Hi Kathy,

Try IPC-SM782A pg. 35, figures 3-31 and 3-32, at least as a starting point.
The breakaway tab width is extreme, IMHO, at around 0.5" including routed
slot, but if you've got real estate to burn, go ahead.  When we do not acres
to spare, I've used an 0.093" slot and a 0.107" tab for a total of 0.200" to
the board edge.  Some fabricators allow 0.062" router bits, but since 0.200"
seems to be around the minimum distance to the board edge on many reflow
conveyors, or so I'm told, there isn't much to be gained with the narrower
slot.

Some assembly houses have their own standard breakoffs that use 0.025" or
0.020" holes and closer spacing.  These seem to leave cleaner edges on
finished assemblies than the larger holes, but I don't know if they're
harder to break apart or not.

Remember to keep these holes non-plated and relieve copper on all layers
0.015" or so from the hole edges as well as the solder mask.

Maybe someone on the assembly side can give their perspective.  My bias is
from the board and assembly design standpoint.

Good luck!

Regards,

Seth Goodman
Goodman Associates, LLC
tel 608.833.9933
fax 608.833.9966

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