references would be welcome
too. This is short-run production, so cost per board is a concern.)

Thanks,
Richard

 

Fibreglass board will wreck a bandsaw blade very quickly...

 

I recently had some small boards made that used a combination of V
scoring (done with the router, with a pointed bit) for straight edges
between the boards and side rails (to hold the boards together for
parts placement and soldering), and routing for the corners and curves
and between boards in the row. This left rough edges only in the
middle of 2 straight sides per board, that I clean up by sliding the
broken board along a sheet of silicon carbide paper. Where the routed
slot meets a V groove, the V groove aligns with the board side of the
routed slot. So I sand till I can't see where the broken edge meets
the routed edge and I have good dimensions.

On another board that was designed by someone else, for us, they used
the routed with tabs, but curved the routed line inward about a half
slot width on each side of the tab so the tab broke just below flush
with the board edge. Of course, this means that it broke sticking way
out on the other side, but this was not another copy of the board, but
a temporary support rail that was discarded.

I think you need about a diameter of board between the holes.

It takes a carbide tipped blade and makes a lot of dangerous dust.

For small runs, you can use a carbide tipped hook knife to pull a V
groove out of the board along a straight edge guide.

If you break boards apart, either along V scoring or drilled holes, it
is a good idea to keep surface mount parts a few board thicknesses
away from the break line, to reduce the chance of pulling the
metalization off the parts or cracking them in half. It is also a
good idea to have the resistors and capacitors along the edge run with
both pads equally far from the edge, instead of with one pad along the
edge and one away from it, for the same reason.

 

All good tips, thanks.

It sounds like tabbed routing might be the best approach for the
smallest scale (which is important here), allowing us to have parts
closer to the edge without the stress. Maybe with a throwaway strip
between the parts for the indent technique you described.

Or, I'll tryout your technique of v-scoring with a router before
assembly, if we can't find a shop that'll do it for cheap enough.

Thanks,
Richard

 

V-groove would be the way to go. Very little waste (you do have to
keep traces away from the very edge of the board) and you can assemble
and perhaps even test the little b*ggers before you have to break them
apart. No tooling, and only a tiny cost per board (perhaps less than
the cost of the PCB area otherwise lost to kerf).

Shear is theoretically possible on an assembled SMT-only board but
requires a lot of unused space around the cut on the component side
for the clamp to come down.

The only way I'd consider a saw is if I could find an appropriate
'table saw' type configuration with a $$ solid carbide slitting saw
blade. Something like this:

http://www.discount-tools.com/ful-2450-4.cfm

I think there are purpose-made depanelizing saws, but I've never had
occasion to use one.


Best regards,
Spehro Pefhany

 

Olimex (http://www.olimex.com) will panelize and separate boards at no extra
cost. They start with standard 160mm x 100mm boards (single- or
double-sided) and make as many of your design as will fit, for a single
fixed price. For instance, I recently had them build a 1" x 1.5" design;
that works out to 25 boards, for a total of about $43 including shipping.
(The price varies slightly depending on the presence of nonstandard drill
sizes.) I'm pretty sure I've had them do boards as small as yours, in the
past.

The shipping is a bit slow because they come from Bulgaria, and the quality
is not quite as nice as what I've had done locally, but it's well within
spec, and the price is very hard to beat.

I have not yet found anyone else who will panelize for free.

On the original topic, though: you can use tin snips to cut fiberglas board.
It helps if you warm the board up with a heat gun first.

 

Walter, thanks.

Actually Olimex is the most promising so far. I'd mailed them for
details, but they are closed for the holiday season. (PCB Pool also
offers this, but their prices are outrageous.)

Olimex will depanelize, but I really don't want a bag of tiny PCBs in
the mail - they'll be very hard to assemble. I'm hoping that if I
provide the routing template as a layer file, they'll do tabbed routing.
How did they deliver your tiny boards?

The mailing is no problem; I can be patient.

Thanks for the pointer on tin snips; I'd read about that one too, but it
seems very unlikely with a populated board (lots of flex, it'd seem).

For the tolerances we need, having them pre-tabbed/scored by machine is
looking to be the only practical option, with in-house pre-scoring with
a v-bit running a distant second place.

Thanks,
Richard

 

Sacrificial connectionstrips are a good way to work with small boards.
You can have the board house route most of the board outlines, with a
web of disposible strips connected just enough places to hold the
little borads well enough for assembly.

Our board house does that as part of the outline routing option. It is
just an alternate router bit.

 

Something's impelling me to say that, if you go the V-score route,
don't break them apart with only your fingers and thumbs - find
some kind of fulcrum or blade or something to set them on and just
push down.

Probably pretty obvious, now that I think about it. Oh, well.

Thanks,
Rich

 

V-scoring is nice, but SMT parts near the score lines can be damaged,
especially if the board thickness is 0.031" or less.

I like using tabs with three 20 mil drill holes in each tab along the
board edge. I can post a sketch of the tab w/ drill holes if you want.
Easy to break off and the jaggies aren't too bad. Easily touched up
with a bit of sandpaper. Hmm, perhaps v-score the tabs.

