Whiskers on Components, Worth a Read..

[LaserSteve]

Saw this over on S.E.D. worth twenty minutes of your time:

http://nepp.nasa.gov/whisker/reference/tech_papers/2012-Panashchenko-IPC-Art-of-Metal-Whisker-Appreciation.pdf

Steve

[klunkerbus]

Some of my own personal tin whisker evidence.   In purging some old stock a few years ago, I found more than one part from the 1970's with tin whiskers.  These chips had probably sat undisturbed for 10 years or more when I found them. 

EDIT: Going back through my notes, this was a T.I. 74161 chip with a date code of 1976.  Unfortunately, most of the tin whiskers broke away in handling the part for the photo.   

[chris_leyson]

Thanks LaserSteve, only ever seen one tin whisker. Was given a hermetically sealed relay that had failed, filed the top off and wow what's that, a whisker 2mm or 3mm long and maybe 50um wide growing out of a solder joint and connecting to one of the relay contacts, get the lighting right and you could see it with the naked eye. Should have taken a picture and more important I should have recorded who manufactured the relay.

[jitter]

I found this statement in that document quite remarkable:

Whiskers are making a mockery of 60 years of research.

Page 22.

[ZeTeX]

Whiskers? You mean whiskey?

[saturation]

Thanks for the good link.  Conformal coatings and potting save the day, its sine qua non to avoid whiskers.  Hot melt glue could provide the same benefit and its far better than nothing.

[Alex Eisenhut]

Just curious; how would a coating prevent something physical from happening inside the material that causes the whiskers to grow? Or is the coating there to deflect the whiskers?

On another note, there is also silver migration, in old Tek scopes they used ceramic strips with silver terminals, the high voltages drove silver whiskers across the terminals and ZAP.

http://www.reprise.com/ash/clients2/parts_shop/ceramic_strips.html

[saturation]

Yes, deflect.

There are many hypotheses on how whiskers grow, but a coating keeps the whisker confined underneath the coating and prevents it from touching another metal contact thus eliminating electrical shorts.  The material of the coatings can matter as some are 'poor' in that its doesn't provide enough thickness or tough enough that it wears away over time and allow the whisker to grow through it.



In the end, given the volume of metal in the whisker versus where it grows out off, its not enough to cause deterioration of the metallic contact in many decades, if not longer.  Nearly all the mischief is caused by the 'beard' of metal that touches other parts of the circuit and causing electrically anomalies.
 

Just curious; how would a coating prevent something physical from happening inside the material that causes the whiskers to grow? Or is the coating there to deflect the whiskers?

On another note, there is also silver migration, in old Tek scopes they used ceramic strips with silver terminals, the high voltages drove silver whiskers across the terminals and ZAP.

http://www.reprise.com/ash/clients2/parts_shop/ceramic_strips.html

[batteksystem]

Whiskers? You mean whiskey?

Whiskers are inner-glow as a result of electro-migration, have nothing to do with whiskey. However, if you drink too much whiskey, you may think that they are related.

[FlyingHacker]

My cat has whiskers.

[FlyingHacker]

Seriously, though. This is a real issue. I have found whiskers in old potentiometers in old radio. Pain in the butt, they are.

[timb]

Seriously, though. This is a real issue. I have found whiskers in old potentiometers in old radio. Pain in the butt, they are.

There's a simple solution: Stop shaving around your electronics!

Just the other day, I noticed some whiskers on a board I had recently cleaned and couldn't figure out how they got there. I had left it sitting next to the bathroom sink overnight to dry after a scrub with 90% Isopropyl Alcohol; I guess the next morning (while shaving) some timb whiskers must have fallen on the board!

[b_force]

Fascinating. 

[TerraHertz]

Another good paper: http://nepp.nasa.gov/whisker/reference/tech_papers/2011-NASA-GSFC-whisker-failure-app-sensor.pdf
That's from 2011, and uses quite a few pictures the same as the other paper from 2012.

[saturation]

Yes, that's the controversial cause of the unintended acceleration in Toyotas report.  FYI:

https://en.wikipedia.org/wiki/Whisker_(metallurgy)#Toyota_accelerator_position_sensors_false_positive

Another good paper: http://nepp.nasa.gov/whisker/reference/tech_papers/2011-NASA-GSFC-whisker-failure-app-sensor.pdf
That's from 2011, and uses quite a few pictures the same as the other paper from 2012.

[CatalinaWOW]

Read the whole paper.  Conformal coating does not always contain whiskers.  The thicker it is the less likely penetration is (and the more maintenance/repair trouble it causes).  It does reduce the likelihood of shorts because it does almost always prevent a single whisker from penetrating from the outside, and it helps keep detached whiskers from shorting adjacent conductors.

Shorts remain possible from two causes in conformal coated boards.  Two whiskers can contact each other after penetrating the coatings from within.  A detached whisker can contact two penetrated whiskers.

In the absence of a meaningful theory, or a repeatable series of of test data reflecting actual durations and environments no one has any idea how likely any of this is.  Is conformal coating better than nothing.  Absolutely.  How much better?     Is a conformal coated lead free board as reliable as a non-coated PbSn board?   

[saturation]

Until its cause is precisely known, remediation is a matter of risk reduction.  It may not always work, but it probably works most times. 
http://nepp.nasa.gov/whisker/background/index.htm#q6

That said we can still design for the mission critical that take failures from various possibilities into account.  In space, Voyager and Hubble are or were operational continuously for 20+ years without electronic servicing.

Many T&M equipment from the late 1980s onward from top makers like H&P or Fluke, built without such failure modes, are still operational to its factory specification, without repair, suggests mitigation works to a major degree.

  Is conformal coating better than nothing.  Absolutely.  How much better?     Is a conformal coated lead free board as reliable as a non-coated PbSn board?   

[rsjsouza]

Despite I have seen this presentation before, that was a cool refresh down this whisker lane. Nothing like physics to give our status quo a kick in the pants...

Over the years I always heard and experienced radios losing sensitivity over time - usually blamed on the vacuum tubes getting "weakened" by the loss of filament material or deposition of oxides and other foreign elements. The "solid state" technology was presented as the ultimate solution for these (and many more) issues - and here we are, many years later, fearing "weakened/destroyed" equipment once again...

Despite the durability/reliability of the passive components was much smaller, I can't help but wonder if whiskers had their share in weakening old radios and we just didn't know about...