Andrew,
A subtle fact in the article was that the service tech said the PCB worked with the 6.8uFs in the same design which I ASSUME were built under the same processes as the 15uFs... If that was the case then an assumption can be made that the 15uF caps more poorly manufactured then the 6.8uFs because they under went equivalent process and electrical stresses and failed.
Typically voltage failure curves are developed with conditioned or reflowed parts to insure the data does not contain cap mfging induced defects and is roughly indicative of the part's final application.
Paul
Paul Edwards
Process/Quality Engineering
Surface Art Engineering
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Andrew Shieh
Sent: Saturday, January 29, 2011 7:58 AM
To: [log in to unmask]
Subject: Re: [TN] Tantalum Capacitors and double sided reflow
Perhaps fortuitously, I am a bit slow on keeping up with my trade
magazines. I just read a not-so-recent "Tales from the Cube" article in
EDN magazine related to this topic.
http://www.edn.com/article/509092-What_a_cap_astrophe_.php
Andrew Shieh
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
> Sent: Friday, January 28, 2011 3:18 PM
> To: [log in to unmask]
> Subject: Re: [TN] Tantalum Capacitors and double sided reflow
>
>
> http://stevezeva.homestead.com/files/Rpt_Molded_Tant.jpg
>
> http://stevezeva.homestead.com/files/Crispy_Tantalum.jpg
>
> http://stevezeva.homestead.com/files/Holy_Sh_t.jpg
>
> Steve
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of
> Whittaker, Dewey (EHCOE)
> Sent: Friday, January 28, 2011 2:20 PM
> To: [log in to unmask]
> Subject: Re: [TN] Tantalum Capacitors and double sided reflow
>
> Tantamount to incredible.
> Dewey
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
> Sent: Friday, January 28, 2011 12:04 PM
> To: [log in to unmask]
> Subject: Re: [TN] Tantalum Capacitors and double sided reflow
>
> Hi Paul - excellent example!
>
> Dave
>
>
>
> Paul Edwards <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 01/28/2011 12:30 PM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> Paul Edwards <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> 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
> [log in to unmask]
>
>
>
>
>
> Phil Bavaro <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 01/27/2011 03:25 PM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> [log in to unmask]
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> [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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