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April 2005

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Subject:
From:
John Burke <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, John Burke <[log in to unmask]>
Date:
Thu, 14 Apr 2005 09:28:22 -0700
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Hi Genny, to use your phrase "Out of the loop" I think, would be my opinion
of your vendor conclusion.

Out of all of the finishes ENIG will produce the weakest result due to the
SN/NI / Ni/Cu interface. It will definitely be an issue with area array
devices given the higher processing temperatures.

The supplier obviously did not understand the mobility of solder on a silver
surface either. The mobility of the solder tends to allow it to travel to
where it is required - into the  joint area and if you print as your vendor
did in the non "real world" way described it will definitely "blob" because
of this surface mobility -. The rest of the pad will look as if the solder
did not wet - actually it did, and then it flowed into the "blob" area. If
the vendor had bothered to confirm this with EDX  on the "de-wetted" area he
would have known that.

I did the first beta site tests and initial production runs with the
original alpha level silver back in '96, have used Atotech chemistry and
McDermid, have never had a problem with it, and have been building 17 inch
by 22 inch boards 36 layers using double sided BGA technology.

Apart from the inherent weakness of the joint structure in ENIG, there is a
definite issue caused by the fluid dynamics in high aspect ratio holes
particularly when the boards are using 10 mil vias in thick boards. This
leads to skip plating which can take a couple of forms.

Problems due to skip plating with ENIG were the reason for moving over at my
last company (same effect as black pad - different trigger mechanism)


John

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Genny Gibbard
Sent: Thursday, April 14, 2005 8:54 AM
To: [log in to unmask]
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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