Bench Talk

Solve Problems with Discrete Capacitor Component Sourcing

(Source: Vlyaks / stock.adobe.com)

Selection and sourcing of discrete passive components is often an understated stage in circuit design, especially in printed circuit board (PCB) design. Current versions of through-hole and surface mount technology (SMT) for capacitors, resistors, and inductors have been established for so long. Furthermore, since there are well-established manufacturers of these components, they are generally an afterthought. However, passive components, especially capacitors, are now commonly the first to fail in a circuit because of this neglect. This blog serves as a guide to understanding a few common failure modes with capacitors in circuit design and suggests some avenues that may help mitigate these failures.

The First and Often Last Line of Defense

Capacitors are regularly used to decouple filters as a noise bypass for impedance matching, voltage smoothing, charging resonant circuits, and many other purposes. Hence, these passives are often used in power, high-speed digital, digital-to-analog converters (DACs), analog-to-digital converters (ADCs), RF, and precision analog circuits. Though the capacitors used in high-speed digital and precision analog circuits are often small and relatively reliable, they are generally only being subjected to relatively small voltages and currents. In contrast, capacitors used in power and RF circuits are often exposed to relatively high voltages and currents and potential transients.

Potential Capacitor Pitfalls

Given the use of more advanced processing technology, machine learning (ML) and artificial intelligence (AI), and the widespread integration of advanced electronics into virtually every application, any circuit can be placed in an environment where it is potentially exposed to surges and transients that could cause current and voltage overloads of a capacitor. This may be especially common in industrial, military, space, avionics, naval, and land mobile applications, but could impact essentially any circuit that isn't installed in a relatively benign electrical environment. The truth is, with the growing prevalence of wireless technologies and the extensive electrification of most systems, there are few benign electrical environments for most circuits.

Another common failure for capacitors is related to their particular sensitivity to overvoltage, which could be compounded by temperature variations. Wear and tear over time at the extremes of a capacitor's operating limits can degrade a capacitor and even cause early failure. The leading cause of capacitor failures is a breakdown in the dielectric, which can be degraded by the issues mentioned above, and typically, dielectric degradation is just a matter of time. Once this happens, capacitors usually fail quickly, and often explosively. The fact that capacitors are widely used to protect and filter potentially hazardous signals and voltage spikes to downstream components means that dielectric degradation can often be catastrophic for an entire circuit. Capacitors often show signs before such failure by leaking fluids, showing corrosion, expanding from internal pressure, or exhibiting excessive temperature levels.

Designing for Success

Since there is an inherent need to account for conditions beyond the ideal operation of a circuit to ensure capacitor health, it may be advisable for many applications to provide a capacitor with a significant operating margin. For instance, selecting a higher voltage rating for a capacitor than is strictly necessary may help reduce early failures. Some capacitor types are intentionally designed to be more reliable than others, and these are often governed by their application standards. It may also be worthwhile to select a critical capacitor or one that can be subjected to harsher than intended environmental conditions; for instance, placed near a heat sink or exposed to high levels of humidity. Bottom line, it is good to select a capacitor designed with standards for an application with harsher environmental testing criteria.

Capacitors are also listed with temperature coefficients and tolerances, and this may be useful to consult if there is a suspicion that a circuit may be exposed to wide temperature extremes, as temperature can significantly impact the operating capabilities of many dielectrics used in capacitor fabrication.

Some capacitors are designed to handle higher levels of current than other capacitors. This often equates to the size of the capacitor, so choosing a higher current handling capacitor may not be applicable given certain space constraints. It may be possible to use a through-hole capacitor with higher current handling and include manufacturing steps to place that capacitor bent over other surface mount passives or use other placement techniques to make use of any 3D space that may be available. As it is often desirable to place capacitors near the nodes they are designed to protect, alternative placement techniques could be advantageous in optimizing capacitor operating specifications while ensuring proximity to key nodes.

Using environmental housing or shielding for capacitors exposed to harsh environmental conditions may be helpful as well. For instance, capacitors that could potentially be close enough to heat sinks or components that may radiate a significant amount of thermal energy could benefit from shielding or additional thermal management to ensure these capacitors operate well below their electrical and thermal operating range.

Another important factor to consider is that capacitors are often relatively delicate to shock and vibration. Excessive mechanical forces can cause damage to a capacitor's solid dielectric or the fine conductive structures internal to a capacitor. SMT capacitors are naturally more resilient to shock and vibration than through-hole capacitors. However, if there may be extensive vibration or shock, providing some vibration or shock mitigation, such as shock suspension to the circuit board or even shock-absorbing mountain material to a through-hole capacitor, could help mitigate these concerns.

Conclusion

Capacitors are essential circuit elements for a good reason. They are used in almost every type of discrete circuit, from power to precision analog, RF, and digital. Sourcing and selecting capacitors to ensure they operate reliably throughout their lifetime is a growing challenge, especially considering the vast array of potential hazards standard circuits face. Taking steps to select appropriately robust capacitors and even adding fabrication steps to protect critical capacitors better can go a long way in ensuring the longevity of these fundamental passive components.