Sorry Eric, I'm afraid that I can't answer your specific questions, as I am
not very experienced on the assembly side of the operation (last did
assembly in 1986 when it was still all through hole). What I do know is that
the modern OSPs are capable of withstanding up to 6 heat cycles, ie you can
glue cure, wave solder, paste reflow and handsolder without losing
solderability. I don't know what the cleaning process in between will do -
you would have to ask the OSP supplier like Cookson for compatibility -
Entec 106A is a Cookson product that they support from all angles - OSP
chemistry, paste, flux, cleaners etc.
I was involved with surface finishes a number of years ago, and found the
ignorance in some (assembly) companies astounding, whilst others fully
understood what they were dealing with.
One OEM company wanted to do accelerated ageing by using steam ageing - a
technique developed to artificially age reflowed tin lead, but pounced on as
the 'be all and end all' method for ALL surface finishes. Not surprisingly,
it failed miserably and the company dismissed OSP as an alternative. After
much persuasion they tried again, and 8 weeks after supplying product to
them, they told me that OSP had failed due to its inability to solder after
the first side. What they didn't tell me, at first, was that they tried to
solder side 2 four weeks after side 1, because this simulated what happened
in their production. Sad to say they have now closed that factory, losing
over 1000 jobs, and all their work is now manufactured by CEMs somewhere in
the far east. I think the inability to understand the basics of JIT
contributed to this greatly.
On the positive side, I worked closely with a (very) major telecomms
company, who saw the benefits of OSP and worked on the basis of never put a
board into assembly unless it can be SHIPPED within 24 hours. They are still
a very successful Telecomm company despite the worldwide downturn.
I have read all the other mails responding to my plea, and I think that the
general message is that surface finish should be related to need and data,
not fashion and opinion.
I would also like to answer Dave Hillman's point
-> 'Field repair - OSPs don't provide the long term solderability on pwa's
which have been in the field for a number of years which sometimes are
subjected to upgrades/changes. It is typically an uphill struggle to have
good solderability for HASL for servicing an assembly which has been in the
field for 5-10 years anyway and OSPs just give up the ghost way to early in
comparison to metallic surface finishes.'
I don't understand this issue as all the solder joints on an assembly were
made at the time the board was first soldered, any surface finish on the raw
card has been incorporated into the solder joint or disappeared through
volatisation at the point of soldering. Surely any rework soldering will be
on a solder joint, not on an ancient surface finish? However, I am glad to
hear that Rockwell use OSP as one of their finishes, on the basis of the
right finish for the right reason.
Rich Fudalewski (and Bob Furrow) stated: 'The other area where OSP can fail
is with via in pad (VIP) product. If there is any residue remaining in the
holes it will cause a degradation of the OSP coating. determine to not be
the OSP process at fault, but rather things like unseen soldermask residues
or incomplete removal of the tin etch resist. Doug, this seems to be a
problem with your board manufactures control of the process.'
I couldn't agree more with this, and have seen even worse issues with
immersion tin, where the thiourea emerges from the VIP during the first side
reflow. I have seen issues with immersion silver where the undercut of the
soldermask traps residues that corrode the finish, and problems with ENIG
where nickel foot has almost shorted between pads. I return to my original
point - only OSP is reworkable in these situations. All of the other
finishes will end up with scrapping the raw card rather than rework, and
even though companies go bust if they cannot control their processes, scrap
is always paid for by us the end customers somewhere down the line (even if
its to pay higher national insurance or tax to cover the unemployment
benefit of those who lost their jobs).
I hope not to have preached too much, and that Rich F has his faith restored
that Fab people do respond ! I am glad to have rekindled some interest
anyway!
Dougal Stewart
email: [log in to unmask]
phone: +44 1896 822204
mob: +44 7984 629667
----- Original Message -----
From: "Eric Dawson" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, April 18, 2002 1:40 PM
Subject: Re: [TN] Immersion Silver
> Hi Dougal,
> OSP sounds attractive but can I seek your views on a couple of points?
> We double side reflow and clean between operations to avoid baking the
flux
> on. These are complex boards and can take several days to progress through
> the line. Will the OSP stay fresh for the second side reflow or would we
> have to treat the surface? Will cleaning take it off? (Prozone).
> Regards
> Eric Dawson
>
>
> > -----Original Message-----
> > From: Dougal Stewart [SMTP:[log in to unmask]]
> > Sent: Thursday, April 18, 2002 10:57 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] Immersion Silver
> >
> > I would like to know why people don't use OSP - from a board shop's
> > viewpoint, it is the best finish to put on a board - why ? because it is
> > very simple chemistry, puts no thermal shock on the product, is applied
on
> > a
> > SHORT conveyor line, and can be easily reworked unlike ANY of the other
> > finishes. The assembly guys don't like it if they start assembling a
> > product
> > and then stop because they don't have the parts to finish the job, but
> > anyone that puts a board into assembly without the ability to finish it
> > within 6 hours obviously knows nothing about world class manufacturing
> > practise. As for shelf life, if your stock is even 2 years old, send the
> > boards back to the board shop and for a small fee they just put them
back
> > down the line again and you have a finish that is good as new, without
> > having undergone a vicious chemical attack to remove the finish.
> > Just my thoughts !
