Leaded Ceramic Capacitors Still Have Legs in Today’s Surface Mount World

Corey Antoniades

Monday, September 7th, 2015

Leaded Ceramic Capacitors don't need a PCB

Mark Twain’s famous quip “Rumors of my death are greatly exaggerated” may also apply to Leaded Ceramic capacitors. Surely you might expect that leaded components would entirely give way to chip-size surface-mount alternatives, right?

Well, turns out, such devices continue to offer significant advantages over surface mount multilayer ceramic capacitors (MLCCs) and other capacitor dielectric technologies.

Specifically with respect to MLCCs leads provide advantages such as greater immunity to mechanical stress, reduced microphonic noise, and greater volumetric efficiency.

Radial and Axial Leaded MLCC Types

Internally, a leaded MLCC has the same structure as a surface-mount device. Unlike the surface-mount component, however, conductive wires are attached to both termination surfaces. These not only ensure a reliable electrical connection but also provide robust physical support to the capacitor.

Radial Leaded MLCCs

Leaded Ceramic Options

Various Lead-Forms and Spacing

Typically, radial leaded MLCCs have exposed wire leads attached to the end of a ceramic chip capacitor. You will find that capacitors with a bare non-encapsulation or conformal coating or molded encapsulation. Various case sizes and capacitance values are offered, with a variety of lead forms and spacing to suit a wide array of applications.

The lead’s parallel configuration provides a “plug-in” feature that allows for use with high-speed automated component insertion machines. Radial-style capacitors are also ideal for applications where a PCB is not desired, and point-to-point wiring is used instead.

Axial Leaded Ceramic Capacitors

Axial leaded multilayer ceramic capacitors have wire leads that protrude from opposite ends of the ceramic capacitor. We see these devices either spanning a short distance on a PCB or in point-to-point wiring with small gaps. Axial leaded capacitors offer a reduce mounting profile when positioned parallel to the board with leads bent into a radial configuration.

Flex Cracking

The Issue

The most common failure mode among surface mount MLCCs is internal cracking of the ceramic layers caused by flexing of the PCB under thermal and mechanical loads. We sometimes referred to these as “flex cracks.” They tend to originate on the inside edge of the mounted termination band and propagate through the ceramic layers. If the crack extends through opposing internal electrodes, any ionic penetration or humidity will allow current to pass leading to a short-circuit failure.

When working with surface mount chips, there are three options for reducing failure due to flex cracks.

“Fail Open” Technologies for Ceramics

  1. FT-CAP, Flexible Termination
  2. FE-CAP, Floating Electrode
  3. OM-CAP, Open-Mode

Mechanical isolation is the most effective solution to flex cracking.

Solution: Mechanical Isolation

Solve Flex Cracking with Leads

Leaded MLCCs are less susceptible to flex cracking

Leaded components benefit from inherent mechanical isolation and so are vastly less susceptible to flex cracking. The wire leads minimize the transfer of board stresses to the rigid body of the ceramic chip.

High Vibration

The shock-absorbing qualities of the leads also enhance the capacitor’s ability to resist high-g impacts and high-amplitude vibrations. As such, leaded capacitors are well suited for use in environments such as aerospace engine compartments, geophysical probes, hybrid and electric automotive motor drives, a wide variety of defense applications, and deep-well drilling.

Singing Capacitors

Electrostriction MLCC Effect

Electrostriction on Surface Mount MLCCs

The Issue

MLCCs made with Barium Titanate, the most common material, typically undergo electrostrictive movement when placed under an electric field. If the electric field is alternating from 20Hz to 20KHz, the deformation from the electric field can cause an MLCC soldered directly a board to emit sound. The capacitor acts like a mini-loudspeaker.

The inverse, piezoelectric effect, is also true. That same MLCC may also act as a microphone in the event of mechanical shock or vibration, adding electrical noise to signals in your circuit.

Stop ceramic capacitors from “Singing”

The wire lead material on leaded capacitors mechanically isolates the MLCC portion of the device from the circuit board. This isolates mechanical vibrations between the PCB and capacitor body. Being isolated significantly reduces the characteristic singing.

Additional Advantages MLCCs with Leads

Unlike surface-mount components, which must be mounted on a printed circuit board (PCB) using solder or conductive adhesive, leaded capacitors can be used in many more versatile ways.

Leaded Capacitors are compatible with both solder and conductive adhesive, and can also be crimped or welded into a circuit. When used for noise suppression in electric brush motors or power distribution, these mounting options allow the capacitors to be easily positioned. The capacitors can be mounted in the areas closest to the noise source by welding or crimping directly to motor terminals, housing assemblies or bus bars.

Visit KEMET’s Leaded Ceramic Page for more information.

Conclusion

Leaded ceramic capacitors offer the versatility engineers need to push the limits of electronic design and further enhance the performance of equipment used in the most physically demanding applications. Leaded devices, it would appear, have plenty of life in them yet.