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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