Mark,

        One of the best descriptions I've seen and read is in the "Printed
        Circuit Handbook" by Clyde Coombs. (McGraw Hill) It has pictures
        and all.
        Personal Experience-Only one time that I know of. One of my 
        projects had Electrolytic Ni/Au as a surface finish. There were
        three variables that created it. 1) pwr and grnd side by side with
        tight spacing. 2) Exposed copper edges on the traces. 3) PWA was
        used in a highly acidic or caustic environment. (Medical field
        that was highly corrosive. I'm not a doctor so I won't get into
        details!) Those variables created a de-plate / re-plate situation
        that developed the shorting condition that you speak of. (Dendritic
        growth) The end user redesigned the bd to move pwr and grnd apart
        and used conformal coating over the body of the rest of the bd.
        (Probably better for the patient.) I heard of no more problems.

        Groovy.

        Hadco Printed Circuits
Tech Center Two / Watsonville                


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Dave Hoover              (408) 728-6677 
Senior Process Engineer  (408) 728-1728 Fax 
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______________________________ Reply Separator _________________________________
Subject: Dendritic Growth?
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    10/6/95 4:05 PM


Background:
     
We had a PWA fail in the field after approximately 6-7 months.  The unit 
was powered on approximately 1000-2000 hours.  Analysis of the PWA 
revealed a short had developed between two parallel traces underneath an 
IC.  The traces are .006" wide and .010" apart.  The short was not visible 
(even under a microscope).  The short was approximately 2 1/2 ohms in 
value.  Once a razor was run between the traces the short was eliminated 
and the PWA was functional once again.
     
Someone I spoke to at a PWB Fabricator suggested it might have been 
dendritic growth.
     
Questions:
     
What is dendritic growth?  
Does causes it on a PWA?
Where can I get more information about it? 
Can it be attributed to the PWB fabrication?
     
     
Any and all assistance is GREATLY APPRECIATED!
     
     
Best Regards,
     
Mark Lettang
3M
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(507)359-0418