Hello Everyone,

I have similar experience to what James is describing with Immersion Silver and Immersion Tin finishes.  They are easy to damage with exposure to chlorides, sulfides, sulfates, etc.  Discoloration of the tin and silver finishes is a sure sign that they have reacted with one of these type of compounds.  The end result is a surface that is difficult to solder.   There are ways of refreshing the immersion tin and silver surfaces which can restore good solderability, but it involves the use of wet chemicals and it is difficult to do to a high volume of circuit boards.  


Best regards,

Tony Lentz

FCT Assembly
Field Application
[log in to unmask]
970-566-0360 Mobile

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of James Head
Sent: Tuesday, May 5, 2015 2:49 AM
To: [log in to unmask]
Subject: Re: [TN] Chloride and Sulfate damages to circuit boards

This may be off on a tangent from your query.

Both Chlorides and Sulphates will attack both an immersion tin and an immersion silver finishes on PCBs.  Both of these are target gasses for our Gas Detection products so we use alternative finishes.

The result of Chloride and Sulphate exposure to these finishes was a severe tarnishing of the PCB finish, coloured black, after we conducted a series of short tests.  We didn't do any long term performance testing of the boards to see if there were any electrical performance changes.

It was far simpler from our view to state that these finishes were not acceptable for our products as alternative finishes are available: lead-free HASL, ENIG and Electroless Gold.  It avoids doing any long term tests to check and make sure.  The tarnishing may not have an effect on performance, nor operation, but customers may still notice it, particularly when calibrating or servicing products and it's a lot easier to minimise the number of calls to technical support from customers by avoiding these finishes.

Regards,

James


James Head BEng CID+ MIIE MIET
Senior PCB CAD Engineer
Crowcon Detection Instruments Limited
172 Brook Drive
Milton Park
OX4 4SD
Telephone: 01235 557700
E-mail: [log in to unmask]

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