 

Don't forget the old-fashion way. In case of single side boards, I us
Press n' peel (the blue stuff) buy a paper phenolic copper clad 12x12 fo
about $12.00. Iron on the Press n' Peel. Put in in the old Ferri
Chloride, and etch the boards myself. You can score the paper phenoli
boards with a stanley knife and break clean. Gives you 144 sqare inces (
lot of small boards).
I just did 28 small controller boards last night .75" x 1.5" in an hr tim
+ $8.00 board. For low power boards, I still like the phenolic copper cla
better, you can cut it drill it, file it, and your toold stay shar
forever. Never lost a drill bit on these! se
http://www.web-tronics.com/pp1212.html

Stefan V

 

Just add that to the cost of cutting the boards apart. Don't use a
big (12" or 14") bandsaw, get one of the $89 8" tabletop models, so
you can use a thinner blade that will cut a smaller kerf. Also,
consider a scroll saw, also a very small kerf, but perhaps not as
fast.
Also, in any of these cutting methods, use a dust collector, and
don't breathe anything put into the air that the dust collector might
not get.

 

Good info, thanks. These guys are local too. I'll keep that in mind in
case this can be one-sided (i.e., larger). As-is, it'll need 2-sided
with PTH.

Thanks,
Richard

 

On Wed, 28 Dec 2005 10:42:50 -0800, "Walter Harley"

(snip O/P)

Futurlec (www.futurlec.com) depanelise free. I regularly order batches
comprising 30 double-sided boards each 1.3" * 0.8" and the shipped cost is
$US41.

Re the O/P, I appreciate he is concerned about stuffing tiny boards and appears
to favour population before separation. I certainly prefer stuffing tiny boards
in a small board holder rather than separating assembled boards from a panel.

 

Yeah, I hadn't thought about the issue of the dust generated during
cutting too. By the time I rig this, it'll be cheaper / safer / easier
to just pay the board house.

We're definitely planning to assemble and test on the larger board, then
snap off for packaging. What drives your suggestion for V-score over
tabbed routing? I don't have enough data to weigh cost yet, but is
there much difference in the waste? (Routers seem to be in the 0.12"
range.)

As for unused space around the perimeter, even reserving a 0.1" border
would take a huge bite out of the yield. So anything that minimizes the
perimeter space will be welcomed.

Do you know board houses that'll do v-score or tab routing for
proto-volume, and a reasonable price? I'm working through my list
slowly (it's hard to find this detail info), and Olimex is the closest
thing to reasonable so far. Volume is a problem, because at this size
2-3 boards makes a huge batch of devices, and the one-time fees make the
per-piece costs high.

Thanks,
Richard

 

In production we stuff the boards while still panelized. They are
scored. We then use a "pizza cutter", which is a rotary diamond saw on
a fixture that can slide back and forth along rails, to depanelize
along the score lines.

 

I'm planning to use standard thickness, which IIRC is 0.062". Good food
for thought.

Yes, I'd very much like a sketch, thanks.

After all these comments and some noodling, it looks like tabbed routing
is a strong option, maybe with the indent technique mentioned by John
Popelish. Instead of a whole strip of throwaway, see if I can make the
tab to snap on both sides, recessed from the edge to hide the jaggies.
Separate the first break by hand, and remove the second stub with
pliers. Like this:

| -ooo- #1 |
|-----/ ||| \-----|
tab
|-----\ ||| /-----|
| -ooo- #2 |

The question then becomes how small to make the tab and how close to
space the holes in the tab (balancing ease of snap vs. sag during
assembly), which I expect will be a matter of experimenting. I imagine
with parts this small you can't go less than one tab per side, unless
I'm missing something.

And of course, where to get these made in small proto quantities...

Thanks!
Richard

 

As a bag of tiny PCBs

Olimex's edge tolerances are not that great. In fact that's actually my
main complaint with them - the edges are pretty rough.

Budgie's comment about Futurlec was pretty interesting. I've used them as a
parts supplier (though their online catalog is awful, at least last I
checked) but I had no idea they did PCB manufacturing. Have to check that
out - might be a bit quicker than Olimex.

 

There is basically *no* waste with V-groove, provided you don't intend
on putting parts and traces right out to the edge. 60 mils is usually
enough. I have not tried it with >2 layers, you might need a bit more
for that.

Most moderate/small volume PCB suppliers will do it for a nominal
charge. Not so many prototype companies.


Best regards,
Spehro Pefhany

 

Good pointer - they have very good pricing ($99 for 200 pcs)! I wasn't
aware they did PCBs. Do you find their routing tolerances are good?

Yes, this is SMT on both sides, and I expect it'll be much easier to
paste and pre-heat / solder as a larger panel. One-offs could be
managed, but panelizing seemed cleaner.

Have you run into problems de-panelizing, or just not interested since
you have a good one-off technique? How is your holder constructed?

Thanks,
Richard