Alistair,

Are your test pads circular lands or are they vias?  What is the
thickness of your solder mask?  Solder skips in wave soldering on test
pads that don't have an opening to the top of the board are not
uncommon.  Since flux is applied to the board prior to hitting the
solder wave, the flux in the well made by the solder mask can prevent
the solder from touching the pad.  My guess is that the reason you don't
find anything wrong with the ENIG pads is because there is nothing wrong
with them I assume that you've tried to add solder to the pads after
wave soldering and they accept solder so telling you that there isn't a
solderability issue.  If this is the case than what you are experiencing
is a physical soldering issue caused by flux entrapment in a "well".

 
Regards,
George
George M. Wenger, Andrew Corporation
Reliablity / FMA Engineer
Base Station & Subsystems Group
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [log in to unmask]
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Alistair Murray
Sent: Wednesday, August 11, 2004 3:59 AM
To: [log in to unmask]
Subject: [TN] 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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