Steve,
You mention the different joint characteristics also seen with Sn85Pb15
plated leads. I have had a similar experience with gull wing leads with this
particular finish. The appearance of the joint after reflow soldering is as
you describe, with a slightly discernible margin between the lead plating
and the solder forming the joint. This is accentuated by the fact that the
pad is not much wider than the lead footprint, thus there is little
paste/flux activity happening up the side of the lead to promote stronger
wetting forces.
Even under magnification this can be construed as non-wetting, meaning
falling foul of the requirements of IPC-A-610C where a statement "Evidence
of a properly wetted fillet" for the side joint is used. Our (and our
customers') inspectors become over cautious, especially when working to
class 3, and end up rejecting them.
Acceptable joint integrity can be difficult to prove without microsection
evidence. This can be impossible to provide when, as a contractor, I only
have a small quantity of valuable assemblies in process and every one is
accounted for. Instead of sectioning it is deemed less costly to touch up
the joints with an iron and solder wire to give an acceptable appearance to
our customer. This annoys me as we shouldn't be doing the rework, adding
cost, potential reliability issues, unpredictable manual intervention, etc..
etc.. etc..
After much argument I am now getting microsections done to show that the
joints are good, however, do you or does anyone else know of any sites that
show typical appearance of joints formed to this particular lead finish?
When my microsection results are available I would be happy to share them
with anyone in a similar predicament.
Best regards,
Peter Barton
===== Original Message from [log in to unmask] (TechNet E-Mail Forum.) at
9/05/02 12:32
>Good Morning Henry,
>To answer your question, yes I have experienced non-wetting with Pd along
>with Sn85Pb15 & Sn80Pb20 lead finishes. The first question is, "where does
>the non-wetting occur?" Typically solder plated or solder dipped leads
>during reflow experience mixing of the solder screened onto the substrate
>and the lead itself providing a smooth transition from the solder to the
>lead. With alternate lead finishes such as Pd, the only solder available to
>form the joint is that which is applied via printing, dispensing or wire
>core. This provides little or no solder on the top of the foot creating a
>transition line along the lead outline.
>The second question is, "does your solder joint exhibit good wetting at the
>heel and the along the foot?"
>The most casual observer will say the solder joint is a defect but under
>heightened magnification and scrutiny, the joint is probably acceptable but
>looks different.
>TI used to have a user's manual on Pd lead finish available for download. I
>suggest that you get a copy of that document and also begin a training
>exercise for your operators and inspectors regarding the differences in
>solder joint wetting characteristics dependent on lead finish.
>
>Steve Sauer
>Mfg Engineer
>Northrop Grumman, Xetron
>
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-----------------------------------------------------------------------
Peter Barton
Senior Process Engineer
ACW Technology Ltd
Dinas Isaf West
Tonypandy
Mid Glamorgan. CF40 1XX Wales
Tel: 01443 425200 Fax: 01443 436882
International Tel : +44 1443 425200 Fax : +44 1443 436882
E-mail: [log in to unmask] Website/URL: www.acw.co.uk
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