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,




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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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