TIN WHISKERS

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Introduction

Unknown by many, famous of its past. What is this phenomenon and why is it relatively unfamiliar to a large part of people in the electronic industry?

Phenomenon

Pretty obvious to see the hairs rising from the terminal surface. These hairs have a thickness of 0.006 to 10 μm (usually ± 1 μm). Because of their diameter and nature to grow in any direction, originating from the tin part of solder, they are called “tin whiskers”. First reports on tin whiskers origin from approximately 1940s, so it is not a new thing! However, back in last century there was somewhat of a mechanisme working against the growth of these whiskers. The lead in the solder largely hindered the whiskers originating from the metal surfaces. So, back in the day tin whiskers were rarely cause of any failure. However……

Incidents

Galaxy IV
This telecommunications satellite 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 whiskers formed in the pure tin plating caused 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”. The false alarm was caused by a tin whisker that short circuited the logic board responsible for monitoring the steam pressure lines.

Toyota accelerator position sensors false positive
In September 2011, three NASA investigators claimed that the tin whiskers they identified on the Accelerator Position Sensors[16] of sampled models of Toyota Camry could contribute to the “stuck accelerator” crashes affecting certain Toyota models during 2005–2010

Properties

• Length: up to 10mm (usually <1 μm)
• Diameter: 0.006 to 10 μm (usually ± 1 μm)
• Direction: Grows from the solid materials
• Speed: 0.03 to 9mm / year
• Incubation time: from a few days
• Shape: indefinite, often straight “pen” but nodules (“cauliflower”) is possible
• Electric power: growth is observed on powered and unpowered units

Effects

As these whiskers are conductive, shorts can be caused by this phenomenon. Because the hairs are very thin, one would expect little to no current being able to flow. In test, currents up to 300 Amps are being generated by shorts caused by tin whiskers. So this issue remains to be a real life fact caused problems to longevity and reliability of our electronic assemblies.

Lead Free

As the whiskers grow from the tin part of solder joints and or surface platings, it will be clear that eradicating lead from the solder joint and swapping it for tin will promote whisker growth.

Mitigating tin whiskers

To date it is still unknown what causes the growing of these hairs. So there is no method to keep the hairs from growing. The only known fact against the growth of whiskers is where temperature rises above 150˚C, growth stops. As this is not a feasible solution for electronic assemblies, the only manageable mitigation remains:

Conformal Coating

Coating will not stop the growth of tin whiskers. It will however overcome the whisker causing shorts. Knowing this, it is a given fact conformal coating is currently a very important application.