tin concentration / wisker formation

[jedward]

Pure tin forms wiskers and is prohibited as a coating in aerospace applications.

Can x-ray fluoresence be used to determine tin concentration in thin coatings?
(thin being under about 0.0004 inch)

Are NIST traceable standards available for calibration of the equipment for this measurement?

 

[WKTaylor]

jedward...

Suggest You review SAE AIR5444 Investigation of Whisker Formation on Tin Plated Conductors. Lots of embedded references may lead to an answer... or at least provide in-depth info!

Regards, Wil Taylor

 

[IRstuff]

Is there some reason you can't ask a manufacturer?

http://www.niton.com/Libraries/Docu...ing_Thickness_Plating_Bath_Analysis.sflb.ashx

http://www.niton.com/Libraries/Document_Library/Manufacturing_Quality_Control_Brochure.sflb.ashx

TTFN

FAQ731-376

 

[jedward]

1) XRF measurements give results that are "averaged" over some thickness,about 5 to 15 microns, depending upon the x-ray energies involved. If the thickness of the pure tin is less than this averaged thickness, the results will never report any pure tin because it will include some of the elements of the substrate.

2) The thickness of pure tin required to generate wiskers is not to be found in the literature and possibly only a few microns of pure tin could be sufficient to generate the dreaded wiskers. This seem improbable but the stakes are high.

3) If there was an allowable thickness of pure tin that would not generate wiskers, some calibration method would be possible.

4) Our requirement is to provide assurance that none of the many parts (procurred from a variety of sources) can generate wiskers in our instruments; in the absence of a minimum thickness for generating wiskers, a method for measuring very thin pure tin would be nice although possible not by X-ray fluoresence.

5) If we cannot perform this test, vendors will not be able to either.