Bob,
You'll going to get many different answers to your question. The type
of surface finish one uses on PCBs would be chosen based on the design,
assembly, service reliability, and possibly repair considerations. As
you indicated the finishes being used on PCB's today are winding up
being the use final finish on deployed products. For many years bare
copper or rosin/resin coated copper was the primary surface finish for
PCB's. However, because companies had large inventories and PCB's might
sit on the shelf for several years before being assembled there was a
need for a surface finish that protected the solderability for long
periods of time (i.e., >> 6-12 months). SnPb HASL protected the
solderability for much longer periods (some will time in > 5 years. The
shelf life of the new "flat Pb-Free" surface finishes like IAg, ISn,
ENIG, OSP, etc. is high dependent on the storage environment. If you
are really concerned about the final finish on your PCB features during
use then I would suggest that you stencil print and reflow solder paste
on all exposed features during SM assembly. That way all the non-solder
mask coated traces and features will have the same final finish.
We use IAg (immersion silver) as a surface finish on our PCBs but we do
not use it as a solderable surface finish! We use it as an indicator
that the copper it was plated to is solderable. The solder pastes and
fluxes we've historically used were developed to reduce copper oxide and
unless our boards have gone through an aggressively harsh thermal
process the fluxes do a reasonable job. As for protect during use, if
our products were deployed in harsh environments that would attach and
eat through copper features and traces there aren't many surface
finishes that would be very helpful. Even if there were good protective
PCB surface finishes we'd have to worry about the surface finish on each
and every component on our assemblies. As much as conformal coating is
hated by most people who have to use it, it is probably a necessity for
harsh environmental exposure.
Regards,
George
George M. Wenger, Reliability Engineer
Andrew Corporation
40 Technology Drive
Warren, NJ 07059
(908) 546-4531 or (732) 309-8964 (Cell)
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert L. Lazzara
Sent: Tuesday, July 26, 2005 4:23 PM
To: [log in to unmask]
Subject: [TN] PCB Finishes: Final or otherwise
Virtually all lead-free bare PCB finishes are intended as interim, pre-
assembly finishes. Yet I also see a growing occurence of circuit
features that are sent into the post-assembly world without protection
of assembly solder, solder mask or conformal coating. Which begs the
question:
Are the lead-free PCB surface finishes safe in the field as final
finishes?
There's a design before me at this writing: It has nearly 100 test
points. All immersion silver. They'll go through assembly as immersion
silver, then they'll go out into the field as immersion silver. The
assembler is very interested in knowing how long the silver can sit on
his shelf and still promote soldering, but hasn't any interest in how
long immersion silver will sit in the field and still protect copper.
Immersion silver isn't being singled-out: Trade it with OSP or imersion
tin, and I don't feel any better or worse.
In-fact I have yet to find any producer of immersion silver, immersion
tin or OSP that rates their products as final, in-service finishes.
But it's happening...
I'm not too concerned about lead-free HASL or even ENIG, but the
immersion and OSP coatings seem like long term risks in circuit
reliability.
Anybody else thought about this?
Is there a paper that can be recommended?
Bob Lazzara
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