Hi ,

More inputs:

OSP require Nitrogen in soldering process.
%of PTH solder filling  is lower
Press fit  process need more force
3-6 month max shelf life
Baking under 100 degC
Number of soldering cycles is less

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ryan Grant
Sent: Friday, January 04, 2008 2:52 AM
To: [log in to unmask]
Subject: Re: [TN] OSP w selective gold

Hi Genny,

OSP is actually the "preferred" finish with selective gold because you
do not need to mask the gold in order to apply the finish.

Of course, there are gotchas!
The OSP solution tends to precipitate out onto the rollers of the line,
which then transfers as big globs onto the board, if the line is only
run occasionally.  Suppliers who run the OSP line continuously don't see
this problem very much.

Certain OSP formulations carry copper into the solution and transfer it
onto the gold, causing it to discolor.  (The more common OSP
formulations don't have that problem.)

OSP has a limited shelf life.

OSP does not like to be baked.  Baking your boards for 24 hours at 125C
will render them unsolderable, so beware!

On the positive side, you can have boards recoated with OSP to restore
the solderability.  So if you store boards for 2 years, or bake them to
unsolderability, you can send them back to the supplier and they will
restore the solderability by micro-etching the copper and reapplying the
OSP.

Which brings up another gotcha.  Since each OSP application involves a
micro-etch, too many times through the OSP application will
significantly reduce the copper thickness in the vias.  So you will want
limits on the number of times boards are allowed to go through OSP
application.  Sometimes the board shop will re-work boards through the
OSP line before you even get them the first time!

Many board shops do electrical test before OSP because the OSP causes a
high number of false failures.  This means the micro-etch in the OSP
process can open up marginal vias, but there won't be a test to catch
the opens.  Instead, the open vias will be caught after you assemble the
product.

The OSP micro-etch presents a "risk" of chemical entrapment if your vias
are tented on only one side.

Solderability through the wave is marginal and more difficult to get top
side fillets.

Exposed copper surrounding a solder joint is common.  While this isn't a
problem, just be aware that the solder will not wet all the way down the
sides of pad, or to the corners, etc..

Solderability is not as good as ENIG or imAg, so marginality in the
soldering process will appear sooner.

The OSP can usually be removed by the stencil/board wash system.
Getting washed boards back to the line is not especially time critical,
but don't expect to wash boards and put them back into inventory to sit
for a long time before re-use.

The copper will discolor like it is oxidizing (because it is) after each
reflow.  Don't be alarmed!  It still solders.

OSP will not hide the galvanic effect that occurs during the gold
plating process, so some SMT pads will appear oxidized because their
surface texture will be rough.  Don't be alarmed!  This is typical when
there is a large difference in surface area between the gold plated pad
that is connected to a non-gold plated pad.  It doesn't noticeably
affect solderability.

There are probably other gotchas, but I can't think of them right off
the bat...


In spite of these gotchas, OSP still works quite well and has the huge
advantage of simplicity for the PCB supplier; and you are soldering to
copper instead of nickel compared with ENIG.  For most shops, this is
the cheapest finish, especially if you require selective electroplated
gold.  You don't need any "special" handling criteria except what was
stated above.  Nitrogen is NOT needed for soldering to OSP, and I really
haven't seen any no-clean solder paste that has a problem with OSP.  In
other words, soldering to OSP is not difficult.

Good luck.
Ryan

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Genny Gibbard
Sent: Thursday, January 03, 2008 3:22 PM
To: [log in to unmask]
Subject: [TN] OSP w selective gold

We have never used OSP for anything, but we may be required to switch
one of our products to it.  Can anyone point me in the direction of some
good solid info regarding working with this finish?  
How common is it, especially if you need selective gold?  Are there any
solder process pitfalls to watch out for?  
This is for leadfree products.

Thanks in advance,
Genny.

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