Replating bare boards is a simple process with immersion silver. I have boards in my desk that have been there for two years and only the top two boards are oxidized. I have taken boards out of the middle of the pile and put solder on them, (not recommended for production) but you get the point. Anti Oxidants are in both Sterling (Alpha Level) and Mac Dermids solutions.
 
On assembled PCB's tarnish starts right after the first thermal excursion the boards see so we make it a habit to run both sides as soon as possible to improve our statistical chances of good solderability. Our rule is three days. But we have waited several weeks the boards did not look pretty but they worked. I recommend vacuum sealing assemblies which you will need to leave around, this will also help extend the solderable/test window.
 
For testing Beryllium/Copper probes are recommended for Immersions silver. Your Pogo pin Co. can help. Your flux is probably not very probe able right now although Indium and Aim make some very good pastes that extend (stays soft) the success rate. Look to see if the flux where the pins are hitting is chipping away and yellowish it actually shatters after a couple of tests when the pin finally breaks through.
 
Dave Chapman
Manufacturing Engineer
Circuit Service Inc.
 
-----Original Message-----
From: Mike Fenner [mailto:[log in to unmask]]
Sent: Wednesday, January 08, 2003 11:00 AM
To: [log in to unmask]
Subject: Re: [TN] Immersion Silver oxidation

This may be something you checked or knew before coming on the 'net, but I have to mention just in case: Silver oxide is electrically conductive.
 
How well compared to elemental silver I don't know and I don't have a reference to hand. I think it might be worth checking  and/or ensuring that this is an oxide problem before you go down the road of trying to remove  from assembled boards as I the only ways I can think aren't attractive.
 
If different pin profiles don't work then the alternatives  are
Chemical de-oxiders: would seem to be out on the grounds that immersing the boards in aqueous acids and rinsing them off is off limits (unless you are using water wash chemistry in your flux/paste).
 
Use flux: Possibly you could paint on flux, reflow and either probe through the residues (some fluxes allow this). Or remove flux residues. The thinking is that fluxes are an assembly product and therefore "safe"at this stage, and the oxides must be reduced by  flux well enough to solder -  this might be good enough to produce a positive test.
 
Abrasion: locally abrade the test points.
 
And that's it ..... unless other 'netters know better.
 

Regards

Mike Fenner

Applications Engineer, European Operations
Indium Corporation
 T: + 44 1908 580 400
M: + 44 7810 526 317
 F: + 44 1908 580 411
 E: [log in to unmask]
W: www.indium.com 
Leadfree: www.Pb-Free.com

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Sherif Refaat
Sent: Wednesday, January 08, 2003 3:35 PM
To: [log in to unmask]
Subject: [TN] Immersion Silver oxidation

Hi Technetters,
 
I've got a situation here where boards coated with Immersion Silver were partly assembled before the Holidays and the assembly was completed this week. So, the boards were left in the factory, uncontrolled environment, for over two weeks. The circuitry on the side that was assembled first exhibits oxidation. The oxidations does not allow the In-Circuit test probes to have good contact with the test pads, hence causing lots of false opens.
 
Any recommendations regarding how to remove Immersion Silver oxidation from assembled boards? The Immersion Silver Chemistry supplier is located overseas and because of the time difference I can not get an immediate answer. I heard about a more pointy test probes that may be able to penetrate the oxidation but the factory does not accept this solution.
 
Can any one help?
 
Thanks in advance.
 
Sherif,
 
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