PLEASE forgive me if this is posted twice.  I got "undeliverable
message" the first time around...

Taylor, Chris H. wrote:
>
> 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]
>

Chris...

We have not used OSP on our boards, but our assembler has asked us to
use immersion gold (2 - 7 microinches over nickel).  I just received
some info on the process (from Tyco in Utah) and supposedly the cost the
same (from this board mfg) for HASL, immersion Au or OSP. A couple of
the advantages are no special handling, no shelf life control and added
strength in PTH barrels from the nickel. The only drawback I can see is
possible solder bath contamination.

A couple of technical papers were mentioned:
1.  "An Investigation into the Characteristics of Solderable Coatings
for Printed Circuit Boards", K.J. Whitlaw'
2.  "Effect of Au on the Reliability of Fine Pitch Surface Mount
Joints", J. Glazer, P.A. Kramer and J.W. Morris Jr.

I'm sorry this doesn't address your question directly, but I think it
provides an option that might help with the problem. You might want to
check with your vendors to see if they have immersion gold capability
and if it is a cost adder.


Ben Davis
Systems & Electronics, Inc.
[log in to unmask]

################################################################
DesignerCouncil E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE DesignerCouncil <your full name>
To unsubscribe:   SIGNOFF DesignerCouncil 
################################################################
Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
################################################################