Actually,
There may be a lot of industry confusion between "black pad" and "skip
plating".
Skip plating will take the form of what appears to be random black pad
occurrences particularly in a BGA field with small via holes.
This is an important differentiation, black pad typically should be general
and somewhat catastrophic in nature whereas skip plating will have the same
chemical signature of hyper corroded nickel, but be somewhat random in it's
nature.
The first one I have never tried to switch on or off.
The second one I have sucessfully switched on and off using a test vehicle
called FDAT (fluid dynamics assessment tool) but which since it is a test
vehicle I dubbed the "Bernoulli bus"
This consists of a BGA test pattern comprised of a typical BGA pad and
"dogbone pattern" with the via holes in the interconnect becoming ever
smaller (within the same BGA field) the hole sizes ranging in the type 2
test vehicle between 20 mils down to 8 mils in 2 mil steps. The land annular
ring around the via holes being scaled to the hole size.
This test pattern was repeated across a panel, and put through a typical
manufacturing process - the board was 110 mils thick and the two types of
board were made with and without internal layers.
This test vehicle will find the "edge" of the fluid dynamics capability of a
PCB line and yes it is possible to find the point at which the line produces
skip plating.
Of course if you are not trying to produce boards using very small via holes
in very thick boards, then you may never have the problem.
John
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Frank Kimmey
Sent: Thursday, April 14, 2005 10:48 AM
To: [log in to unmask]
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]
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]>
To: [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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