What you've described has all the characteristics of dendritic growth.
     More specific detail about the operating conditions and board
     construction would be required to know definitively.

     For flex circuits the traditional model attributes the dendrite growth
     to ionic contaminants at the adhesive interface of the coverfilm and
     base laminate. These contaminates arise from exposure to process
     chemicals prior to encapsulation of the traces with coverfilm. During
     use the contaminants respond to moisture exposure by creating
     conductive pathways between oppositely charged circuit traces.

     In the past few years failures attributable to the adhesive
     composition have been encountered. In these cases examination revealed
     that the positively biased traces (anodes) had tarnished and the
     negatively biased traces (cathodes) had grown dendrites visible at
     100X. These developed in <300 hours at 100V, 85C, 85%RH.
     Cross-sections of the dendrite structures revealed that the coverfilm
     adhesive itself was acting as an electrolyte, providing the path for
     growth, rather than arising from any contaminants at the interface.

     We worked with Seagate Technology, Bloomington, MN a couple of years
     ago to develop qualification tests to insure that our adhesive systems
     are immune to this phenomenon. Various types of adhesive from several
     other suppliers were tested and several constituents were identified
     that commonly lead to this effect. Chief among them are excessive
     levels of BF3 catalyst or various sulfur compounds.



     Andrew P. Magee - Applications Engineer
     Rogers - Circuit Materials Unit
     Tel: (602) 917-5237
     Fax: (602) 917-5256
     E-Mail: [log in to unmask]




______________________________ Reply Separator _________________________________
Subject: Dendritic Growth?
Author:  mmlettan (mmlettang)
{NAME:mmlettang|EMS:INTERNET|MBX:[log in to unmask] at MCIMAIL
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
[log in to unmask]
(507)359-0418