found a good paper regarding nff.... http://www.prognostics.umd.edu/ 
calcepapers/08_Qi_No_Fault_Found_Intermittent_Failure.pdf
we all should be aware of the potential fall out, even no failure in  
return....
         jk
On May 29, 2016, at 11:31 PM, George Wenger wrote:

> Michael,
>
> I agree with Tan Geok Ang. I fully understand why your mfg/assy  
> facility in EU is balking at the finish mismatch. However, I do not  
> agree at all with their suggestion that you change the board finish  
> to immersion tin. Immersion tin surface finish has serious shelf  
> life and solderability issues and I know that sometime after you  
> switch to immersion tin you'll be wishing you hadn't switched
>
> Although Guideline #7 in "The TIN Commandments" published by AMP  
> indicates "Mating tin coated contacts to gold coated contacts is  
> not recommended.", before I would switch your board surface finish  
> I would ask your customer if they have ever seen any issues with  
> your boards related to finish mismatch. Since this finish mismatch  
> is only used for mechanical mounting I don't think the finish  
> mismatch will cause functional issues. Especially since you  
> indicated you been using this finish mismatch for some time and if  
> that is the case I assume someone would have seen a problem if  
> there was one Also, there isn't much gold on ENIG surface finish  
> (only a couple of micro-inches) and there probably won't be any  
> considerable amount of corrosion
>
> If my mfg/assy facility told me I had to board surface finishes I  
> would push back really hard and tell them that if they are worried  
> about the gold/tin finish mismatch they should just take a  
> soldering iron and a piece of solder and "tin" the few ENIG pads on  
> the board where the hardware is mounted (i.e,, wet the ENIG pads  
> solder which will dissolve the small amount of gold and eliminate  
> the finish mismatch).
>
>
>
> Regards,
>
> George Michael Wenger
>
> George M. Wenger
> Failure Signature & Characterization Lab LLC
> 609 Cokesbury Road, High Bridge, NJ 08829
> (908) 638-8771 (Home) (732)-309-8964 (Cell)
> [log in to unmask]
>
> ----- Original Message -----
>
> From: "Tan Geok Ang" <[log in to unmask]>
> To: [log in to unmask]
> Sent: Sunday, May 29, 2016 10:44:50 PM
> Subject: Re: [TN] ENIG Board Finish with Tin Plated hardware
>
> In long term , ENIG with Sn will cause fretting corrosion. BUT  
> don't change the ENIG board finish to Tin surface finish as it  
> creates hell lots of other issue during manufacturing.
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Yuan-chia Joyce  
> Koo
> Sent: Friday, 27 May 2016 11:43 PM
> To: [log in to unmask]
> Subject: Re: [TN] ENIG Board Finish with Tin Plated hardware
>
>> "We've built all these assemblies, many, many times without issue."
> you means Siemens didn't have the dissimilar metal coupling check?
> Build is not an issue, corrosion in the field and resistance change  
> might be... it is reliability issue. if your stuff only work on one  
> shot deal (like bomb), and you just engage the two parts upon short  
> usage life, not much of issue, if you are in the field for long time
> - like more than 3 month, you should consider dissimilar metal  
> coupling from the start of design phase - use some good material/  
> process engineer to select mating parts properly (according to  
> standards).
> better not be one of those electrical switches... not smart one  
> either...
> jk
> On May 27, 2016, at 10:43 AM, Kuczynski, Michael wrote:
>
>> We've been using ENIG finish for our PCB's and installing tin plated
>> hardware (self clinching, broaching nuts and standoffs)
>>
>> We use anywhere from 1 or 2 pieces, to maybe a dozen, of this  
>> hardware
>> on various assemblies.
>> These items are used just to hold or mount some items in place (no
>> serious stress on these items).
>>
>> Our mfg/assy facility in EU is balking at the finish mismatch and
>> suggesting we change the board finish to immersion tin (which we've
>> never used).
>>
>> We've built all these assemblies, many, many times without issue.
>>
>> Is there any other merits I should consider, before I tell them  
>> please
>> build them as is?
>>
>>
>> Michael Kuczynski
>> Mgr. ECADS - Electrical 511 Benedict Ave.
>> 914-524-3118 (Ph) Tarrytown, NY 10591
>> 914-524-3322 (Fax) Mail Stop P25-1-C
>> [log in to unmask]<mailto:[log in to unmask]>
>>
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