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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