Anecdote #4- Tin Whiskers on Shield of GPS Units

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Anecdote #4: Tin Whiskers on Shields (Enclosure) of GPS Modules
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Source:  Dr. Henning Leidecker/NASA Goddard Space Flight Center

Date:       May 2002

A recent failure analysis of two GPS receivers revealed the presence of tin whiskers on the metal shield enclosing the receiver electronics. The assemblies were to be used in high altitude applications and were expected to demonstrate successful use of commercial off the shelf (COTS) technologies. During failure analysis of the parts, the analyst noted an accumulation of metal debris (including solder melt balls) on the bottom of the GPS enclosure. Further inspection revealed a bunch of ‘twinkles.’ 

The unit consisted of four satellite receiver modules mounted to a PC board, two on one side and two on the other. Each was (roughly) 8 cm by 4 cm by 1cm, and was mounted within a metal box, presumably for RF shielding reasons. The RF shields were implemented by a press-on lid. The metal boxes were tin-plated, and each lid was covered with tin whiskers! It is possible that the side walls also contained whiskers, but the inspection focused only on the lids, which were easier to inspect. One lid was apparently more densely covered than the others. 

It is easy to miss these whiskers, which show up only as a sort of ‘twinkle’ in the right kind of lighting. Another analyst was called over to review the assembly and was told that there were tin-whiskers on the tops of these tin-plated lids --- and he could not find them at first (no discredit to him in any way!). Only when he reoriented the unit, to get just the right lighting, could he see the ‘twinkles.’ Even then, one could not "see" these as whiskers, until one put one's eye to within several inches, and got the lighting right, and knew what one was looking at. 

It would have been easy to brush the whiskers away without ever seeing them, as one was working on the units. Even removing the lid could result in the loss of many, or all, of the whiskers, depending on how the lid was handled as it was removed. 

Inspection of one lid using a binocular microscope indicated 5 whiskers (lengths ranging between 1 and 3 mm) in a 6 mm by 4 mm area. This gives a density of (5 whiskers)/(0.24 cm**2) = 20 whiskers per square cm, and allows an estimate of the total number of such whiskers per lid: (8 cm)*(4 cm)*(20 whiskers/cm**2) = 640 whiskers per shield top. 

There were many places within the receiver where connectors (apparently consisting of uncovered metal) were spaced by 0.5 mm and less. The whiskers could have easily bridged across those connectors. 

During the inspection, the "press-on" tops of the RF shields were not removed, so it was not possible to confirm whether there were whiskers on the inside surfaces of the lids next to the receiver electronics. And furthermore without removal of the lids it was not possible to determine if the electronics inside may have been protected from whisker induced shorting by (for example) a conformal coat.

The investigators were reasonably certain that the whiskers they observed were not producing the original high-altitude failures that initiated the failure analysis. However, the whiskers certainly constituted a threat to un-interrupted, glitch-free operation of those units!

Responsible NASA Officials:

   Michael Sampson/NASA GSFC Code 306
   Dr. Henning Leidecker/NASA GSFC Code 562
Additional Researchers: 

   Jong Kadesch/Orbital Sciences Corp.
   Jay Brusse/Perot Systems

Last Updated:

June 9, 2008

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