Dennis,

You're probably going to get many answers some indicating OSP aren't as
solderable as other finishes, doesn't have a long enough shelf life,
doesn't have a low enough surface resistance for applications that
require good contact or EMI and shielding requirements.  However I have
a different take on OSP coating.  I think one of the fundamental reasons
users stated to move away from OSPs is because OSP coatings don't change
the color of the copper features that they coat.  What I mean by that is
that PCB fabricators just like any manufacturer, don't have perfect
controlled processes that provide 100% yield.  They sometimes leave an
extremely thin tin residue (or solder if they use solder as an etch
resist) behind on features after tin-strip process.  They can also leave
behind an extremely thin solder mask residue on features during the
solder mask operation. Although these thin residues will cause soldering
problems, the residues aren't thick enough to change the color of the
copper feature so you don't know they are there until you try to solder
the product.  Many PCBs are made as SMOBC (solder mask over bare copper)
and the finial surface finish is applied after solder mask.  It really
doesn't matter what metallic surface finish you use (e.g., ENIG, ISn,
IAg, HASL) they all change the color of the copper features.  The color
change makes it easy to look at a board and tell if the PCB fabricated
had a tin-strip or solder mask problem.  If you see features with a
copper color the surface finish didn't plate or wet the copper feature
because there was.  All of the HASL operations I've seen have an
inspector looking at panels as they come off the HASL process looking
for features that didn't coat with solder.  These PCB fabrication
defects are therefore caught during fabrication and don't reach the end
user.  With OSP boards the fabricator doesn't know if there are features
that have a process residue.  The way the user finds out is to screen
solder paste, place components, and reflow.  The large telecom company I
retired from converted many of there PCB codes to OSP in the early
1990's because it was a flat surface finish for fine pitch components
and the supply chair people said great because it was cheap.  Well by
the end of the 1990's they began switching away from OSP as fast as they
switched to it initially.  They reason was because the assembly people
were finding all of the PCB fabrication defects that their fabricators
used to find.  You find out quickly that saving a few pennies or even a
few dollars per board doesn't offset the expense of scrapping populated
boards that cost hundreds to thousands of dollars.

Regards,
George
George M. Wenger, Andrew Corporation
Reliability / FMA Engineer
Base Station & Subsystems Group
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [log in to unmask]
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dennis Manowski
Sent: Friday, January 14, 2005 1:57 PM
To: [log in to unmask]
Subject: [TN] OSP Board Finish

Does anyone know why the use of SMT PCB's with OSP finish has dropped
off
as much as it has ?

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