Silicon ChipTechno Talk - April 2021 SILICON CHIP
  1. Outer Front Cover
  2. Contents
  3. Subscriptions: PE Subscription
  4. Subscriptions: PicoLog Cloud
  5. Back Issues: PICOLOG
  6. Publisher's Letter
  7. Feature: The Fox Report by Barry Fox
  8. Feature: Techno Talk by Mark Nelson
  9. Feature: Net Work by Alan Winstanley
  10. Project: DIY Solder ReFLow Oven with PID Control by Phil Prosser
  11. Project: Programmable Thermal Regulator by Tim Blythman and Nicholas Vinen
  12. Project: Frequency Reference Signal Distributor by Charles Kosina
  13. Feature: KickStart by Mike Tooley
  14. Feature: Max’s Cool Beans by Max the Magnificent
  15. Feature: Max’s Cool Beans cunning coding tips and tricks by Max the Magnificent
  16. Feature: AUDIO OUT by Jake Rothman
  17. Feature: Circuit Surgery by Ian Bell
  18. Feature: Make it with Micromite by Phil Boyce
  19. PCB Order Form
  20. Advertising Index

This is only a preview of the April 2021 issue of Practical Electronics.

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Articles in this series:
  • (November 2020)
  • (November 2020)
  • Techno Talk (December 2020)
  • Techno Talk (December 2020)
  • Techno Talk (January 2021)
  • Techno Talk (January 2021)
  • Techno Talk (February 2021)
  • Techno Talk (February 2021)
  • Techno Talk (March 2021)
  • Techno Talk (March 2021)
  • Techno Talk (April 2021)
  • Techno Talk (April 2021)
  • Techno Talk (May 2021)
  • Techno Talk (May 2021)
  • Techno Talk (June 2021)
  • Techno Talk (June 2021)
  • Techno Talk (July 2021)
  • Techno Talk (July 2021)
  • Techno Talk (August 2021)
  • Techno Talk (August 2021)
  • Techno Talk (September 2021)
  • Techno Talk (September 2021)
  • Techno Talk (October 2021)
  • Techno Talk (October 2021)
  • Techno Talk (November 2021)
  • Techno Talk (November 2021)
  • Techno Talk (December 2021)
  • Techno Talk (December 2021)
  • Communing with nature (January 2022)
  • Communing with nature (January 2022)
  • Should we be worried? (February 2022)
  • Should we be worried? (February 2022)
  • How resilient is your lifeline? (March 2022)
  • How resilient is your lifeline? (March 2022)
  • Go eco, get ethical! (April 2022)
  • Go eco, get ethical! (April 2022)
  • From nano to bio (May 2022)
  • From nano to bio (May 2022)
  • Positivity follows the gloom (June 2022)
  • Positivity follows the gloom (June 2022)
  • Mixed menu (July 2022)
  • Mixed menu (July 2022)
  • Time for a total rethink? (August 2022)
  • Time for a total rethink? (August 2022)
  • What’s in a name? (September 2022)
  • What’s in a name? (September 2022)
  • Forget leaves on the line! (October 2022)
  • Forget leaves on the line! (October 2022)
  • Giant Boost for Batteries (December 2022)
  • Giant Boost for Batteries (December 2022)
  • Raudive Voices Revisited (January 2023)
  • Raudive Voices Revisited (January 2023)
  • A thousand words (February 2023)
  • A thousand words (February 2023)
  • It’s handover time (March 2023)
  • It’s handover time (March 2023)
  • AI, Robots, Horticulture and Agriculture (April 2023)
  • AI, Robots, Horticulture and Agriculture (April 2023)
  • Prophecy can be perplexing (May 2023)
  • Prophecy can be perplexing (May 2023)
  • Technology comes in different shapes and sizes (June 2023)
  • Technology comes in different shapes and sizes (June 2023)
  • AI and robots – what could possibly go wrong? (July 2023)
  • AI and robots – what could possibly go wrong? (July 2023)
  • How long until we’re all out of work? (August 2023)
  • How long until we’re all out of work? (August 2023)
  • We both have truths, are mine the same as yours? (September 2023)
  • We both have truths, are mine the same as yours? (September 2023)
  • Holy Spheres, Batman! (October 2023)
  • Holy Spheres, Batman! (October 2023)
  • Where’s my pneumatic car? (November 2023)
  • Where’s my pneumatic car? (November 2023)
  • Good grief! (December 2023)
  • Good grief! (December 2023)
  • Cheeky chiplets (January 2024)
  • Cheeky chiplets (January 2024)
  • Cheeky chiplets (February 2024)
  • Cheeky chiplets (February 2024)
  • The Wibbly-Wobbly World of Quantum (March 2024)
  • The Wibbly-Wobbly World of Quantum (March 2024)
  • Techno Talk - Wait! What? Really? (April 2024)
  • Techno Talk - Wait! What? Really? (April 2024)
  • Techno Talk - One step closer to a dystopian abyss? (May 2024)
  • Techno Talk - One step closer to a dystopian abyss? (May 2024)
  • Techno Talk - Program that! (June 2024)
  • Techno Talk - Program that! (June 2024)
  • Techno Talk (July 2024)
  • Techno Talk (July 2024)
  • Techno Talk - That makes so much sense! (August 2024)
  • Techno Talk - That makes so much sense! (August 2024)
  • Techno Talk - I don’t want to be a Norbert... (September 2024)
  • Techno Talk - I don’t want to be a Norbert... (September 2024)
  • Techno Talk - Sticking the landing (October 2024)
  • Techno Talk - Sticking the landing (October 2024)
  • Techno Talk (November 2024)
  • Techno Talk (November 2024)
  • Techno Talk (December 2024)
  • Techno Talk (December 2024)
  • Techno Talk (January 2025)
  • Techno Talk (January 2025)
  • Techno Talk (February 2025)
  • Techno Talk (February 2025)
  • Techno Talk (March 2025)
  • Techno Talk (March 2025)
  • Techno Talk (April 2025)
  • Techno Talk (April 2025)
  • Techno Talk (May 2025)
  • Techno Talk (May 2025)
  • Techno Talk (June 2025)
  • Techno Talk (June 2025)
