Author Topic: Whiskers - have you seen them?  (Read 4582 times)

0 Members and 1 Guest are viewing this topic.

Offline jahonen

  • Super Contributor
  • ***
  • Posts: 1046
  • Country: fi
Whiskers - have you seen them?
« on: July 03, 2010, 02:33:01 pm »
This might me pretty old stuff for someone but I think it is anyway quite interesting phenomenon worth of mentioning of very pure metals, like tin or zinc. When I first read about whiskers, I out of interest grabbed some random D-connectors for closer inspection under the microscope at work, and there it was - very thin filaments of tin sticking out from the tin plating. Not one, but several ones. No question about it.

NASA has quite nice site about this, with large collection of pictures.

Problematic aspect about this is that whiskers make short circuits when the whisker grows long enough. This growth may take some time, like years. Although thin, they can still withstand enough current to cause problems (see the relays destroyed by whisker initiated arc flash!). NASA site says that even nuclear reactor was shut down due to whisker short circuit, kinda scary. I believe RoHS has make situation worse since lead was banned. Lead essentially prevents the whisker growth.

Regards,
Janne
 

Offline saturation

  • Super Contributor
  • ***
  • Posts: 4788
  • Country: us
  • Doveryai, no proveryai
    • NIST
Re: Whiskers - have you seen them?
« Reply #1 on: July 03, 2010, 03:06:43 pm »
Yes, have heard of it but have not seen it.  Makes for an interesting justification for prophylactically replacing a device in service after some years to avoid failure during service.



http://en.wikipedia.org/wiki/Whisker_%28metallurgy%29
Best Wishes,

 Saturation
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 14060
  • Country: gb
  • 0999
Re: Whiskers - have you seen them?
« Reply #2 on: July 03, 2010, 03:54:12 pm »
Applications such as military, aerospace, nuclear and some medical devices are exempt from RoHS so it's not a problem in safety critical applications.
 

Offline tecman

  • Frequent Contributor
  • **
  • Posts: 434
  • Country: us
Re: Whiskers - have you seen them?
« Reply #3 on: July 03, 2010, 04:01:08 pm »
Recently had a board with whisker shorts.  Very fine, had to be seen under a scope.

Went to the PCB manufacturer for test and eval.

Paul
 

Offline djsb

  • Frequent Contributor
  • **
  • Posts: 589
  • Country: gb
Re: Whiskers - have you seen them?
« Reply #4 on: July 03, 2010, 04:02:06 pm »
I was hoping to repair an old FM stereo generator at work that had been stored away since the 1960's. Had to abandon the idea after I found out it used germanium transistors and that internal whiskers where a common form of fault on such old equipment. Lost interest after that.

David.
« Last Edit: July 03, 2010, 04:09:15 pm by djsb »
David
Hertfordshire,UK
 University Electronics Technician, London PIC,CCS C,Arduino,Kicad, Altium Designer,LPKF S103,S62 Operator, Electronics instructor.  http://debuggingrules.com/ Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.
 

Offline jahonen

  • Super Contributor
  • ***
  • Posts: 1046
  • Country: fi
Re: Whiskers - have you seen them?
« Reply #5 on: July 03, 2010, 05:31:31 pm »
Applications such as military, aerospace, nuclear and some medical devices are exempt from RoHS so it's not a problem in safety critical applications.

I think that there are plenty of safety critical industrial stuff which must comply to the RoHS. And if you looked the NASA site, there are a numerous whisker-related failures in satellites and nuclear power plants, and those were even before the whole RoHS was even invented.

Another problem is that there might not even be whisker-free plated version of a component available, which makes it even more difficult even if you don't need to care about RoHS.

Regards,
Janne
 

Offline saturation

  • Super Contributor
  • ***
  • Posts: 4788
  • Country: us
  • Doveryai, no proveryai
    • NIST
Re: Whiskers - have you seen them?
« Reply #6 on: July 03, 2010, 05:34:57 pm »
Are whiskers relatively easy to remove?  Are they fine enough to be brushed off?
Best Wishes,

 Saturation
 

Offline NiHaoMike

  • Super Contributor
  • ***
  • Posts: 6097
  • Country: us
  • "Don't turn it on - Take it apart!"
    • Facebook Page
Re: Whiskers - have you seen them?
« Reply #7 on: July 03, 2010, 08:18:58 pm »
The bad part about RoHS, apart from forcing lead-free too early, is failing to regulate what really matters. For example, they should phase out NiCd batteries in favor of superior NiMH batteries. And at some point in the future, phase out small lead acid batteries in favor of LiFePO4 batteries.

Then again, engineers don't understand politicians and politicians don't understand engineers.
Cryptocurrency has taught me to love math and at the same time be baffled by it.

Cryptocurrency lesson 0: Altcoins and Bitcoin are not the same thing.
 

Offline tecman

  • Frequent Contributor
  • **
  • Posts: 434
  • Country: us
Re: Whiskers - have you seen them?
« Reply #8 on: July 04, 2010, 06:07:44 pm »
Are whiskers relatively easy to remove?  Are they fine enough to be brushed off?

Whiskers are actually dendrites.  They are generally attached on one end, but a firm brushing will remove them. 

If they have started to form, brushing will only delay the return.

paul
 

Offline jahonen

  • Super Contributor
  • ***
  • Posts: 1046
  • Country: fi
Re: Whiskers - have you seen them?
« Reply #9 on: July 04, 2010, 06:20:34 pm »
Are whiskers relatively easy to remove?  Are they fine enough to be brushed off?

Whiskers are actually dendrites.  They are generally attached on one end, but a firm brushing will remove them.  

If they have started to form, brushing will only delay the return.

paul

They are not dendrites. Whiskers grow from the base, whereas dendrites grow from end.

And yes, you can brush them off. They are very fine and can be easily removed mechanically. I think that there must have been dozen failures of TV-tuner boxes when tin-plated casings have developed whiskers inside and those naturally short out internal components. Fault was mysteriously fixed by opening the tuner box, which caused the whiskers to detach. Actual fault remained a mystery for service technician. I'm currently on vacation but when I get back to work, I'll try to take some whisker photos of the D-connectors I mentioned.

A quote from NASA Whisker site FAQ:

Quote
People sometimes confuse the term "whiskers" with a more familiar phenomenon known as "dendrites" commonly formed by electrochemical migration processes.  Therefore, it is important to note here that whiskers and dendrites are two very different phenomena. A "Whisker" generally has the shape of a very thin, single filament or hair-like protrusion that emerges outward (z-axis) from a surface.  "Dendrites", on the other hand, form in fern-like or snowflake-like patterns growing along a surface (x-y plane) rather than outward from it.  The growth mechanism for dendrites is well-understood and requires some type of moisture capable of dissolving the metal (e.g., tin) into a solution of metal ions which are then redistributed by electromigration in the presence of an electromagnetic field.  While the precise mechanism for whisker formation remains unknown, it is known that whisker formation does NOT require either dissolution of the metal NOR the presence of electromagnetic field.

Regards,
Janne
« Last Edit: July 04, 2010, 06:27:44 pm by jahonen »
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf