Hello All

Hmmmmm.

We have designed boards using OSP for years and it has worked well for
the assembly sites that use it. You have all discussed using OSPs on
pads where screws and various types of mounting hardware will be used.
We have found that there is no issue with OSP in these types of
applications. We have known that any surface conductive surface has to
be made truly conductive by applying solder paste and reflowing along
with the other components.

With aqueous fluxes there is no problem. However, If you are stuck using
no-clean flux only, you have found the "Achilles heel" of OSP. There is
no way we know of to penetrate the no clean flux residue to resurrect
minimal conductivity without having to resort to "unnatural acts" (we
have tried too!). Selective application of solder or nickel/gold to
these areas while keeping OSP everywhere else is expensive. Thus for
these designs we use HASL.

Oh well.

Gerry

-----Original Message-----
From:   Taylor, Chris H. [SMTP:[log in to unmask]]
Sent:   Thursday, May 14, 1998 9:10 AM
To:     [log in to unmask]
Subject:        [DC] OSP Over Hardware Ground Lands

With the use of OSP becoming more commonplace, a concern has been raised
as to how chassis ground lands surrounding hardware mounting holes are
protected from corrosion in a high reliability commercial assembly. Is
OSP adequate for long term corrosion protection for a ground land? Per
my understanding, OSP is used to provide short term protection to SMT
lands, has a shelf life and is not intended to be left on the PCB
assembly after SMT reflow.

In the past, HASL (hot air solder leveled) was adequate in providing the
necessary corrosion protected metallic contact for hardware conducted
chassis ground. If a PCB is produced with OSP instead of HASL, then
there is no provision for the ground lands to be solder coated. One idea
that comes to mind is to include the lands in the solder paste artwork.
Is this practical? I realize that the PTH (plated thru hole) will still
lack solder coating, but a circular relief can be provided within the
land and around the hole to allow it to be an NPTH (non plated thru
hole).

How are my colleagues addressing this issue?
Chris Taylor
LXE, Inc.
125 Technology Parkway
Norcross, GA 30092
(770) 447-4224 X3242 v
(770) 447-6928 f
[log in to unmask]

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