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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