> > Dougal Stewart
> >
> > email: [log in to unmask]
> > phone: +44 1896 822204
> > mob: +44 7984 629667
> > ----- Original Message -----
> > From: "Jack C. Olson" <[log in to unmask]>
> > To: <[log in to unmask]>
> > Sent: Wednesday, April 17, 2002 4:22 PM
> > Subject: Re: [TN] Immersion Silver
> >
> >
> > > Hmmm,
> > >
> > > Well, at some point in EVERY new technology it was used by less
> > > than 3% of the world, eh? Guess I'm just a trailblazer....
> > >
> > > naw, just kiddin ya.
> > > I subscribed to a magazine called Circuitree, and the last issue
> > > came bundled with another publication called "The Board Authority"
> > > which has some very well written (and well researched!) articles
> > > on alternate finishes. I can't see any downside to using silver
> > > in our applications, and I've looked all through the archives for
> > > Technet, DesignerCouncil and PCDlist forums, still no show-stoppers,
> > > so we tried it for the first time on our last design. Yes, HASL and
> > > NiGold do work fine, but maybe we can get the same functionality
> > > for cheaper, and do our fab vendor a favor in the process, since
> > > they seem to prefer it (less steps? more reliable? not sure) maybe
> > > the fab guys can answer that one....
> > >
> > > Jack
> > >
> > >
> > >
> > >
> > >
> > > "WEEKES, MICHAEL HS-SNS" <[log in to unmask]>
> > > 17Apr2002 09:16 AM
> > >
> > >
> > > To: "'TechNet E-Mail Forum.'" <[log in to unmask]>
> > > "'Jack C. Olson'" <[log in to unmask]>
> > > cc:
> > >
> > > Subject: RE: [TN] Immersion Silver
> > > Retain Until: 05/17/2002 Retention Category: G90 - Information and
> > Reports
> > > Caterpillar Confidential: Green
> > >
> > >
> > > There must be some reason why less than 3% of the world has moved
toward
> > > immersion silver. Why doesn't HASL or NiGold work for your
application?
> > >
> > > Mike
> > >
> > > -----Original Message-----
> > > From: Jack C. Olson [mailto:[log in to unmask]]
> > > Sent: Wednesday, April 17, 2002 7:39 AM
> > > To: [log in to unmask]
> > > Subject: Re: [TN] Immersion Silver
> > >
> > >
> > > The PCD magazine list went down before I could get an
> > > answer to this question, does anyone have experience
> > > with silver and contact switches?
> > >
> > > ==============================================
> > >
> > > Most sources say a silver surface will not be solderable
> > > after a month or so unprotected. (Even in a baggie it will
> > > be okay a year later?)
> > > Anyway, you say it is good for switches, which would solve
> > > some of our hard gold plating problems.
> > >
> > > But I would think the "unsolderability" of the aged silver
> > > surface (unprotected in the unit and tarnished?) would
> > > affect the reliability of the switches, wouldn't it?
> > >
> > > Jack
> > >
> > >
> > >
> > >
> > >
> > >
> > > Rob Mongey <[log in to unmask]>@freelists.org
> > > 4Apr2002 06:08 AM
> > >
> > > Please respond to [log in to unmask]
> > >
> > > Sent by: [log in to unmask]
> > >
> > >
> > >
> > > To: [log in to unmask]
> > > cc:
> > >
> > > Subject: [pcdlist] Re: Immersion Silver
> > > Retain Until: 05/04/2002 Retention Category: G90 - Information and
> > Reports
> > > Caterpillar Confidential: Green
> > >
> > >
> > >
> > > Hi all,
> > > Good to see Immersion Silver is starting to take off. We've been
> > using
> > > it
> > > in here for about two years, and it is now our prefered board finish
(We
> > > get silver finish unless we specify differently.) More common useage
> > will
> > > mean that maybe more Far Eastern shops will start supplying it.
> > >
> > > We started to use Silver because it is so adaptable to what we
need
> > > our
> > > PCB surface finish to do, namely:
> > >
> > > 1. Good solderability. (Silver excels at this, ask any vendor).
> > > 2. Flatness for fine pitch SMT. (Silver is as flat, if not flatter
than
> > > Gold)
> > > 3. Mechanical strength for carbon pill switch contacts / Elastomer
> > > contacts. ( Flash Gold is weak, and hard Gold gives SMT solderability
> > > problems)
> > > 4. Stability for compression power contacts. (We have power contacts
on
> > our
> > > PCBs that mate to battery contacts via a spring compression. We found
> > that
> > > the nickel under gold can cause problems with electrochemical
migration
> > > toward our battery contact.)
> > >
> > > Immersion silver is the most suitable choice that can do all four. I'd
> > use
> > > gold as an alternative, but we'd have to compromise our designs
> > somewhat.
> > >
> > > We specify our Silver finish as 0.1 Microns Min to 0.2 Microns Max,
over
> > > bare copper. Anyone else do it differently? Or do we trust our board
> > shops?
> > > :)
> > >
> > > Regards,
> > > Rob Mongey.
> > > CAD
Technician.
> > >
> > > Robert Mongey <[log in to unmask]> Tel : +353 (0)91 774300
> > > Bio Medical Research Direct : +353 (0)91 774338
> > > BMR House, Fax : +353 (0)91 774302
> > > Parkmore Business Park West, Mobile : +353 (0)87 2485262
> > > Galway
> > > Rep of Ireland.
> > >
> > >
>
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