The term "dentritic growth" is becoming more synonymous with 85 C /85 
     RH electrical testing. Some refer to it as "hydrolytic stability" as 
     well. This results in basically "shorts" in systems with tight 
     spacings and high voltages. The limiting factors really become the 
     substrate and covercoat types, as the minimum spacing at 100 volts to 
     be acceptable will depend on which combinations of materials are used. 
     Not all materials are equivalent!!!  See recent proceedings from May 
     '96 IPC meeting in Phoenix.


______________________________ Reply Separator
_________________________________
Subject: DES: Track spacing for 100V on internal layers -Reply
Author:  "Dennis Ostendorf" <[log in to unmask]> at -FABRIK/Internet
Date:    6/2/97 8:50 AM


From: Dennis Ostendorf
Date: Mon, Jun 2, 1997 8:50 AM
Subject: DES: Track spacing for 100V on internal layers -Reply 
To: Duane B. Mahnke; TechNet
Cc: Gary_Willard-G10982
Gary,
     
You should not have a problem with .015 inch spacing for 100V (unless 
you have contaminant(s) between the traces).  In fact, I believe the 
spacing is conservative & could be reduced.
     
     
Dennis Ostendorf
aaeee2o @snds.com
     
     
>>> Gary Willard-G10982
<[log in to unmask]> 06/02/97 
06:15am >>>
     
     
        Hello All
     
        I am in need of some input on a design issue relating to track 
        spacing for 100V operation for the internal layers of a 4-layer 
        PCB.
     
        This PCB is 2.0mm thick with a 1mm core and 35um copper. This
     
        design has to withstand 85% humidity / 85deg temperature testing
     
        for 500 Hours some of which is under 'power-up' conditions.
     
        The current design has an epoxy based solder mask and conformal
     
        coating and has an internal spacing of .015", which I believe 
        should be adequate as IPC-D-275 suggests a spacing of only 
        .004-.008" for class 3 assemblies, this figure of .015" has been 
        put to question so I am seeking further input.
     
        my questions are:-
     
1)      What does class 3 mean in terms of general environmental testing
     
        and in particular 85/85 testing.
     
2)      Does any body have practical knowledge of what a 'better 
spacing'
        may be.
     
        Any input would be most appreciated.
     
        Regards  Gary Willard  -  Substrate Design Engineer
                                  Motorola (UK)
     
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