Lee,
Best written stuff I can give quickly would be BabHui Lee of Multek's article in Nov. 2003 Circuit Tree.
Actually for myself it never was thinner gold as a solution.
My experience was that when Black Pad was identified the only cure always seemed to be replacement (not replenishment) of the baths. I concluded that there was something about the Nickel baths that was the underlying cause (not scientific). Discussion held in the mid to late 90's with various PCB suppliers both in the US and Asia brought me to the conclusion that the facilities that replaced their baths after 3 replenishes seemed to have grossly less occurrences of Black Pad with their customers. This impressed me enough to suggest this process to some manufacturers who then reported an improvement. I don't think I am qualified to make chemical or metallurgical statements that are definitive or should be accepted without further documentable experience. That said my personal understanding has lead to the conclusion that Black Pad may be caused by an unstable Phosphorus ratio in the Nickel that manifests as rapid oxidation and contamination of the nickel prior to application of immersion gold. Once again I wish to reintegrate that I am not an Engineer and am only trying to share what little understanding I have on this issue.
Hope it helps and my apologies if it doesn't
FNK
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Lee parker
Sent: Thursday, April 14, 2005 12:36 PM
To: [log in to unmask]
Subject: Re: [TN] ENIG - no more problems?
Frank
I have been involved in the black pad issue since the middle '90s.
I agree with you the occurrence is now much less than eight or nine years ago; however, other than reducing the gold thickness, I have not seen any convincing data of the mechanism or the remedy. Most of what I have experienced are hand waving augments that are void of supporting data and in most cases conflicting.
You mentioned "we now have a better understanding of the likely causes". Could you go into some detail both on the likely causes and the supporting data; other than of course reducing the thickness of the gold.
Best regards
Lee
J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389
----- Original Message -----
From: Frank Kimmey<mailto:[log in to unmask]>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Thursday, April 14, 2005 1:47 PM
Subject: Re: [TN] ENIG - no more problems?
Genny,
I hate to argue with folks like Vladimir, Ed and George because I truly respect their opinions and knowledge (they are on my list of who to go to for help).
That said, I also need to tell you about the success of ENIG and IAg for replacing Pb.
As far as ENIG and Black Pad all the statements of recreating Black Pad are true (I also have never heard of anyone successfully causing it on demand). Also true though is the fact that we (the industry) now has a better understanding of the likely causes and by following corrected processes have seen a large reduction (to the point of elimination in some facilities) of the occurrence of Black Pad. I personally have not had a Black Pad incident in more than 3 years (close to half of our designs are ENIG).
We spent 2 years trying to qual IAg and found some finishes better than others (due to thickness of deposit).
The result was only one finish (of the 3 available at the time) was qualified for use on our product. Unfortunately some of the finishes resulted in the formation of Cuprous Oxide on the outer surfaces rending the pads unsolderable. From our approved supplier we have seen NO solderability issues in the more than a year we have been using IAG.
All rambling aside, we expect to replace Pb HASL with ENIG as drop in for legacy and use ENIG and IAg for designs going forward. Bottom line should be if you have good suppliers then you should be able to confidently make the change from HASL to Pb free finishes.
Hope its helped,
FNK
Frank N Kimmey CID+
Principal PCB Designer
Powerwave Technologies Inc
Office 916-941-3159
Fax 916-941-3195
Cellular 916-804-2491
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Lee parker
Sent: Thursday, April 14, 2005 9:40 AM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] ENIG - no more problems?
Genny
If the problem with black pad is resolved, then the suppliers should be able to produce the problem at will and then demonstrate a corrective action that prevents the issue form occurring without any side effects. To the best of my knowledge this is not the case.
What has taken place is that most suppliers are now recommending a gold thickness of two micro inches or less based on imperial data; there have been few reported cases of black pad when the gold is restricted to two micro inches or less. Unfortunately, no one has given me a convincing mechanism to explain the phenomena. Also as the gold thickness is reduced the probability of soldering issues is increased.
Best regards
Lee
----- Original Message -----
From: Genny Gibbard<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, April 14, 2005 11:54 AM
Subject: [TN] ENIG - no more problems?
Just listened to a Lead Free presentation by one of our assembly materials
suppliers. They tested different pastes on different board finishes using a
variety of different patterns, spacings, aperture sizes, etc,...
By far and away, ENIG was the best finish for performance, for wetting,
defects, voiding, ...
The guy making the presentation said that he felt a lot of the initial
concerns re ENIG - black pad being the biggest - have been resolved, and
that they are no longer a concern, that they have determined what the cause
of it is and eliminated it. Anyone want to comment on that? I'm not sure
if he is correct, or just out of the loop.
It was interesting as well: ImmAg got a very bad rating from them in
performance, even though many on this forum really praise that finish. It
exhibited dewetting (they printed solderpaste pads on an area with no
soldermask, and the solder did not stay and spread where it was printed, it
crept together with other pads into a big blob, like you might see a water
drop do) and the amount of voiding was impressive!!
About 4 years ago I joined this forum, and one of the reasons was because I
was looking for info on ENIG. We had done a prototype run of a couple of
boards and they failed miserably. They were never analyzed in depth, so it
wasn't figured out what exactly caused the failure. It was the excuse we
needed to not choose this finish and some of the extra costs associated with
it.
However, with the Pbfree push now, I don't know if we should re-examine
whether to use the finish. For products where we needed a flat finish, we
have gone to ImmSn, but we still use HASL wherever possible.
Genny
"Genius may have its limitations, but stupidity is not thus handicapped"
Elbert Hubbard
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