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From:
George Wenger <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, George Wenger <[log in to unmask]>
Date:
Wed, 27 Sep 2017 16:32:12 -0400
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Steve,

I agree with Bev about not ever hearing anyone asking for TEM for a surface finish.  My question to your customer would  be why they are asking for TEM analysis of Immersion tin pads?  Are they having soldering problems?  If they are then XRF, x-section metallography, SEM-EDX and solderability testing should tell you everything you need to know.

I'm at the age that I don't mind speaking my mind so here goes.  In the 48+ years I was involved in failure analysis of PCB solder assemblies Immersion Tin was my least favorite surface finish because it is prone to solderability issues.  When it is freshly plated to at least 1um (40 micro-inches) it has a good solderability life for at least a year if stored appropriately.  However, if it is thinly plated it can have very poor solderability because when tin is plated on copper the "glue" layer that allows the plating to stick to the copper is the copper-tin intermetallic compound (IMC) that forms.  When there are solderability issues it usually isn't because there is tin oxide because most fluxes used in telecommunications soldering are designed to reduce tin-oxide.  Solderability issues usually occur because the copper-tin IMC has grown to the surface and it has oxidized and oxidized copper-tin IMC is not soldereable using any of the typical fluxes used in the telecommunications industry.  The tin-copper IMC grows with time and temperature and if the immersion tin plating was thin it IMC can be very close to the surface and will grow and reach the surface in not very long storage times.  Once it reaches the surface the IMC oxidizes and isn't soldereable.  You can look at the product and it will still look like an immersion tin plated surface but it won't be solderable and you won't know that until you try to solder it. 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Wednesday, September 27, 2017 3:50 PM
To: [log in to unmask]
Subject: Re: [TN] TEM Analysis

Steve,
First time I have ever heard someone asking for a TEM for a surface finish!
I know that at least in certain instances TEM can be used for determining crystal structure, but my return question is “what would be the point?”.
XRF, x-section, EDX and solderability testing should be sufficient.

IF you do want TEM done, approach the University of Toronto or CANMET in Hamilton.

Regards,
Bev

Sent from Mail for Windows 10

From: Steven Kelly
Sent: Wednesday, September 27, 2017 8:39 AM
To: [log in to unmask]
Subject: [TN] TEM Analysis

Hi all,
One of my customers has asked me to do a TEM analysis on immersion tin pads on a flex we make.

 1.  what will this tell me that XRF, x-section and EDX does not  2.  If it does tell me something – suggestions of a source to get the work done.

Thanks. Steve Kelly

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