What are the selection criteria and application guidelines for vibration resistant capacitors?

Selection criteria for vibration-resistant capacitors include factors like the type of application, operating environment, and specific vibration characteristics. Consider the frequency and amplitude of the vibrations, as well as the duration of exposure. Look for capacitors with robust construction, larger solder pads, and extra-high bases for better vibration damping.

Aluminum electrolytics

Axial-lead and soldering star aluminum electrolytic capacitors provide high vibration stability by using extra-rugged leads (∅ 1.0 mm) and strong internal attachment of the winding in the can.

Double fixation of the winding element includes being axially compressed between the can base and the cover disk, and a corrugation of the can wall. Welded connections between the winding lead tabs and the case further increase vibration resistance.

Using those construction techniques enables large axial lead and soldering star aluminum electrolytic capacitors to have high vibration stability without the need for additional clamping or gluing to the circuit board (Figure 1).

Figure 1. Vibration-resistant aluminum electrolytic capacitors can be designed for horizontal (axial lead) or vertical (soldering star) mounting. (Image: TDK)

Vibration resistance of axial-lead capacitors is specified in IEC 60068-2. The test requires a maximum acceleration of up to 20 g. Designers should guard against the possibility of resonance based on the capacitor mounting conditions and circuit board design.

Conductive polymer aluminum electrolytics

The solid polymer electrolyte in conductive polymer aluminum capacitors is a significant contributor to vibration resistance and increased capacitance stability. These features and their better temperature stability and higher ripple current ratings compared to the liquid electrolytes found in traditional electrolytic capacitors make these capacitors popular in automotive electronic control units (ECUs).

The industry is moving toward smaller vibration-resistant conductive polymer electrolytic capacitor designs with diameters of 6.3 mm for compact ECU designs. At the same time, new packaging and soldering options can increase vibration resistance from 10 g to 30 g.

The 3x increase in vibration resistance is a result of three factors, including larger vibration resistance structures on the outside of the capacitor, improved board attachment that eliminates the need for potting or adhesives, and auxiliary soldering tabs that improve the strength of the solder connections (Figure 2).

Figure 2. With the proper packaging and mounting, the vibration resistance of conductive polymer electrolytic capacitors can be tripled. (Image: Panasonic)

Capacitors can cause vibrations

Not only are many capacitors required to handle high levels of vibration, but the multi-layer ceramic capacitors (MLCCs) used in power converters can be vibration sources due to the piezoelectric effect.

In a switching power converter, the MLCCs can experience a high-frequency voltage change that causes a corresponding deformation in the capacitor due to the piezoelectric effect in the ceramic. The deformation of the MLCC creates a vibration in the circuit board that can result in amplification and audible buzzing.

There are several approaches that can minimize MLCC buzzing. For example, mounting two capacitors on opposite sides of the circuit board can result in noise cancellation. Minimizing the size of the voltage swings, when possible, can also reduce the buzzing.

Some MLCCs are available with low-distortion dielectric materials that experience smaller deformations. In addition, there are several MLCC mounting options, like interposers and metal mounting terminals, that can be used to reduce the transmission of MLCC deformations to the circuit board and control noise. Those mechanical solutions can have a larger impact (up to -25 dB) compared with the use of low-distortion dielectrics, which deliver about -7 dB noise reduction (Figure 3).

Figure 3. There are several material and mounting options for reducing the vibrations generated by MLCCs. (Image: Texas Instruments)

Summary

While aluminum and conductive polymer electrolytic capacitors can be sensitive to vibration, MLCCs can be a source of vibration. The solutions are similar in both cases. Capacitors are available that have been optimized for use in high-vibration environments found in industrial, automotive, and even some consumer applications. They feature ruggedized designs and special mounting structures. PCB integration is always an important consideration.