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August 2004

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From:
"McFaddin, Wade" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, McFaddin, Wade
Date:
Fri, 13 Aug 2004 08:08:06 -0500
Content-Type:
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text/plain (187 lines)
Hi Peter,

When you say that you had the PCB analyzed for contamination, by what
method?  As I mentioned to Alistar, the analyst looking for a thin layer of
nickel oxide on the surface of a thin layer of gold must be very careful in
the techniques employed to "see" this oxide layer. It is very easy to miss,
I have seen it over-looked quite a few times by expert analysts. I have seen
conditions where the ENIG surface would not wet in random areas of the PCB,
and when we performed the analysis we found nickel oxide (in some cases a
very thick nickel oxide layer) present.

I would be happy to talk with you offline if you would like more
information.

Wade McFaddin
Nextek Inc.
256-772-1995  ext.1064

-----Original Message-----
From: Peter Barton [mailto:[log in to unmask]]
Sent: Friday, August 13, 2004 7:48 AM
To: [log in to unmask]
Subject: Re: [TN] Solder skipping on ENIG finish

I have a supplementary question relating to this answer. Could a low amount
of gold in ENIG also be attributed with poor solderability of assemblies at
second side reflow?

We have an assembly that is ENIG finished, and solders perfectly well on
side 1. When turned over to second side, placed and reflowed, a lot the
joints are not wetted to the pads. There is sufficient solder, and the
components themselves have wetted. If the process is reversed i.e. side 2
reflowed first, followed by side 1 then there is non wetting on that side.

Could it be that if the immersion Au is particularly thin that the first
side heating process could promote oxidation of the underlying Ni and this
oxidation migrate through the thin Au?

I have had the PCB analysed and no contamination can be found.

Regards,

Peter Barton

===== Original Message from [log in to unmask] (TechNet E-Mail Forum) at
12/08/04 09:15
>Alistair
>
>The presence of Nickel oxide on suspect pads shows up the problem. There
>is a lot of material on the NET/ archives and lots of gurus out there
>who advise higher gold in ENIG to cure this problem. The problem occurs
>due to low Gold in ENIG. U can ask for 0.1microns minimum.
>
>Best wishes
>Anil
>
>
>
>
>-----Original Message-----
>From: Alistair Murray [mailto:[log in to unmask]]
>Sent: Wednesday, August 11, 2004 1:29 PM
>Subject: Solder skipping on ENIG finish
>
>Hi to all. This is my first visit to this forum and I apologise if this
>subject has been aired and exhausted in previous correspondence.
>
>My customer has an issue with a simple double sided PTH board (no
>surface
>mount pads) with ENIG finish all over. After standard wave soldering, he
>is
>experience random skipping of test pads on the solder side. Normally,
>this
>would not be a problem but some of these test pads do not provide a good
>electrical contact when subjected to electronic testing. Again, this
>phenomenon is quite random.
>The wave soldering parameters have been modified but with little success
>as
>the skipping persists in a random fashion.
>We have carried out SEM analysis on good and suspect pads and have
>concluded
>that (1) there are no unexpected elements present thereby eliminating
>contamination such as solder resist (2) evidence of nickel oxide is
>present
>on the suspect pads (3) there is a physical difference in the material
>structure of the good and suspect pads - the good test pad has an
>homogenous
>structure with no discontinuities whilst the suspect pad is not
>homogenous
>and has a series of parallel lines which are boundaries in the material
>deposition (4) SEM analysis on the test pin found no insulating
>material.
>
>Can anyone explain how such skipping can occur and how it can be
>prevented?
>The random failure to make electrical contact on non soldered pads is
>puzzling. We are convinced that there are no insulating elements
>present.
>Could the different structures be a clue? Could the deposition rate
>differ
>from pad to pad? Could the process control parameters of the nickel and
>gold
>solutions be a factor here?
>
>Thank you for your anticipated help.
>
>Alistair F Murray
>Technical  Manager
>Artetch Circuits Limited
>Tel: 01903 725365
>Voice Mail Ext. 1260
>e-mail: [log in to unmask]
>www.artetch.co.uk
>
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-----------------------------------------------------------------------
Peter Barton
Senior Process Eng
ACW Technology Ltd
Dinas Isaf West
Tonypandy
Mid Glamorgan. CF40 1XX  Wales

Tel: 01443 425200
Fax:  023 8048 4882
International Tel : +44 1443 425200
International Fax : +44 23 8048 4882
Website/URL:  www.acw.co.uk

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