Doug,
There are a couple of drawbacks to OSP from an assembly standpoint. One is
that we would get occasional "busts" where the solderability of the pads was
poor on a few pads on the board. You can not determine this prior to
assembly and since it is a low level occurrence, when it did occur, you
could have a lot of money worth of components on the board prior to catching
the problem. You can repair the defects that are obvious by scraping the pad
, but because you could not be sure of the integrity of the rest of the
solder joints, we chose to scrap the boards rather than assume the
reliability risk. This gets expensive very quickly. On most occasions it was
determine to not be the OSP process at fault, but rather things like unseen
soldermask residues or incomplete removal of the tin etch resist. However,
on at least one occasion boards were shipped without the OSP coating
applied.
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. Again, it results in scrapping expensive product.
OSP in general worked very well for us for over a decade, but the problems,
although infrequent, were costly and hence we have shied away from OSP in
recent years. It is still considered an acceptable finish, but it is not
preferred.
Thanks,
Robert Furrow
Printed Wiring Board Engineer
Supply Chain Networks
Lucent Technologies
978-960-3224 [log in to unmask]
-----Original Message-----
From: Dougal Stewart [mailto:[log in to unmask]]
Sent: Thursday, April 18, 2002 5:57 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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