[SI-LIST] Re: Voltage rating of a Ceramic capacitor

Ravinder,

I must correct the misinformation you have received concerning the voltage 
rating of a ceramic capacitor and in particular for an X7R type dielectric.

First, ALL ceramic capacitors are tested to 2.5 times rated voltage so your 
6.3V rating has been 100% tested to 15.75 volts.  Further, the typical X7R 
MLC (multilayer capacitor) will not breakdown until you reach a voltage above 
500V!  The typical breakdown voltage I see is in the 500V to 800V range!

Second, the voltage coefficient for X7R caps at ten (10V) volts is less than 
a negative 5% change in capacitance value and at five (5V) volts it is less 
than a negative 2% change in capacitance so this will not be a problem.

Third, the typical temperature coefficient for X7R for this voltage range 
will not change the capacitance value by more than a negative fifteen (15%) 
percent from -55C to +125C and is most likely to be less than -10%.

Fourth, the biggest factor from a SI standpoint is the change in capacitance 
value with frequency.  This should be qualified on a part by part basis.  The 
typical X7R will drop ten (10%) percent of its value from zero to 1MHz so get 
a good high frequency gain/phase meter and compare your tantalum and the X7R 
you plan to use then adjust accordingly.  If your frequency is extremely 
high, consider using a porcelain microwave cap.  An NPO cap has almost zero 
capacitance drop across this same frequency span.

Remember an MLC is a miniature thick film hybrid circuit so test and mount 
the cap with the plates normal (vertical) to the PCB trace.

Finally, ceramic caps like tantalums usually fail by shorting.  If you 
require high reliability, then request the lot be HALT (highly accelerated 
life testing) qualified which is usually a maximum failure rate of between 5 
to 8 caps out of 55 tested from the lot you are interested in buying.   For 
extreme high reliability (zero failures), have the caps 100% HALT sorted.  
HALT testing will always find more failures than Mil testing.  X7R will 
always have a higher reliability than NPO and don't use Z5U or Y5V.  There is 
not enough room here to discuss this further so email me direct if you need 
more info on reliability.

Harold L. Snyder, Jr.
Scientist & Consultant

Begin Included Message:
==========================

> Subj: [SI-LIST] Voltage rating of a Ceramic capacitor
> Date: 6/14/2001 5:13:41 PM Central Daylight Time
> From:    ajmani@xxxxxxxxxx (Ravinder Ajmani)
> Sender:    si-list-bounce@xxxxxxxxxxxxx
> Reply-to:    si-list@xxxxxxxxxxxxx
> To:    si-list@xxxxxxxxxxxxx
> 
> 
> 
> I have been asked to replace Tantalum bulk capacitor in a design with a
> suitable Ceramic capacitor.  The Ceramic capacitor is X5R type, which I
> believe is more stable than Y5V.  However, its voltage rating being 6.3V, I
> am not feeling very comfortable to use it in the 5V application.  Can
> anyone advise me about the minimum voltage rating I should have for a bulk
> Ceramic capacitor in a 5V application.
> 
> Regards, Ravinder
> PCB Development and Design Department
> IBM Corporation
> Email: ajmani@xxxxxxxxxx
> ***************************************************************************
> Always do right.  This will gratify some people and astonish the rest.
> .... Mark Twain
> 
> 

==========================
End Included Message.







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