September 12,2004
You hit the nail on the head. We think the problem is poor copper plating process control that allowed excessive build-up of copper around the PTH. After tin/lead plating, and scotch brite abrasion, the excess plating flaked off and caused conductive tin/lead/copper slivers to be randomly dispersed throughout the PWB. Cleaning process was inadequate since it die not remove all slivers and ionic cleanliness test prior to solder mask did not detect the condition, nor would I expect it to. See attached photo of the MIL-P-55110 cross section as example.
----- Original Message -----
From: Dehoyos, Ramon<mailto:[log in to unmask]>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Friday, September 10, 2004 12:53 PM
Subject: Re: [TN] PWB Conductive Contamination Under Solder Mask; What Can Cause it?
As stated by another Technetter in a previous. during surface prep prior to solder masking, boards are buffed for better adhesion of the solder mask and the overhang caused by the copper etch breaks off, taking with it, lead tin plating from the pads and lands and just copper from the traces. These fragments are washed off during the next process, but a few stay behind.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Whittaker, Dewey
(AZ75)
Sent: Friday, September 10, 2004 9:59 AM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] PWB Conductive Contamination Under Solder Mask; What
Can Cause it?
I'm sorry I came in on this late in the thread. The first discussions
appeared to point at dendrites, as so eloquently pointed out by Doug and
Susan, but could be tin whiskers. Where there pictures to look at? There are
some risk mitigating scenarios you could employ to save the current PWBs and
potentially eliminate it from happening to new ones.
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of - Bogert
Sent: Thursday, September 09, 2004 3:46 PM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] PWB Conductive Contamination Under Solder Mask; What
Can Cause it?
September 9, 2004
Thanks for the information.
I don't think dendrites are the problem since we also have some brand new
PWB'S with the same conductive contamination that were never populated or
conformal coated or exposed to any voltage other than the short time
exposure to 50 VDC used during the shorts and opens test performed by the
PWB manufacturer. My understanding is that humidity, without voltage
applied, can cause tin whiskers (if tin is pure), but not dendrite growth.
----- Original Message -----
From: Douglas O. Pauls<mailto:[log in to unmask]<mailto:[log in to unmask]>>
To: [log in to unmask]<mailto:[log in to unmask]<mailto:[log in to unmask]<mailto:[log in to unmask]>>
Sent: Thursday, September 09, 2004 9:19 AM
Subject: Re: [TN] PWB Conductive Contamination Under Solder Mask; What Can
Cause it?
1) What process or material problem could have caused the conductive
filaments to form, and how to prevent it from happening again? My best
guess is contamination residue from the etch-back process.
**Les, I think Susan Mansilla hit the nail on the head. Almost all the
SEM
shots of dendrites that I have seen had the same tin, lead, copper that
you
see. I agree that the cause is most likely insufficient tin/lead etch
resist removal. The only caveat might be - are you sure the dendrite grew
"under" the mask, as opposed to "through" the mask? I have seen some
cases
were poorly developed solder resist had dendrites grow through the bulk of
the solder mask, rather than under it.
2) If the PWB passes shorts testing and end-item functional test, is there
any concern with continued use in a humid environment given that the
contamination is trapped under a layer of solder mask as well as a layer
of
Type UR conformal coating.
**Any concern? Maybe. What is the design life of the product and what
are
the consequences of failure? You have seen the failure mechanism on some
product, so it CAN occur. How often will the unit be powered in the
field?
How constant will the humidity be while power is on? Urethane coatings
are
pretty good at moisture inhibition and solder masks are a combination of
acrylics and urethanes. Moisture diffusion will be slower. Growing
dendrites under a polymeric layer takes a much greater forcing function to
occur. How much risk depends on how bad the contamination is.
Any info available on how long solder mask and conformal coating can
retard
moisture entry? I know that neither material will prevent eventual
moisture entry. Is there any data that shows how long these materials
will
retard moisture from causing dendrite growth problems?
**None of the data that I have done has quantified the link. Dr. Craig
Hillman, University of Maryland, CALCE Center, has done some work in this
area and presently has such a project ongoing, if memory serves.
Doug Pauls
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