First reported within the 1940s, Metal Whiskers (MW) are spider-web-skinny conductive filaments that sprout (most normally) from tin, zinc, and cadmium steel surfaces. A “tin whisker” may presumably effectively be manually fashioned with melancholy soldering systems, nonetheless steel whiskers no doubt grow on their have, too. They’re a phenomenon that many engineers have to not responsive to. Silver, gold, lead, indium, and other metals have also grown whiskers. Hair-love steel structures erupt out from one or several grains of a steel surface. The whiskering transport mechanism is attributable to “grain boundary diffusion” (the clarification of which would require a foray into supplies science and aren’t attempted right here).[i]
Presumably the phenomenon is continuously known as Tin Whiskers (TWs) because tin appears to be like to be to be extra ample and extra at risk of grow whiskers. Whatever the reason, steel whiskers grow over time, providing a transient circuit direction between steel-plated surfaces. Within the 1950s and ‘60s, Bell Labs experimented with components throughout the periodic table to choose if adding another aspect to tin would dampen whisker snort. They found that adding 0.5 – 1% by weight of lead works, even supposing other components perceived to amplify whiskering.
Jack Ganssle, in Embedded Muse 350, components out that, “Older solders had been a combination of lead and tin, and the lead appears to be like to be to enormously mitigate (even supposing doesn’t exclusively attach away with) whiskers. Make a choice the Pb, as we’ve accomplished to video display RoHS, and whiskers can grow. In type parts are so small, and SMT components have this kind of small lead pitch, that TW shorts are even extra seemingly.” Metal whiskers have been found on relays, connectors, ceramic chip capacitors, bus bars/rails, RF enclosures, circuit breakers, potentiometers, mounting hardware (e.g., nuts), and even the axial leads of a diode.i
Being so skinny, shouldn’t steel whiskers most attention-grabbing dissipate?
You may presumably deliver that steel vapor arcs may presumably be the most widespread result. No longer moderately; steel whiskers can elevate tens of milliamps before melting. NASA missions have failed and reactor vegetation shut down attributable to tin whiskers. Heart pacemakers have been recalled attributable to tin whiskers. Tin whiskers can also simply not continuously fast a circuit nonetheless can motive on-going, complex-to-decide disorders. Whiskers invent not desire a PCB, connector, or bright fresh/voltage sources to originate, either. Data centers have had disorders with zinc whiskers under raised floor tiles in server rooms. The whiskers under the floors are benign and may presumably effectively be left in express, nonetheless love asbestos, irritating the subject fabric is the notify. Whiskers can change into airborne and procure lodged in nearby equipment.
The growth assignment of whiskers begins with an unpredictable incubation length of minutes to years, followed by a length of snort as like a flash as 15 nanometer per second to 1 mm per year.[ii]
Metal whiskers can motive:
- intermittent shorts if Iwhisker> Isoften; or, rule of thumb: fresh is accessible to the whisker on the expose of better than tens of milliamps
- permanent shorts if Iwhisker soften; or, rule of thumb: accessible fresh to the whisker is lower than tens of milliamperes
- steel vapor arcs are initiated in a vacuum when the voltage is extra than dozens of volts and up-to-the-minute is better than tens of amps. In this case, arcs are kindly of withstanding hundreds of amps of up-to-the-minute before vaporizing.
What can we invent about tin whiskers?
Nobody has been in a region to attach away with whiskering, as the phenomenon will not be yet fully understood. NASA advises avoiding pure Tin or Zinc if that that that you simply may additionally have confidence. Metal whiskers are complex to behold with the naked glance. In areas the place steel whisker “infestations” are that that that you simply may additionally have confidence, such as raised floors in server rooms, folk have to defend their lungs from inhaling loose whiskers and originate inspections some distance flung from susceptible equipment. E.g., steal floor tile, express it steel-aspect up, and glance it with a laser or vivid light whereas taking a look parallel to the bottom tile surface towards a darkish, non-reflective backdrop. It doesn’t have to be a floor tile, on the opposite hand. Whiskers can originate on shielding, plated housings or chassis, or between any steel surfaces.
In step with Lyudmyla Panashchenko of NASA Goddard House Flight Heart, who has been engaged on steel whiskering problems for years, there aren’t any particular-lower solutions we can observe to attach away with whiskering. We can exclusively lower the risk of whiskers forming by avoiding the utilize of whisker-inclined surfaces such as pure tin, zinc, or cadmium. A brass (Cu-Zn alloy) substrate under tin has shown a increased propensity to whiskering, as have supplies with a serious mismatch of Coefficient of Thermal Expansion (CTE) to that of tin throughout sessions of temperature biking.
Panashchenko’s advice is to “belief nonetheless verify” the producers through the use of X-ray Fluorescence (XRF) or Vitality Dispersive X-ray Spectroscopy (EDS) for folk that are to be particular of the supplies intelligent. In any other case, conformal coatings can sluggish whisker snort or block whiskers from electrically shunting some distance-off conductors. Lastly, that you simply may additionally steal away or replace tin, zinc, or cadmium finishes if it is realistic to invent so. A hot solder dip with lead-tin (Pb-Sn) solder can also simply not continuously be an option, on the opposite hand.
The miniaturization of electronics is motive for notify with respect to whiskers. A millimeter-long whisker is enough to fast two pins; lower currents and voltages can motive permanent fast circuits if whiskers invent not soften. Metal whiskers may presumably effectively be causing inexplicable mess ups. Up to now, no one is conscious of what precisely initiates whisker snort. We invent know that the atoms of the outside steel that provide a whisker can near from an trusty distance some distance flung from the condominium of whisker snort, as there may be puny depletion of subject fabric within the vicinity of the whisker snort. We also know that whiskers grow outwards from the outside with additional subject fabric gathering at the unsuitable, not the tip. Regardless, the components that open whisker snort are unknown, even after nearly 60 years of research on the subject.
[i] L. Panashchenko, “The Art of Appreciating Metal Whiskers: A Handy Guide for Electronics Mavens“, IPC Tin Whisker Symposium, Dallas, TX, April 2012
[ii] J. Brusse, M. Sampson, “Zinc Whiskers: Hidden Reason within the wait on of Tools Failure“, IEEE IT Professional Journal, 2004 Nov/Dec, Vol. 6 No. 6, pp 43-47