Hi Leif and folks! You are not alone as other folks have reported this 
silver sulfide/resistor issue.  The issue is a combination of product use 
environment, resistor construction materials and resistor construction 
configuration. See Marie Cole's paper in the 2010 SMTAI Conference 
proceedings titled "Harsh Environment Impact on Resistor Reliability" , 
paper SMTAI10HE2.3. 


Dave Hillman
Rockwell Collins
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Leif Erik Laerum <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
05/25/2011 09:08 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Leif Erik Laerum <[log in to unmask]>


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Subject
Re: [TN] [SPAM] - RE: [TN] Silver Sulfide contamination - Found word(s) 
list error in the Text body






  George,

The silver sulfide corrosion was verified by using SEM, EDS and FTIR. We 
had a test lab make this analysis. The corrosion causes a change in the 
resistor value and eventually an open. This causes our equipment to fail 
of course. This is an environmental issue local to a particular customer 
or area. This determination is based on the fact that this is the only 
place where we see these issues and also based on what we know about the 
local air quality.


On 5/25/2011 8:37 AM, Wenger, George M. wrote:
> Leif Erik,
>
> When you indicate that you confirmed the failure due to silver sulfide 
do you mean that you can see silver migration or silver sulfide corrosion 
products shorting features or corrosion causing an open circuit?  I 
couldn't tell from your email if the exposed silver on the resistor 
network was corroding and causing an open circuit or if silver had 
migrated causing a short circuit.  It is not uncommon to see tarnish and 
or silver sulfide corrosion on products with exposed silver.  However, the 
only failures we've seen were primarily due to ionic residues left behind 
from assembly processes causing creep corrosion.  When residues aren't 
present we still see various degrees of silver sulfide on silver surfaces 
from a light tarnish to a heavy Brown/Black appearance depending on the 
amount of air pollution and exposure time but no migration or detrimental 
corrosion.
>
> Regards,
> George
> George M. Wenger
> Senior Principal Reliability / FMA Engineer
> Andrew Corporation - Wireless Network Solutions
> 40 Technology Drive, Warren, NJ 07059
> (908) 546-4531 Office (732) 309-8964 Mobile
> E-mail: [log in to unmask]
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Leif Erik Laerum
> Sent: Wednesday, May 25, 2011 9:12 AM
> To: [log in to unmask]
> Subject: [TN] Silver Sulfide contamination
>
>    Technetters,
>
> We have confirmed that a returned product from the field has failed die
> to silver sulfide contamination.
>
> The conclusion from the analysis is:
> Based on the analysis performed, the "black" contaminant material is
> silver sulfide, a common silver corrosion product. This corrosion often
> occurs when silver plating is exposed to atmospheric pollution that
> contains high levels of sulfur compounds (exhaust from fossil fuels).
>
> We have confirmed that in particular resistor networks are susceptible
> due to the silver content that apparently is exposed on the top of the
> component. We have not observed issues with any other component types.
>
> The obvious answer to how to protect a product in this environment is to
> use conformal coating. However, from what I have read there are only
> very few coating materials that can actually protect from silver sulfide
> corrosion. Epoxy based conformal coatings is the only ones that will
> successfully pass ASTM N 809 test. My question to the experts is if
> there are other solutions or coatings you have been successful with or
> if someone has actually solved this issue in different ways. It seems
> this should be an important issue for the automotive electronics 
industry.
>
> Some one suggested using solder to cover the component, but I can not
> understand how that could work.
>
> As always, thank you for your insight.
>


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