Tech Corner

By Sven Miguel Rodriguez, corporate product manager, capacitors, & Christian Kasper, technical expert, capacitors, both from Rutronik

Solid as a rock: electrolytic for rugged applications

Vibrations are not usually associated with electronic assemblies. However, it is not at all uncommon for these assemblies to be subjected to strong mechanical vibrations. And with electrolytic capacitors, this can lead to sudden failure. Some capacitor suppliers, though, have managed to come up with solutions to this problem.

Vibrations do not only occur in applications with vibrating parts such as internal combustion engines. Electronic components are also exposed to vibrations in industrial punching machines, fast-moving linear actuators, pumps, and gearboxes. All of them generate a vibration profile that can cause an electrolytic capacitor to fail.

A common failure is broken capacitor connections. The breaking point is either directly at the solder joint to the board or inside the capacitor at the connection from the terminals to the anode and cathode foil.

Usually, a capacitor fails completely unexpectedly without significant measurable changes to its electrical properties. The reason for this is that there is little indication of whether the component is exposed to excessive vibrations and, therefore, prone to failure. At best, an increased residual current may be detected if the mechanical stress has damaged the oxidation layer between the winding and the case. If sufficient electrolyte is available, this damage is repaired by the self-healing effect. If, on the other hand, microcracks occur in the capacitor connections, a measurably increased ESR value can usually only be detected shortly before breakage.

Another failure is caused by rubbing of the winding inside the case wall, which also leads to capacitor failure in the long run.

Reliable, vibration-resistant capacitors according to requirements

In order to offer reliable components that can withstand loads of up to 30 G, some capacitor suppliers have developed series featuring a vibration-resistant design. But when is a component considered vibration-resistant?

A common, standardized vibration test profile used by suppliers for testing may look like the one in the table below.

It indicates in which frequency range and with which amplitude the capacitor withstands up to which acceleration. However, since every application has different vibration frequencies, resulting in very different profiles, suppliers also offer individual tests and approvals based on specific customer requirements. This ensures that the approved component meets the respective requirements.

If the capacitor supplier is involved in the development process, they can design the components precisely to ensure they can be used at their load limit. This prevents over-specification and allows a better design-to-cost ratio.

The electrolytic capacitor market offers, depending on the mounting method, various designs and approaches to increase vibration resistance.

Designs and approaches for the ideal result

Axially mounted THT capacitors tolerate vibrations better than radial THT or SMD modules. Since horizontal mounting means that their center of mass is significantly lower due to the reduced height, this results in less leverage forces acting on the connecting pins.

However, vertical mounting is much more common. In this case, the center of mass sits higher, which makes a vibration-resistant design more demanding. One approach is to reduce the free space in the capacitor case. To do so, the winding is wound tighter and the inner diameter of the case reduced. This leaves less room for the winding to move and reduces rubbing effects. Another option is to taper the case at points from the outside so that the winding is fixed inside.

In both cases, the wire diameter of the connecting pins must be reinforced to ensure they can withstand the occurring forces without being damaged. Greater stability is provided by additional retaining pins without electrical function, as is the case with snap-in pins. They generally make sense for large and/or heavy components. If the capacitor is subject to extremely strong vibrations, it is advisable to additionally fix it to the circuit board with an external holder, which is usually made of plastic. Alternatively, the components can also be potted or fixed with adhesive. In this case, however, it should be clarified in advance with suppliers whether the ingredients of the adhesive or compound could have a negative impact.

With potting, heat can no longer be dissipated that well due to poorer thermal conductivity, which leads to a shorter service life. In both cases, bonding and potting, material incompatibility of the adhesive and the compound could also occur in the component. Therefore, the utmost care must be taken when selecting the materials to be used.

For SMD capacitors, the approach is similar to that for THT components. This restricts the movement of the winding in the case. However, to prevent the connections from breaking off, the SMD components offer yet another option: They are equipped with a base plate with additional plastic bars attached to the sides. This fixes the capacitor case in place. Vibration-resistant models also have enlarged solder pads to mechanically strengthen the solder joint. Other external supports are not used. However, bonding and potting as with THT modules are also possible with SMD designs.

Supported by the board layout

In addition to selecting vibration-resistant components, it is also important to pay attention to vibration safety in the circuit board layout. This prevents the creation of areas with strong vibrations. The force acting on the capacitor can be minimized by shifting the resonance points. But this does not solve the vibration problem. Therefore, vibration-resistant capacitors are of key importance. Suppliers are constantly working on their further development, and we can expect to see new series in the future that are able to withstand even higher forces.