Phil- You didn't indicate where the FA lab found the lead/tin composites
(on the cap, on the board, etc...).
Anyway, it is not likely that you have an electromigration problem with
solder. Electromigration occurs as a result of an electron flux flow
(known as electron wind). This occurs naturally along grain boundaries
in all metals exposed to any energy field, not just electrical fields.
Electromigration occurs when the electrons in the metal are drawn off at
the anodic terminal faster than the cathode can replace them. Voids will
develop at triple points (where three or more grain boundaries intersect
simultaneously) and the metal "moves" toward the cathode. High frequency
studies in the gigahertz range have shown the tendency for solder
electromigration; again, the key here is to look for voiding in one area
and agglomerates in another.
What makes more sense is dendritic growth, which is a different
phenomena altogether. This occurs when the metal (most likely tin in
this case) combines with a liquidus material (usually water, but also
some organics can be at work). This material forms a hydroxide compound
which is very mobile. Its mobility is generally caused by differentials
in electric potential between where this hydroxide is and some
neighboring area. The electric potential can be as simple as 2V on a
capacitor at 500ua, and an adjacent IC running at [log in to unmask] More likely, if
the capacitor has the ability to trap organics near the solder joint,
this combined with moisture from a humidity test could be enough to
nucleate the site for dendritic growth. Then, with this latent problem,
all it takes is a little temperature elevation to start the growth
moving across the cap (if say, it was a filter cap with 5V on one side
and ground on the other).
If what you're seeing is 'feathery" it is not electromigration, but
dendritic growth.
Hope this helps...
Bill Davis,
Sr. Scientist
(408)-325-7868
[log in to unmask]
-----Original Message-----
From: Phil Bavaro [mailto:[log in to unmask]]
Sent: Tuesday, May 25, 1999 4:51 PM
To: [log in to unmask]
Subject: [TN] Electromigration of Tin Lead?
We have run a "no clean, low solids" operation for almost ten years now
without any contamination issues. Recently a shorting failure has
occurred
across a capacitor on a few field returned circuit boards.
The failure analysis report came back as electromigration of both tin
and
lead based on 1) a SEM photo showing the feathery migration tracks
similar
to silver migration and 2) a spectrum plot showing Sn and Pb peaks.
Questions for Technet:
Does lead actually electromigrate along with the tin as an alloy? I
thought the metals separate during electromigration and the tin would be
the culprit.
This occurs on a potted board but only in one location which happens to
be
a high frequency switching application. Why wouldn't it happen all over
the board uniformly?
Is 2 volts enough to cause the electromigration to occur?
Our solderpaste has antimony (0.4%) and silver (0.6%) in it as well as
63%
tin ( and the rest is lead). Shouldn't the spectrum be showing these
components as well or is anything under 1% not discernible?
Is it possible for the bare board to be the culprit as it is a HASL
board?
Any insight would be appreciated.
Phil Bavaro
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