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

Harold,

I'd be very interested in your reasons for recommending against Z5U.

Thanks,
Mark Alexander



HaroldLSJ@xxxxxxx wrote:

> 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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