Hi Don:
     
     I am not sure who wrote about the pink ring either, but in my 
     experience, thermals limit the spread of the pink ring rather than 
     limiting pink ring itself, by limiting the cross sectional area of the 
     oxide that is available for chemical attack.  In other words, the 
     total area of oxide across the top of the thermal ties is much less 
     than the total area of oxide available, if the site of attack was a 
     solid oxided reference plane.  In cross sections, many times the pink 
     ring has spread throughout an internal land, but not all the way 
     across the ties connecting the land to the rest of the reference 
     plane, I think one of the reasons is because there was not enough area 
     of oxide on the ties to sustain the oxide reduction reaction.
     
     Greg Schroeder
     Hadco
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Re[2]: DES: VIAS - Thermal vs. Direct
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    9/24/96 6:27 PM


     
I'm not sure who wrote this, but I would like to hear more.  How does 
a direct connect via increase the incidence of pink ring?  Perhaps my 
understanding of what creates pink ring is in error but I would think
the geometries and chemistries at the hole wall is the same either way.
     
Also, what do you TechNetters say about pink ring?  Is is a functional 
defect or not?  How many board failures have you seen attributed to pink 
ring (whether thru delamination, via failure, or whatever)?  Please note 
the word failures - not rejections (although that might be interesting 
to know as well).  As we all know, sometimes requirements are based on 
"that's the way we've always done it" rather than on real data.
     
[log in to unmask]
Don Walker
     
>    By eliminating thermals at via locations you significantly increase 
>    the probability of creating pink ring.  Depending on who you talk to 
>    will determine if pink ring is a functional defect or not.  If your 
>    finished specification for your board manufacturer states "No pink
>    ring allowed" you are setting him up for failure.  If pink ring is a 
>    non issue with your company and this is reflected in your
>    specification this is a non issue.  The diameter of the center pad, 
>    width of the spokes and diameter of the clearance around the center
>    pad are significant factors that contribute to the amount of pink ring 
>    present.
     
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