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Re: Potential issues with oaa_ lib



On Sun, 29 Aug 2010, Lorenzo Marcantonio wrote:
> In drill files the tool specified is *always* the finished hole, no
> exceptions. This is because PCB drills usually are actually cutters and 
> special
> compensation applies. And of course there is plating which is a process
> variable, too. Buried and blind vias are another kind of hole too, but
> anyway you just state the *finished* hole size.

Well, yes and no.  Often (and for PCBNEW as it stands) no sizes are
placed in the drill file, just tool numbers.  It is the drill report
that identifies sizes.

Fabricators accept either finished or drill size, as long as it is
specified.  When restricting drill sizes to a given drill rack, it is
desirable to specify drill size rather than finished size for TH holes.
When ignoring drill racks, it is desirable to specify finished size
rather than hole size for TH holes.

Also, it is not useful to specify finished hole size for vias.
Particularly on high-speed boards it is desirable to specify drill size
(barrel O.D.) for vias for two reasons: 1, controlling the barrel o.d.
is necessary for controlled impedance vias; and, 2, finished via hole
size should be permitted to drop to zero.  Often, even when considering
TH holes as finished size, fabricators will still consider vias (or a
hole beneath a set minimum as vias) as drilled hole size.

For NPTH and mechanicals the finished hole size is the drilled hole
size.  So in this case, drilled hole size is always specified.

It is common to separate holes (and tool numbers) into PTH, NPTH and
vias and to mark the drill report as finished or unfinished hole size
and mark the hole type as PTH, NPTH or VIA.

When holding to a drill rack, the drill report should specify drill
size for all holes.

Also, converting between the two is a simple matter of establishing the
minimum and maximum barrel plating and hole manufacturing tolerance.
Plating and tolerance only varies somewhat by whether the hole is blind
or buried, laser, chemical or machine drilled, and whether the hole is
fabricated as a PTH as a sublaminate buildup.

> In conventional processes you can ask for holes from 0,4 mm up in 0,1 mm
> steps; large holes (typ >6mm) are usually milled using
> a circular path (often during the final cut, if not plated). Smaller
> holes are done using microvia processing with laser evaporation
> (usually).

Some fabricators, particularly in North America, for lower cost, only
use an imperial drill rack composed of fractional or wire gauge drill
sizes.  Some lower cost fabricators restrict to a given drill rack even
when metric drill sizes are used.

> For these reasons it's the manufacturer job to 'compensate' tool size
> (and tolerances are stated in their process description). So if you want
> a 3.2 mm hole, just state a 3.2 mm tool, the board manufacturer will do
> whatever it would be done to achieve it. It's a CAM issue, not a CAD
> one!

For a usable board (and particularly for DIY manufacturing) there is no
difference between CAD and CAM issues.


Brian F. G. Bidulock    ¦ The reasonable man adapts himself to the ¦
bidulock@xxxxxxxxxxx    ¦ world; the unreasonable one persists in  ¦
http://www.openss7.org/ ¦ trying  to adapt the  world  to himself. ¦
                        ¦ Therefore  all  progress  depends on the ¦
                        ¦ unreasonable man. -- George Bernard Shaw ¦