I am very interested in why you recommended against the use of Z5U or Y5V caps. Thanks, Mary Wilson EMC Design Engineer Astral Point Communications (978) 367-6376 -----Original Message----- From: HaroldLSJ@xxxxxxx [mailto:HaroldLSJ@xxxxxxx] Sent: Wednesday, June 20, 2001 1:33 AM To: si-list@xxxxxxxxxxxxx; ajmani@xxxxxxxxxx Subject: [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. ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list Old list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list Old list archives are viewable at: http://www.qsl.net/wb6tpu