Maybe April 17th on the fiscal calendar...
In a message dated 10/17/2016 7:49:24 A.M. Eastern Daylight Time,
n4py3@earthlink.net writes:
I didn't think it was April 1.
Carl Moreschi N4PY
58 Hogwood Rd
Louisburg, NC 27549
www.n4py.com
On 10/17/2016 12:39 AM, Gary J FollettDukes HiFi wrote:
>
> The definition of electromigration is migration of conductor material in
a circuit trace due to the momentum transfer from charge carriers (usually
electrons) flowing in the conductor traces and the phenomenon bears
characteristics of diffusion. The species undergoing electro migration need
not
be ionic, or charged in any way. The electron flow is simply pushing the
conductor metal out of the way.
>
> Electromigration CAN NOT TAKE PLACE in the absence of significant
current density flow in the conductor in question.
>
> In electromigration, conductors migrate atomistically in a direction
orthogonal to a current flow, causing flow of conductor material in a path
that leads away from the conducting trace. This phenomenon can cause either a
short circuit to an adjacent trace or an open circuit in the trace if
sufficient material migrates. Much effort to reduce this effect, especially in
Aluminum metallization in IC chips, was undertaken in the late 1970’s.
Electromigration on the macro scale, the size of PC boards and associated
parts,
has never been documented.
>
>
> More likely what you saw was solder whisker formation, a recurrence of
what took place decades ago with low Lead solders and which is coming to the
foreground once again with Lead-free formulations. These whiskers can grow
due to heat, stress and other environmental stresses, mostly in purer
metals, but can also take place in certain alloys (no need for current flow)
and cause the formation of short circuiting bridges as you describe. The
small sizes of the whiskers, on the order of the wavelength of light, is what
gives rise to the whitish appearance. These types of whiskers caused
failures in satellites and in some nuclear power plant metering systems. The
term “
electromigration” is incorrectly applied to this phenomenon even in
Wikipedia. They correct their mistake later in their discussion when they
point
out that no current flow is required for whisker formation to take place.
Current flow is required for electromigration to take place, though the
current can be a
corrosion current in an appropriate corrosion cell.
>
> They do show some pretty cool images of such whiskers, many of them
several millimeters in length, more than enough to cause the problems you
experienced.
>
> If you had a small sample of the stuff you removed, I could send you
some nice scanning electron microscope images of them along with their
compositions… That would take about 15 to 30 minute of my time.
>
> 73
>
> Gary
>
>
>> On Oct 16, 2016, at 10:48 PM, A R<raf_3@msn.com> wrote:
>>
>> Just recovered from a failure of my Orion 565 that looks to be a result
of ionic solder migration.
>>
>> Orion was purchased two years ago from original owner's estate. Details
of his operation/use are unknown.
>>
>> Replaced snaphat battery, A9 caps, and intermittent VFO encoder. Orion
performed flawlessly for over a year following those measures, and was
relegated to back-up status about 3 months ago.
>>
>> When it was brought back into service, receiver worked fine, but no
transmitter output on either ANT 1 or ANT 2 outputs. Two master resets were
performed, but no joy. Covers were removed, and interior subjected to the
burnt component "smell test", and visually inspected for any obvious causes
(loose cable connectors, etc.). Discovered powdery substance on the I/O board
that surrounded the entire periphery of TXEN 1 rca jack solder pad
(perfectly circular around solder pad) intersecting the adjacent +13vdc wire
conductor solder pad, with a "track" extending to ground side (anode) solder
pad
of adjacent 5KP15A surge suppressing diode. Removed the powdery deposit
using Q-tips and 90% isopropyl, followed by judicial flushing with distilled
water.Gently and slowly dried with warm (not hot) air, and after 10 minutes
of ambient air dry time, applied power. All sytems are "go", and the Orion
lives again.
>>
>> I have never used the TXEN or TXOUT jacks, but don't know whether the
original owner did. At any rate, the condition (which I attribute to
electromigration of the solder) had to have resulted/propagated with no
externally
applied potential to the TXEN jacks, since the condition only became
"fatal" after two years of problem-free operation/life. The internally applied
potential to the TXEN pad/lead is just +3.3vdc. And, the distance from the
TXEN's pad edge to the diode's anode (ground) pad edge is more than 1/2
inch, and distance from the +13vdc pad edge is approx 3/16 inch. Since the
deposit was uniformly centered around the TXEN's pad (and, not the +13vdc pad
edge), it would seem that the donor was the TXEN pad/solder.
>>
>> Given the wide spacing between these pads/solder and the low potentials
involved in this case, the potential (no pun intended) for similar failure
conditions elsewhere (with closer pad separation) would seem quite
possible. So...I guess my counsel is to do a close visual inspection (under
magnification) of boards when unexplain/unprovoked failures occur. Or, maybe
better...periodic visual board inspections...or, even cleaning...BEFORE
electromigration causes a failure. Fortunately, in this case, the condition
didn't
result in permanent (and catastrophic) damage. An ounce of
prevention....etc.
>>
>> 73,
>> Allen--W7GIF
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