Whiskers are back and killing your electronics! Techno Talk Mark Nelson No, this is not an April Fools’ joke, and has nothing to do with hipster beards. A ticklish problem that plagued electronics in the past has made an unwelcome return. It’s a ‘disease’ that is devious and creeps up undetected. Not every product is affected, and the remedy is not simple. The name of this pestilence is ‘tin whiskers’, and it’s one of the causes of sudden death syndrome in PCBs using SMDs. N ASA summarises these whiskers very succinctly: ‘First recognised as a problem during the 1940s by Bell Labs in the USA, tin whiskers are electrically conductive, crystalline structures of tin that can grow from surfaces where tin (especially electroplated tin) is used as a final finish. Tin whiskers have been observed to grow to lengths of several millimetres and in rare instances to lengths in excess of 10mm. Tin is only one of several metals known to be capable of growing whiskers.’ Destructive and unpredictable The deadliness of this seemingly bizarre growth is considerable. When it occurs on the pins of semiconductor devices, it can short-circuit adjacent leads. Sometimes the short is only momentary (if the resulting current flow is strong enough to melt or ‘fuse’ the whisker), but if the short is maintained, it can result in total device failure. In application environments involving high levels of current and voltage, a highly destructive ‘metal vapour arc’ can occur. Here the solid metal whiskers are vapourised into a plasma of metal ions that are more conductive than the solid whiskers themselves. The growth of these whiskers is also unpredictable, beginning after an incubation period that varies from seconds to years. However, recent research (published at www.doEEEt.com) sets out key characteristics of whisker growth: n The filaments grow most often from tin-plated copper n The most likely driving force for whisker formation is compressive stress in the tin layer. Stress on tin-finishes may also be induced mechanically, and consequently bending, scratches or nicks in the tin may favour tin whisker growth n Whiskers grow spontaneously without any applied electric field or moisture n Tin whiskers can grow either under ambient conditions, under reduced pressure or even in a vacuum n The spontaneous growth of whiskers is an irreversible process. 10 What is not disputed is that whiskers have been proven to have caused the failure of satellites and heart pacemakers, and even caused the shutdown of nuclear power stations. Public safety The whisker problem was not a major issue until a decade or so ago, mainly because the 40% lead content of solder was effective at preventing whisker eruptions from the other 60% – tin. From around 2008, however, tin whiskers became reported as problematic again, triggered by the phasing out of lead in electronic component leads, with many parts using pure tin instead of leadtin alloys. Lead is now banned by the Restriction of Hazardous Substances (RoHS) directive, and although RoHS originated in Europe, this directive now affects virtually every piece of electronics gear manufactured today or planned for the near future. Connectors, passive and active components, switches, and relays must now all be lead-free. As semiconductor maker Maxim states, lead is recognised as a neurotoxin and is known to affect brain development, with children at greater risk than adults. The ban on lead in electronic products followed on from its removal from petrol, and paint, with the public no longer breathing in lead fumes and children no longer able to lick lead-based paint surfaces. This was a well-intentioned but ill-informed move. As Maxim points out, banning lead in electronics didn’t make sense. the public do not ingest it and lead is not a contaminant of groundwater because it is not soluble in water, nor does it break down in the environment (it usually sticks to soil particles). Blame the EU? The leaded solder ban began in Europe, and according to Joe Smetana, now a principal engineer with Nokia in Finland but then a principal engineer and tin whisker expert with French telecoms equipment maker Alcatel-Lucent, ‘The EU’s decision was irresponsible and not based on sound science. We are solving a problem that isn’t one and creating a bunch of new ones.’ It’s hard to fault Smetana’s logic, but despite this, EU officials retaliated that the regulations banning lead and other hazardous substances are needed to protect people and the environment regardless (huh?). In any case, the RoHS directive contains exemptions from the lead ban for companies in certain industries, such as aerospace, defence and, ironically, medical equipment. Back with a vengeance Moving rapidly forward to present times, the use of SMD integrated circuits with their little legs packed more closely together than a millipede’s is near-universal. This means they are at high risk of tin whiskers growing directly on the leads themselves; alternatively, well-nigh invisible whiskers can become fractured and migrate elsewhere on the PCB, causing faults there. So, what can practical electronicists like us do to avoid the tin whisker plague? For commercial products, with PCBs hidden out of sight and populated with densely packed SMDs, the answer is not a lot. But if you build your own projects that employ ‘electronic millipedes’, the remedy is to protect the components immediately after soldering to mitigate the subsequent growth of whiskers. One remedy mentioned in an online forum suggests that a good scrub with a nail brush might do the trick, followed by a ‘conformal’ coat of epoxy or polyurethane varnish. ‘Might’ is the operative word, because Trizo Ltd warns that tin whiskers have managed to penetrate polymer coatings, causing an even greater risk of critical electronic system failures. Alternatively, simply don’t use leadfree solder on your projects! Stick to the good old 60/40 tin-lead formula; it’s far less prone to tin whiskers, perfectly legal for home construction and melts at a lower temperature, meaning your soldering iron’s tip will last longer. Result! (Do remember to wash your hands properly!) Practical Electronics | April | 2021