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Date: | Fri, 6 Oct 95 19:07 EST |
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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
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