Alistair:
There seems a bit of confusion in understanding your message.  It is my understanding that you are reporting some Nickel oxide, and other differences on the "problem" pads?

If so, this points to problems in the fabrication of the board, not in the assembly process.
It MIGHT even go back as far as the soldermask developing, if some mask is left on the pads, this can cause problems.

What are you seeing on the skipped pads?  Is it Gold, Nickel, or Copper?

Rudy Sedlak
RD Chemical Company

Alistair Murray <[log in to unmask]> wrote:
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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