I'd like to add to David's comments too...I tend to agree with him on his
point about OSP's. In order of preference, the board finishes I'd `druther see
on SMT boards that I'd have to build are 1. OSP, 2. Flash Gold, and 3. HASL.

A few more good things that I've observed with OSP's in addition to what David
said are; 1. Less solder bridges on fine pitch - The solder seems to stay
right where it's printed during reflow on a OSP coated board...it's like the
flux has to use some of it's activity to go through the OSP before it reaches
the pad surface and does it's thing, and all that is confined to the area
where the paste actually is. It's almost like the OSP coating that's in
between the pads acts like a little "dam" if you will, preventing the molten
solder from easily crossing over to an adjacent pad. 2. No heat used applying
OSP's - Less board warping...

     There's a couple of caveats with the board houses that say they can do
OSP's. I've run across a few that say they do OSP's, because it's easy to say
that. Only because one can go out and buy gallons of the chemical, pour it in
some sort of tank to dip the boards in, and  voila'...you're doing OSP board
finishes. But that's really not the best way to apply the coating. The
thickness of the coating needs to be controlled. It may not be a big deal if
you're in a water soluble process and use flux so active that you can solder
to dirt, but when you're using a no-clean / low activity flux, you'll have a
hard time soldering if the coating is too thick.

One more board house problem I've run across is that they've done the bed-of-
nails testing (or bare board test) after they've applied the coating...that's
a no-no. The probes will break thru the coating and compromise it's integrity.
OSP's are meant to be applied and stay in an unbroken layer until it's ready
to be soldered, that's how it protects the copper from the oxidizing.

                                                        -Steve Gregory-

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