Jack,

I wouldn't do a countersink hole on a PCB.  What is it for?  The laminate is
nicely sealed by the top layer of resin.  If you c'bore, you dig down into
laminate and end up exposing the glass mat - nasty stuff.  I do a lot of
mechanical stuff and countersinks are the worst kind of hole - depth is very
difficult to control - the bottom of the hole can break out and then the flat
head screw bottoms out into whatever you are fastening to.  If the screw must be
flush, then I suppose you have to do it and take your chances but in .062
material - that's not going to be good odds.  A split washer and a Phillips or
socket head cap screw is a better choice, esp, if you solderplate one of the
holes for a mechanical ground.

Heck, my machinist can't set a C'sink that well with a Bridgeport milling
machine on aluminum!  Chatter and such make the job really iffy.

Bob Landman
H&L Instruments

----- Original Message -----
From: Gfranck <[log in to unmask]>
To: <[log in to unmask]>
Sent: September 27, 1999 10:50 AM
Subject: Re: [DC] PCB countersunk holes


> Jack,
>
> One other gotcha... all the other comments have been good, especially use a
> unique hole symbol.  You gotta do that.
>
> Fab suppliers like to drill the unplated holes on the rout machine.  The
> counterbore requires some z-axis (depth) control, so they will move the board
> to the drill machine to do the c'bores.  Don't let them do this.  They can
> introduce all kinds of registration problems between the center of the c'bore
> and the center of the hole.  Make sure your suppliers know to drill the
> unplated hole and c'bore on the same machine with the same set-up.  Been
> there, done that, not pretty.
>
>
> Jack Olson wrote:
>
> > I have never been asked to provide countersink mounting holes on a PCB
> > before.
> > Since I have never had a drafting class, can I ask if any of you typically
> > do this?
> > I was just going to add a note to the fabrication dwg saying:
> >
> > x.) Countersink mounting holes (8X) 82 degrees x .230 on component side.
> >
> > Is this adequate?
> > Are there any "gotchas" I need to keep in mind?
> >
> > Jack (the new guy)
> >
> >
>