TechNet, Paul, and Eltek Engineers,
The particulate mechanism for causing shorts was described differently in
CircuitTree (January 1997, p 76) where "some of it [black oxide] sloughs off
the board, especially in flexible circuits, and creates particulate larger in
diameter than the separation between lines in the board." This places the
particle on the conductor surface away from the laminate.
I find your interpretation to be very interesting, though, as an embedded
laminate side mechanism, rather than conductor surface mechanism, creating
high resistance.
> [log in to unmask] wrote "The spots are located under the conductors > (laminate side) , they have irregular shapes and are very thin."
Several Thoughts:
1. Is it possible there are two separate mechanisms?
2. Why does one not see a short opposite the laminate on occasion to
confirm the flex-circuit theory?
3. Is it not possible there is residual unetched copper that fortuitously
is oxidized thin enough and compressed into the laminate?
4. Does one see these thin, irrregular entities anywhere else on the board?
In explanation, sloughed off black oxide would seemingly embed itself
anywhere including at the short?
5. Could this be fractured, treated side foil from the laminate
manufacturer's bonding operation winding up right between two close traces?
Personal Opinion of PGW:
It wouldn't be prudent to condemn black oxide without complete understanding of the observations. Further, the new processes do not have the years of run time behind them to identify such issues. As a small tit for tat observation, conveyorized black oxide and the new conveyorized replacements vary little in waste impact and throughtput: It is understood most are running dip oxide, though. If this were the laminate itsef, the logic of change would be weak. I would be very interested in a few hard thought-out rebuttals concerning the merits of eliminating black oxide.
Paul Wilson
Plant Chemist
Circuit Center, Inc.
[log in to unmask]
----------
> > Date: Tuesday, May 05, 1998 03:58:05
> > From: Paul Gould
> > To: TN
> > Subject: [TN] [FAB] High resistance internal shorts
> >
> > Hi Edward,
> > I am also concerned about the same issue where high resistance shorts
> > are experienced on what appears to be a batch related basis. One batch
> > will have zero defects then another will have as many as 10%. We test at
> > 500V and look for more than 1 Gigohm resistance. We too have improved
> > our process to a high degree and can rule out innner layer defects and
> > particulate contamination during lay-up for bonding.
> > Apart from laminate or pre-preg defects which you suggest, there is
> > another explanation given in an interesting article in the May issue of
> > CircuiTree called Black Precipitate Formation in Black Oxide Baths by
> > Ming de Wang and Donna Fawcett. When titanium is used for the tank
> > construction, apparently it acts as a catalyst in promoting the
> > formation of black oxide sludge in the tank and this can lead to shorts
> > between traces. We use titanium coated heaters so this could be a reason
> > for the problem. I am accelerating our evaluation of alternatives to
> > black oxide as a result of this article and looking for alternative
> > heaters in the short term.
> > If anyone has any information to share on this one, I too would be
> > grateful.
> > Best Regards
> > Paul Gould
> >
> > Eltek Ltd. - Process Engineering wrote:-
> > >Hi Technetters !
> > >During the past period we made a lot of improvements in our production to
> > >reduce the scrap due to internal shorts . At this moment 90% of internal
> > >shorts are " isolation shorts " , it mean high resisitivity .
> > >Some of this shorts were located and we found unidentificed spots in the
> > >laminates . The spots are located under the conductors ( laminate side ) ,
> > >they have irregular shapes and are very thin . Such spots are bridging two
> > >adjacent conductors . Such spots are not result of unetched copper since
> > >they are running deep into the laminate ( more than 25 microns ) .
> > >I will appreciate comments on this problem via technet or to my private
> > e-mail .
> > >Best regards
> > >
> > >
> > >Edward Szpruch
> > >Eltek Ltd - Israel
> > >Tel 972 3 9395050
> > >Fax 972 3 9309581
> > >E-mail : [log in to unmask]
> >
> > Paul Gould
> > [log in to unmask]
> > Isle of Wight,UK
> >
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