Bob Smith asked:

>We are looking for inputs on copper coatings, hopefully based on
>experience, regarding the pros and cons of OSP's, Immersion gold,
>and HASL. We primarily use HASL and are using Immersion gold for
>limited applications.

We tried Entek 106B this spring (? from memory) on a board which is
normally a HASL finish for a small (20 pcs) production run. Worked fine
with our Alpha WS-609 paste, and the printability esp. on the 0.5 mm pitch
parts noticeably better, apparently due to better flatness and less
variation in pad height due to tin-lead puddling on HASL pads. 2nd pass
print/reflow was no problem on reverse side of the board.

Internal inspectors here kicked on "exposed copper" where the paste didn't
print the entire pad pattern or where there was slight paste stencil
misregistration. IMHO you will see copper wetted exactly where you put the
paste, but little spreading out on the pad. Suggest full 1:1 stencil
apertures may be best.

Martin-Marietta fixated types might get apoplectic about the dreaded
exposed copper. Me being a libertarian with regard to solder, I liked the
instant visibility of seeing any areas which had not been wetted with
paste. I thought that it made visual evaluation of fine pitch gullwing
leads _much_ easier.

One problem we encountered - the ICT bed-of-nails fixture had contact
problems on the test vias which were _not_ printed with paste. If we change
the boards we will order a new paste stencil which has apertures for the
test points, so the pins will hit solder instead of Entek coating.

Never tried immersion gold, but seems to me on paste/reflow SMT joints I
don't want any intermetallics I can avoid. This is not based on any test
results, though.

A paper entitled "Printed Wiring Board Surface Finishes: Evaluation of
Electroless Noble Metal Coatings" by Ray, Antaki, Wenger, and Machusak of
AT&T Bell Labs was presented at the SMI conference last month. They said
"Small concentrations of Au or Pd at the solder/substrate interface can
lead to the formation of brittle SnAu and SnPd phases which may initiate
cracking and lead to joint failure". They also noted a decrease in maximum
load in pull tests on SOIC solder connections with noble metal preservative
finishes. They commented that further microstructural joint evaluation and
attachment reliability studies are in progress.

The Bell Labs people have historically offered thorough and accurate
reliability studies to this industry, and I'm interested in seeing their
data.

>At this risk of kicking the hornets' nest I am also interested in
>should we require mask between pads on fine pitch. Our specific
>product is 8" x 16", .047" thick with LPI and a dozen 20 mil
>devices with 12 mil pads and 8 mil spaces.

This is a matter of what your PWB supplier will provide as minimum web
width. One supplier of ours said that they could do dams on 0.5 mm pich
pads if we used .010" pads/spaces, but not 0.012/0.008. Our normal is the
0.010/0.010 anyway, so we get dams from like 3-4 suppliers usually no
problems. I think many PWB vendors will allow a 0.006" minimum web located
+/.002 to the pattern.

IMO, these dams offer noticeable benefits, as they provide gasketing for
the stencil in fine-pitch printing. In the absence of mask webs, you may
see more tendency to push paste through the apertures and require more
frequent wiping, or suffer more shorts. We did process some boards without
these (strictly involuntarily) and had moderate difficulty compared to the
boards with the LPI dams.

cheers,

Jerry Cupples
Interphase Corp.
Dallas, TX
214-919-9150
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http://www.iphase.com