----- Original Message -----
From: "Dennis Saputelli" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Thursday, September 26, 2002 12:58 AM
Subject: Re: [PEDA] OT - bd testing


> so how does a 'flying probe' test really work?
> i understand the general idea of a couple of probes walking around the
> comparing connectivity to a 'netlist' made from the gerbers
>
> but it seems to me and i think i read somewhere that this is better at
> finding opens than shorts
>
> whereas the good old bed of nails would (or could?) find both
>
> anybody know the down and dirty secrets of all this?

Not all the secrets but main points

It is quicker (for the board fab co. anyway) to check board for open circuit
tracks as it is easier for the software to find node ends by itself and
therfor define prob targets and optimize the route path for the probes. This
is most prominent on shops that supply QT prototypes. I stopped using a lot
of board shops for this. Miss 1 short, even if you fix it, could mean more
than one open tacks later.

Most test machines will also check for adjacent shorts (using both board
sides & vias) but this takes longer and relies on correct netlist extraction
and node definitions and is usually limited to a small area around the nodes
being tested.

Testing rules are normally defined globbally in order to set pass
parameters. Not always a good thing, but it is not so easy for them to
define individual rules as they are effectively dealing with their
reconstruction of a dumb format (gerber).

The test voltage is usually low <5v and the acceptable parameters for,
expecially open tracks or shorts is usually left as default, which is
sometimes not enough for hiZ shorts on close tolerance boards with badly set
rules.

A lot of companies also use FP testing to verify a board before using BON
testing. Worrying is it not ;-)

Like all other areas the engineer in charge of the FP testing & the front
end guys are the most important link and as in most companies you get good &
bad, and also very varied excuses to why the board comes out bad, even after
testing!.

John

















>
> Dennis Saputelli
>
>
> Dennis Saputelli wrote:
> >
> > it has happened to me twice
> > 2 different shops
> > they charged for test setup, testing, but there were short all over some
> > of them and they were only double sided
> >
> > Dennis Saputelli
> >
> > Jon Elson wrote:
> > >
> > > Bagotronix Tech Support wrote:
> > >
> > > > With a board that complex, your board vendor had better actually be
doing
> > > > electrical test, instead of just charging for it and not doing it...
> > >
> > > What?  This is endemic in the industry?  I've had to can several
vendors
> > > for pulling this stunt.  But, when they get caught by me, with my (in
> > > comparison) quite modest boards, there's no doubt what is going on,
because
> > > the boards come back with 50% failure rate.  IE. 50% of the boards
have at
> > > least
> > > one internal short or open.  I usually get the test report from
Advanced
> > > Circuits,
> > > so I know what their yield is on multilayer boards.  And, it is
somewhere
> > > between
> > > 50 and 75% on most runs.  The 2-sided usually come back at 90% or
better
> > > passing.
> > >
> > > So, it is real hard for me to believe that many fabricators pull these
> > > stunts of
> > > pretending to do electrical test as a routine matter.  I think they'd
get
> > > caught
> > > WAY too often.
> > >
> > > Jon
> >
>
> --
>
___________________________________________________________________________
> www.integratedcontrolsinc.com            Integrated Controls, Inc.
>    tel: 415-647-0480                        2851 21st Street
>       fax: 415-647-3003                        San Francisco, CA 94110

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