Hi,
We made in our plant very extensive study concerning internal microshorts.As
peoples mentioned,only in very few ocasions we were able to find the
short.This is important to point out:there is no evidence,that the shorts
found are representing those that were not located,so be carefull about the
conclusions.
We found some of the following:
1) unetched base copper smaller that 1 mill ( beyond of AOI capability)
2) spots of burnt ( carbonised) epoxy and lot of strange stuff,even flies
and moskitos,hairs etc inside laminates or prepregs.
3) foreign materials introduced by operators.
Ionic contamination was never found as a source.
The problems associated with raw materials were adressed to suppliers of
laminates.
We took some measures such as slowing down etching stage,additional acid
rinse after etching,reduced thickness of black oxide,additional cleaning of
innerlayers by tacky roller before lamination etc.
No one of those measures gave significant results in reduction of innerlayer
microshorts.
Some reliefe come from E.T. when it was found,that the microshorts were
actually external and missinterpretated as internal due to strange way that
short coordinates are printed out.
The electrimigration theory is interesting , but what are the suggested
measures to eliminate it?

Edward Szpruch
Eltek , Manager of Process Engineering
P.O.Box 159 ; 49101 Petah Tikva Israel
Tel  ++972 3 9395050 , Fax  ++972 3 9309581
e-mail   [log in to unmask]

> -----Original Message-----
> From: Kelly M. Schriver [SMTP:[log in to unmask]]
> Sent: ש ספטמבר 11 1999 21:41
> To:   [log in to unmask]
> Subject:      Re: [TN] Internal fab shorts that happen over time...
> 
> Hi Steve and All -
> 
> There sounds like a high probability that Susan's hit the nail on the head
> -
> I've had several wonderful adventures with electromigration over the
> years,
> mostly in envrionmental test and qualification testing prior to shipment.
> I've also gotten some hardware back out of the field that we were able to
> pin down to various aspects of the operating environment, such as sea
> water,
> carbonated beverages, spilled coffee and a variety of other items.  One
> customer had his hardware operatiing in a relatively uncontrolled, moist
> environment in a paper mill -- brutal for unprotected electronics.
> 
> Recommend you perform a really thorough examination at 10-20X
> magnification
> under good lighting to see if you can spot any black fernlike structures,
> white or green encrustations, etc.  Then ship that hummer off to one of
> the
> labs noted previously.      WHATEVER YOU DO, DON'T LET ANYONE DISTURB THE
> EVIDENCE.
> 
> Have a fun day - Kelly
> 
> 
> 
> 
> 
> -----Original Message-----
> From: [log in to unmask] <[log in to unmask]>
> To: [log in to unmask] <[log in to unmask]>
> Date: Friday, September 10, 1999 5:17 PM
> Subject: Re: [TN] Internal fab shorts that happen over time...
> 
> 
> >Hi Steve
> >What you are describing is exactly what will happen with
> electromigration.
> >Of course after the material migrates and shorts out completely, it will
> >work for a while then finally won't at all.
> >
> >1)  Horizontal microsections will some times be able to let us see the
> >dendrites within the board.
> >
> >2)  If you have the condition in one location it is probably present in
> other
> >locations across the board.
> >
> >3)  To get electromigration you have to have moisture and contamination.
> >So what kind of environment do these assemblies see?  Conf Coating?
> >
> >4)  Hind sight would suggest some sort of cleanliness requirements for
> >inner layers prior to lamination - Bellcore requires process
> qualification
> >with SIR type tests.  IPC specs and military specs do not.
> >
> >Susan Mansilla
> >Technical Director
> >Robisan Lab
> >
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