[SI-LIST] Re: Failed Tantalum capacitors - package hermeticity compromised

Ravinder / Seven,
The real problem is the vendors of all caps do not really control the ESR 
precisely.
When "we" (engineers) parallel many Low TA caps, the lowest ESR caps will fail  
especially if you don't have a high wvdc compared to the operating voltage.  
 
Typically, engineers might use many paralleled 16 WVDC caps in a 12 v plane 
(wrong !)   / Vendors will give you wonderful reliability specs, lol.    Try 
this experiment (I have) , parallel 5 or so low ESR caps and cycle them on/off 
- I believe within X "possibly 3000" cycles one will pop !  
 
remember, even with the same vendor on one circuit, Not all Caps are created 
qual !    
 
My Rule:  use a min of 3x the voltage rating for any 
e.g. for a 3.3 v plane , use a min of 10vwdc - it will also be derated over 
temperature and the ESR will also change here too.
two ways to help the reliability issue / watch or control the rise times 
(overshoot , beyond the WDC of the cap.   big ripple voltages too, kill caps .
 
Some vendors offer a "fire proof" (bad term) cap  / it melt open, rather than 
shorting
and going pop !
 
also - moisture could be another issue here , moisture (especially condensing) 
does
not help  MTBF///
 
My Rule:  use a min of 3x the voltage rating for any 
e.g. for a 3.3 v plane , use a min of 10vwdc - it will also be derated over 
temperature and the ESR will also change here too.  
 
Watch clearances around components for cleaning contaminants , I could go on 
about this one ... but any bad layout or poor cleaning could cause un-wanted 
potential problems.   check you smt contract vendor for the rules here.  
"dendrites" !  

Keith Kowal
www.marbleheadvideo.com
 
 > To: steven.salkow@xxxxxxxx> CC: si-list@xxxxxxxxxxxxx> Subject: [SI-LIST] 
 > Re: Failed Tantalum capacitors - package hermeticity compromised> From: 
 > Ravinder.Ajmani@xxxxxxxxxxxxxx> Date: Mon, 26 Jan 2009 18:11:16 -0800> > Hi 
 > Steve,> We have stopped using Tantalum capacitors for some years now. When 
 > we > used them in the past, we were using 3X voltage derating for Solid > 
 > Tantalum capacitors, and 2X derating for Low ESR Organic Tantalum > 
 > capacitors.> > Regards, > Ravinder Ajmani> Server PCB Development> Hitachi 
 > Global Storage Technologies> > > Email: Ravinder.Ajmani@xxxxxxxxxxxxxx> > > 
 > > "Salkow, Steven" <steven.salkow@xxxxxxxx> > Sent by: 
 > si-list-bounce@xxxxxxxxxxxxx> 01/26/2009 05:40 PM> > To> 
 > si-list@xxxxxxxxxxxxx> cc> EPICenter pwb-cca Email Forum 
 > <pwb-cca@xxxxxxxxxxxxx>> Subject> [SI-LIST] Failed Tantalum capacitors - 
 > package hermeticity compromised> > > > > > > Content-type: text/plain; 
 > charset=us-ascii> Content-transfer-encoding: 7BIT> > Recently we have had 
 > some tantalum capacitors rated at 16V fail on turn> of in a 12V circuit. 
 > While some percentage of low ESR capacitors> experience a high surge infant 
 > mortality on first turn on unless> serrated by 50% or more, these capacitor 
 > were sent for additional> electrical screening at an independent test 
 > house.> > Just as a precaution we decided to also sample test the lot for 
 > package> hermeticity by conducting a bubble test. 25 samples were picked at> 
 > random from the lot of 450 and 100% of these failed bubble test.> > Has 
 > anyone had this problem before? Has any had counterfeit capacitors?> > 
 > Steven Salkow> Lockheed IS&S> 3130 Zanker Rd, San Jose> Ca. 95134> (408) 
 > 473-4058> H: (925) 462-1075> steven.salkow@xxxxxxxx> salkow@xxxxxxxxxxxx> > 
 > > > > > -- HTML Attachment decoded to text by Ecartis --> -- File: 
 > header.htm> -- Desc: header.htm> > > > > 
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