Alan,
Boston Brad did a admirable job of explaining alternate surface finishes
and compatability. Especially from the PWB fabricators perspective.
From a specifier, and user, perspective which is where I am soming from I
can tell you the following:
- HASL is no longer a consideration and hasn't been since about 1992 when
it started showing it's ugly side in support of BGA flatness requirements.
The process lends itself to too much variation and operator intervention.
- OSP is a very good alternative, but along with it comes storage, shelf
life, and handling issues. Not to mention the customer perception of bare
copper. We have and still do use OSP's on a large portion of our products.
- ENIG (Electroless Nickel/Immersion Gold) otherwise known as eNi/iAu has
been the recent change in surface finish from OSP's in support of our newer
packaging technology requirements. Be forewarned however that the source
needs to have a robust and well controlled process with experience in this
finish. The Gold is self limiting and therefore is not the isse. It is the
plating of the nickel that can cause issues. Early on, "Black Pad" was the
nasty phrase and much to-do and research was embarked upon for root cause.
This issue is basically non-existant given a well controlled fabrication
process.
- ENIS (Electroless Nickel/Immersion Silver) is probably the next best
alternate surface finish. Note that Boston Brad responded about Immersion
Silver plated directly to Copper, removing the Nickel influence. I do not
have any experience with this, but I question the flatness without the
Nickel.
- Immersion Tin (White Tin) is also gaining ground.

I suggest you get a copy of the CCAMTF (Circuit Card Assembly Materials
Task Force) Phase 1 and 2 evaluations of Alternate Surface Finishes. This
membership includes 22 organizations and 35 individuals. Their two basic
goals are: 1) evaluate alternatives to fused tin-lead PWB surface finishes
and 2) develop guidelines for conformal coating usage. I believe that this
report will help you tremendously.

That's my three cents. Hope I was of some help.

Bill Y.

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