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  • Author Author: baldengineer
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What is the difference between PME and BME Ceramics?

baldengineer

Sometimes when looking on a datasheet for Ceramic Capacitors you might see references to the terms"PME" and "BME"?  What does these mean and why should you pay attention?  These acronyms are describing the metals used for the electrode plates. 

 

Definitions

BME stands for "Base Metal Electrode" while PME stands for "Precious Metal Electrode."  This means PME typically uses metals like Silver-Palladium (AgPd).  BME, on the other hand, will use less expensive metals like Nickel or Copper.  Nickel is the most common while Copper may be used in specialized capacitors like KEMET's CBR Series which are RF Ceramic Capacitors.  Copper has similar conductivity to Silver-Palladium without the extra cost.

 

RoHS

Cost isn't the only reason to give consideration to PME or BME.  Another reason refers to RoHS Compliance.  In order to manufacture a PME ceramic with Silver-Palladium, the dielectric material will have Lead(Pb) added to it.  BME ceramics, on the other hand, do not.  All PME capacitors, that I know of, are inherently not RoHS compliant.  While all BME, at least from KEMET, are RoHS compliant.

 

For example, KEMET's line of Commercial C0G, X7R, X5R, Y5V, and Z5U ceramic capacitors are all BME and RoHS compliant.

  • in reply to DAB

    BME is more robust in stress tests than PME products.This is proofed in hundreds of stress tests, where KEMET applies overvoltage and high temperature for HALT = High Accelerated Life Test.

     

    As the industry require more and more capacitance, the manufacturer increases the number of layers. During the soldering process the CTE mismatch between the PdAG and Barium Titanate leads to a so called "thermal crack", means the horizontal electrode disconnect from the dielectric material . The Nickel electrode from the BME system has a closer CTE to BaTiO3 and in addition create a chemical oxide connection with increase the adhesive force of the electrode to the ceramic material.

  • in reply to DAB

    No, for intents and purposes the K of BME and PME is nearly the same. 

     

    Since so much focus has been on making BME layers thinner and more robust, you'll find that BME parts offer greater CV than PME.

     

    It's not a technological reason or limit.  PME could probably be just as CV "dense" as BME, however, due to RoHS and cost, most manufactures have put little effort on PME development in the recent past.  It's really only used today in military and niche applications.

  • I am curious about the parameters that change between the BME and PME types.

    Do the PME types allow greater charge density or just higher charge stability?

     

    DAB