The ongoing dicussion of pink ring led me to search my archives for the 
following two emails. Let the debate continue.

Mike Buetow
IPC Staff

---------- Forwarded message ----------
Date: 14 Nov 95 08:44:14 -0500
From: R_R_HOLMES <[log in to unmask]>
Subject: Re: Pink ring and hard failures

RE  Pink ring failures

I concurr with Harry Parkinson that there can be a serious risk with pink 
ring.  My experience is that the extent of the delamination  (pink region)  
will not propogate and this region never leaves the conductor surface.  This 
means that pink rink is not a serious risk with respect to internal shorts. 

However....  As Harry points out the risk of PTH opens is very real.  My 
experience shows that the origin of pink ring is drill.  A combination of poor 
innerlayer adhesion and agressive drilling (often drill withdrawal) leads to a 
a crack at the copper to epoxy interface.  A bake after drill will often close 
and perhaps seal the crack.  However if the board goes directly to plating the 
crack can cause problems.  If it is only slightly open, the plating bridges 
the crack after some chemical leaching of the oxide treatment.  This leads to 
the typical pink ring appearance and no problem.  However if the crack is 
sufficently open, the resulting wedge void will not bridge during plating or 
it may only partially bridge.  In this case, there is a serious barrel flaw 
that can easilly lead to a failure.  I have seen this condition.  It is rare, 
 but it definitely occurs.

Robert R. Holmes
AT&T Bell Labs
[log in to unmask]

------------ Begin Original Message -------------
From: ugetit.ENET.dec.com!parkinson 
Date: Thu Nov 9 13:57:04 EST 95
Subject: Pink ring and hard failures

While reading the recent messages on pink ring, one of the replies stated that
pink ring was cosmetic and did not cause hard failures....not true.

I recently had to replace completed PWA's that failed In-circuit test because 
of massive opens in Vcc and ground planes. 

The fault was traced to "pink ring" that was circumferential around the 
plated barrel and if the circumferential ring was a 360 degree circle, there 
was NO connection between that via and the power or ground plane. It was 
random in location as well.

This was confirmed by vertical and horizontal cross section by Susan 
Mansilla at Robisan Laboratory, Inc. I have micro-photographs and lab 
reports of the failure mode.

This was a very severe case, but it did indeed cause hard failures. The boards 
had been "shorts and opens" tested 100%, but this did not pick the failure 
because not every VCC and ground point was tested; the nature of the failure 
would only be detected with either "flip" testing each side of the board or
simultaneous two sided test of each Vcc or ground connection.

Harry Parkinson
Digital Equipment Corp.
603-884-6760


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