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November 2004

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Subject:
From:
Gerard O'Brien <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Gerard O'Brien <[log in to unmask]>
Date:
Thu, 4 Nov 2004 10:12:06 -0500
Content-Type:
text/plain
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text/plain (232 lines)
Anil - I agree that the size of the feature will have an effect on the
deposition rate of any immersion bath. However the deposition rate of I Au
is slow compared to say Ag or Sn and the deposition studies that justified
the 4552 document using "reference grade solid state XRF" equipment
typically NOT found in the PWB industry showed the variation but
insignificant as an overall function of the solderability performance.

For the studies - solderability was tested with a wetting balance and
associated coupon for ease of testing, XRF was run on both coupons and
boards.

Regards

Gerard O'Brien
Photocircuits Corporation
-----Original Message-----
From: Anil [mailto:[log in to unmask]]
Sent: Thursday, November 04, 2004 12:49 AM
To: 'Gerard O'Brien'; 'TechNet E-Mail Forum'
Subject: RE: [TN] Poor ENIG Solderability

Immersion gold by its nature will be influenced by the circuit pattern.
Large ground planes and small circuit patterns close by will not get the
same deposit thickness. This was brought out in a black pad study by
HADCO if I am not mistaken.

By the way is the 4552 committee doing the study on coupons or on
unbalanced circuit design as above?

Since Peter was observing a problem after 1st pass I felt this is
possible as thermal cycling will accelerate oxide production and poor
solderability.

Anil

-----Original Message-----
From: Gerard O'Brien [mailto:[log in to unmask]]
Sent: Wednesday, November 03, 2004 6:53 PM
To: TechNet E-Mail Forum; [log in to unmask]
Subject: RE: [TN] Poor ENIG Solderability

I must disagree must vehemently, the 4552 committee has tested and
continues
to test after 38 months of real time storage a 1 micro-inch deposit
which
continues to solder excellently. Attempting to specify a thickness
exceeding
a self limiting process will only result in the supplier leaving the
boards
in the bath longer which has been shown to be more detrimental than
advantageous. If you are truly getting an ENIG deposit with wild
variations
in pad deposit thickness adjacent to one another then CHANGE YOUR
SUPPLIER!


Gerard O'Brien
Photocircuits Corporation
Co-chairman  4-14 plating committee.

-----Original Message-----
From: Anil [mailto:[log in to unmask]]
Sent: Tuesday, November 02, 2004 11:46 PM
To: [log in to unmask]
Subject: Re: [TN] Poor ENIG Solderability

Peter
ENIG gold thickness is non uniform.

Pads with lower gold thickness may exist next to pads with adequate
thickness. These low gold pads will bring up nickel oxide in the first
pass and be unsolderable in the second.

It may be safer to specify more than 0.1 micron gold on the PCBs to get
rid of problem the IPC workgroup advises higher.

Anil Kher

micro interconnexion pvt. ltd
D3-12A, Corlim Industrial Estate
Corlim, Ilhas , Goa , India 403110
tel: 91-832-2284209/337. fax 2284209/2285271
e mail: [log in to unmask]
Leaders in Gold plated PCBs - selective/cob



-----Original Message-----
From: Peter Barton [mailto:[log in to unmask]]
Sent: Monday, November 01, 2004 9:24 PM
Subject: Poor ENIG Solderability

We have been experiencing poor solderability on ENIG finished PCB's on a
sporadic basis for some time now but have never been able to pinpoint
the
cause. This problem only ever manifests itself at the second side
reflow,
where solder wetting to pads is very poor, or even non-existent. The
problem
has occurred on a few different part numbers.

Brief details of our process are as follows:

Print paste, place  and reflow side 1, followed by batch water washing
(including D.I. rinse but no saponifier)and dry.
Print paste, place and reflow side 2, washing as per side 1.

There is plenty of solder paste present on the pads before placement and
reflow. Where wetting to the pad is poor the solder has wetted the
component
lead. The solder paste is a water washable type, within it's expiry date
etc. As it is fairly active I expect to have sufficient activity to
overcome
any minor impediment to soldering.

The time before paste printing and reflow has been varied, as has the
time
between washing the first side and processing the second side.
Additionally
I have experimented to see if our misprint washing could influence
solderability but none of these factors seems to have any significant
effect.

I have had the water input to the wash process analysed and there is no
evidence of any contaminants that may cause PCB surface contamination.
In
addition PCB's from the same wash batch can vary in solderability.

I have had SEM/EDX analysis done on affected samples to determine if the
Gold finish may be porous thus allowing the underlying Nickel to oxidise
but
the lab reports nothing unusual. Analysis has been done at low eV as
well as
the standard rate for this test. Additionally they report that
Phosphorus
distribution within the Nickel is not unduly high, or concentrated in
any
particular areas.

One finding at SEM/EDX was the identification of high levels of Carbon
and
Oxygen - presumably this is an indication of organic contamination?

I have researched other possible causes of this problem and understand
that
it is possible that partially cured solder resist may outgass during the
first side reflow, thus rendering the metallised surfaces on the second
side
difficult to solder. FTIR analysis is being done to try to determine the
compounds involved but the lab is new to electronics assembly analysis
(as
am I) and I do not have many samples for them.

I do not have the luxury of sacrificial samples now and still do not
have
and real insight into my problem.

In addition (of course) anything that exhibits this problem is a
production
board. Can anyone recommend what further tests can be done without
having to
destroy the assembly. Also are there any other suggestions as to
possible
causes of this phenomenon?

We are based in the UK so any labs that may wish to respond please do so
offline.

Thanks in advance for any pointers,




-----------------------------------------------------------------------
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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