Tim,

I've been using silver (IAg) for about two years now and have been pretty happy with it.  My usage has been with primarily with lead-based designs, though the design I make now are lead and lead-free compatible (military and forward design thinking).  

But the items I cover are about the same with both lead and lead-free design. I've had carry over Here are the pro/cons that I've had with it
Pros:
* I find it better for High speed than ENIG, no nickel barrier.  I tend to use it for anything over 500Mhz.  Typically I do mix-mode RF and High Speed Digital design with it.
*Good planarity for bga devices, just as good as if not better than ENIG 
*Excellent solderability, I've never had a problem with BGAs with ENIG or IAg.
*Generally cheaper and less process steps than ENIG.
*See IPC-4553 Specification for Immersion Silver Plating for Printed Circuit boards
*Good alternative to HASL finish as it also removes heat cycles
*Can have good shelf life, that depends more on your facilities storage capablities

Cons:
*Tanishes easily, use a silver protectant if possible.  Your vendor may do it automatically or you may have to call it out in your notes, talk to your fab house for their recommendations.  This will protect the silver of the board, but it should be noted that the tarnish does not generally affect the solderability of the board, it just doesn't look at shiny.
*Handling precautions should be followed to per the IPC-4553, section 3.8.  Your vendors (FAB and Assembly) should be familiar with the IPC-4553 or be able to provide you their work instructions in regards to working with silver finish boards.  
*Some vendors don't do it yet, and some assembly houses are not yet well versed with storage and handling of silver finish boards.  Both DDI and Streamline that I use are perfectly capable with it.

If I know the bare board will be sitting on the shelf for the next 2-3 years between builds (50 this year, 25 next, 25 year after) I will typically use ENIG.  If they are all going to be assembled at one time or within the next year, I have no issue using IAg.

Hope this helps,

-JJ
 

________________________________________

JEFFREY A. JENKINS, CID+
SR. PCB/CAD Designer 
L-3 COMMUNICATIONS - 
LINKABIT DIVSION
3033 Science Park Rd.
San Diego, CA 92121
Tel:  (858) 552-9832
Fax:  (858) 552-9487
[log in to unmask]

________________________________________
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Tim Boyer
Sent: Thursday, December 04, 2008 12:16 PM
To: [log in to unmask]
Subject: [DC] Pro's and Con's of using Immersion Silver

Currently we use Immersion Gold for our ROHS boards.
We are looking into changing to Immersion Silver.
What differences are there in solderability, storage, long term reliability, operating temperatures,
etc?
Why are allot of companies still using Immersion Gold over Nickel for ROHS boards?

Tim Boyer, CID+

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