Tin Whisker strategy

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Ramon
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Tin Whisker strategy

Postby Ramon » Wed Aug 30, 2017 12:49 pm

The concept "Tin Whisker" is generally an unknown phenomenon. Tin Whiskers is the occurence of thin "hairs" of tin growing from metallic surfaces. These whiskers are only a few µm in thickness but can lead to unexpected failures in electronic assemblies.
A few examples are:
  • Crash Galaxy IV
    The Galaxy IV was a telecommunications satellite that was disabled and lost due to short circuits caused by tin whiskers in 1998. It was initially thought that space weather contributed to the failure, but later it was discovered that a conformal coating had been mis-applied, allowing whiskers formed in the pure tin plating to find their way through a missing coating area, causing a failure of the main control computer.
  • Millstone Nuclear Power Plant
    On April 17, 2005, the Millstone Nuclear Power Plant in Connecticut was shut down due to a "false alarm" that indicated an unsafe pressure drop in the reactor's steam system when the steam pressure was actually nominal. The false alarm was caused by a tin whisker that short circuited the logic board that was responsible for monitoring the steam pressure lines in the power plant.
  • Toyota accelerator position sensors false positive
    In September 2011, three NASA investigators claimed that the tin whiskers they identified on the Accelerator Position Sensors of sampled models of Toyota Camry could contribute to the "stuck accelerator" crashes affecting certain Toyota models during 2005-2010. * Source: wikipedia

A very good example of tin whiskers is attached below.
The growth of tin whiskers is a known subject since approx 1940's and has not seized to exist. However, it does take 1 year for a tin whisker to grow 1mm in length, so with the high replacement rate by functional upgrades, tin whiskers are not a well known source for problems.
Manufacturers who build assemblies for capital equipment (infrastructure, aerospace, defense) however are quite familiar with the subject. Also, the RoHS regulation kicking into effect on July 1st,2006, even made the issue worse. Tin-lead finishes and solder joints were less prone to tin whiskering than leadfree, high-tin solder alloys. So, the problem has become worse with leadfree soldering!

Do not underestimate this issue!
Many manufacturers think these tiny hairs with slow growth rate will no be a hindrance for them. There is a very nice video of shorts caused by whiskers.

Most commonly accepted countermeasures are:
  • Plating purity
  • Conformal coating

But the issue calls for more understanding of the theory. IPC and JEDEC have setup strategies to investigate on this matter.
There are 2 documents available which are following:
  • JESD201A
    Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes
  • JP002
    Current Tin Whiskers Theory and Mitigation Practices Guideline

JESD201A will give you more information about curing/minimizing the issue at the plating level of boards and components. It is a free download through the JEDEC website (registration mandatory)
The JP002 is an informative document, handbook style, that is a reference to the industry. It can be bought through the IPC webstore
Attachments
Knipsel.JPG
Knipsel.JPG (96.46 KiB) Viewed 1971 times
Keep calm and focus on soldering

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