Hi Paul - excellent example!
Dave
Paul Edwards <[log in to unmask]>
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Re: [TN] Tantalum Capacitors and double sided reflow
Dave,
Fine job of explanation of the issues...
I would like to add something...
Like others here I have reflowed 100s of 1000s of Tants in double-sided
reflow without having known issues. However I had particular controller
design we were running with 4 Tants per in a single pass reflow. These
PCAs were 100% tested and out of a lot of ~600 PCAs 2-4 Tants would
magically explode with all the known fireworks...After a lot of F/A
including profiling, I decided to estimate the defect density based on the
manufacturers curves for failures due to voltage stress. The design was
running 75% of rated max working voltage on the cap. When I checked the
voltage on the curve, the defect density for the number Tants we actually
placed was the same as the number we were losing in test. With such a
small number of defects, it was hard decrease the tolerance band around
the defect density curve. So we used the curve to determine what working
voltage would give 0 defects in the lot sizes we were running and changed
to that Tant. Surprise no more exploding caps. The point of this example
is that the reflow process sometimes has nothing to do with device
failures, especially in the case of Tants...
Paul
Paul Edwards
Process/Quality Engineering
Surface Art Engineering
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Friday, January 28, 2011 7:08 AM
To: [log in to unmask]
Subject: Re: [TN] Tantalum Capacitors and double sided reflow
Hi Phil - the issue/concern with Ta capacitors comes from the metallurgy
and internal construction of the component. Typical capacitors are
constructed utilizing two electrodes separated by a dielectric. The
electrodes can be separated into an anode and cathode. The medium used as
a dielectric can be many types of material, potentially ranging from air
to metal. The Ta capacitors are designed with a Ta anode, a solid MnO2
cathode, and a Tantalum Pentoxide (Ta2O5) dielectric. The advantage of Ta
capacitors is their high volumetric efficiency, basic reliability, and
process compatibility compared to other capacitors of similar size. Their
highly stable capacitance and frequency characteristics and typically very
low Equivalent Series Resistance (ESR) make them attractive when used for
filtering in power supply designs. A final advantage of Ta capacitors is
the self-healing property they possess. For any random defect site within
the dielectric (Ta2O5), the current through this defect area causes
localized over-heating. The result of this localized heating depends in
large part on how rapidly it occurs. If the temperature rise due to the
defect current flow is relatively gradual (over a few tens of
microseconds, which is typical for a current limiting situation), then the
MnO2 becomes resistive and the Tantalum is said to have healed itself. If,
on the other hand, the heating is rapid (over a few microseconds, which is
typical of an unlimited power on situation with no series resistance) the
defect site itself may become very hot before the neighboring MnO2 becomes
resistive. This results in a self feeding ignition failure as the hot spot
pulls in additional Oxygen from the MnO2 layer - your basic "metal fire".
For a more complete details on this failure phenomena, Kemet has an
excellent detailed summary of this in their publication: Kemet Electronics
Corporation Tantalum Applications Notes, Surge Step Stress Testing in
Tantalum Capacitors (note - I am not an EE but I did stay at a Holiday Inn
last night...........).
Ok, so why does all of that matter? The thermal excursions of industry
standard soldering processes - reflow, vapor phase, manual - can
cause/create dielectric defects. The Ta capacitor component suppliers
screen/test the Ta capacitors to make sure there is as much thermal
excursion robustness in the components as possible but statistics isn't
their friend when there are millions of devices produced. We conducted
testing here at Rockwell Collins to measure and quantify the "typical"
impact our various soldering processes had on the Ta capacitors. We have
put into place specific rules for Ta capacitors based on those testing
results. My guess is that the comment you have heard "....can't be
reflowed twice..." is based on someone taking a conservative approach on
the issue. I do know that many product designers de-rate Ta capacitors to
achieve the same effect.
Dave Hillman
Rockwell Collins
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Phil Bavaro <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
01/27/2011 03:25 PM
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[TN] Tantalum Capacitors and double sided reflow
I recently heard from a customer who is concerned about our designs
having tantalum caps on the second side of double sided Sn63Pb37 circuit
card assemblies.
This is news to me but maybe something has happened in the world that I
missed.....wouldn't be the first time.
In a nutshell, I am hearing that tantalums cannot be reflowed
twice....yet I read in Kemet literature that the reliability specs are
based on three time reflows for resistance to soldering heat.
And I have reflowed tantalums at lead free temperatures as well without
seeing failures.
Any help would be appreciated.
Phil
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