Hi All,
I recommend to check the option of HASL PCBs with the Tin Bismuth alloy.
Its have low melting temp. and both metals work fine together.
HASL coating is good option for SMT PCBs up to 20 mils pitch.
-
Best Regards
Reuven ROKAH
"Stadem,
Richard" To: [log in to unmask]
<Richard.Stadem@ cc:
GD-AIS.COM> Subject: Re: [TN] PCB Finishes: Final or otherwise
Sent by: TechNet
<TechNet@
ipc.org>
27/07/2005 16:03
Please respond
to TechNet
E-Mail Forum;
Please respond
to "Stadem,
Richard"
This is an excellent response.
Also, always keep in mind that the final finish is not what you solder
to, except for HASL. ENIG, IAg, ISn, and OSP finishes are removed or
absorbed into the solder joint during the soldering process. So anything
that is not soldered will retain its original finish except for OSP,
which will be at least partially removed during several thermal cycles
and wash cycles needed to process this assembly.
This leads to the question: Which finish is best for my particular
product?
As our famous Mr. Pauls would say, (drum roll, please) It Depends.
If you have a double-sided board with SMT/BGAs on both sides and some
through hole parts and some parts that are not immersible and are hand
soldered after all of the other components are installed, I would
certainly not use OSP as a final finish. After two reflow cycles, plus a
possible wave solder or selective solder, possibly a hot gas BGA rework
or two, and the subsequent washes (only an insane person would attempt
to build this with a no-clean process) there will be very little OSP
left on the pads by the time you get around to the hand-soldered
components, and those pads/holes will be somewhat oxidized and difficult
to solder. An immersion silver or ENIG finish would be best for this
assembly. HASL is not considered because I included SMT and BGAs in the
component mix. This means you are shipping a board with possibly several
hundred bare copper test points and unused pads out into the world.
That's asking, nay, begging for trouble.
OSP works quite well for standard single-sided SMT-only boards that are
typically processed with a single reflow, and for that type of assembly
most of the OSP finish will be retained for protection long after the
assembly is shipped. The problem with OSP is that the board
solderability depends on limiting the oxidation of the copper between
the final fabrication step and the OSP application. Some PWB fabricators
do well at that, others not so well, and you get boards with a copper
tone that looks like an 1894 Indianhead penny, rather than a 2005
Lincoln. This does not always mean that the board will not solder well,
but it is usually a good indicator that it won't.
Regarding the shelf life of immersion silver finish, it may tarnish over
time. However, this does not usually reduce the solderability, as you
are not soldering to the silver. You are soldering to the copper
underneath.
I have also noted that immersion silver finish will not tarnish at all
if you simply have the board vendor wrap the board brick with some type
of clear plastic wrap. This is especially important if the boards are
stored in a area where chlorine and flux are present, as they are
suspended in the air, especially around a wave solder machine with a
spray or mist fluxer. This is also especially true if stored near
coastal areas with heavy salt content in the air.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
Sent: Tuesday, July 26, 2005 4:18 PM
To: [log in to unmask]
Subject: Re: [TN] PCB Finishes: Final or otherwise
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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