Basic Guide To Capacitor Selection
Step 2 - Mechanical and Installation Characteristics
Once the electrical requirements are satisfied, attention shifts to mechanical and installation characteristics. This step is often treated as secondary, but in practice it can be just as limiting as voltage or capacitance. A capacitor that is electrically perfect but mechanically unsuitable is still going to be the wrong choice. Real hardware has constraints that schematics do not show, and those constraints tend to assert themselves late in the design cycle when changes are expensive.
Through-Hole Technology (THT) vs. Surface Mount (SMD)
Capacitors are available in a wide range of physical formats, broadly divided into through-hole and surface-mount devices. Through-hole capacitors remain attractive in applications where size and weight are not critical. They are generally inexpensive, mechanically robust, and easy to solder by hand or wave. Leaded aluminium electrolytics in particular offer high capacitance at low cost and tolerate abuse that would destroy smaller components. In industrial equipment, bench instruments, and low-volume builds, these advantages still matter.
Most modern electronics, however, operate under strict size and weight constraints. Consumer devices, embedded systems, and compact power modules do not have the luxury of excess board area or vertical clearance, and as such in these designs, surface-mount capacitors are not a preference but a requirement. SMD parts support automated assembly, higher component density, and predictable manufacturing outcomes. If the product is expected to scale beyond a handful of units, surface mount is the default choice.
Even within surface-mount technology, there is significant variation. Package sizes range from large case electrolytics down to ceramic components that are barely visible without magnification. Applications that require large capacitance values, typically above 10 µF, often rely on aluminium electrolytic or tantalum capacitors. These devices occupy more board space and have greater mass, which makes them more susceptible to mechanical stress. Shock, vibration, and poor board support can crack solder joints or tear pads from the PCBm and this is not a theoretical concern, but a very common failure mode in poorly supported designs.
Large vs. Small Capacitors
The larger physical size of these capacitors does bring practical benefits. They are easier to place manually, simpler to inspect, and generally cheaper per microfarad. For low- to mid-volume production, or where field servicing is expected, this can outweigh the disadvantages. Engineers often accept the mechanical compromises because the alternatives either do not exist or introduce other trade-offs.
For smaller capacitance values, multilayer ceramic capacitors tend to dominate. MLCCs are compact, inexpensive, and available in a wide range of voltage ratings and dielectric types. They are well suited for decoupling, bypassing, and signal filtering, where placement close to the load is critical. Modern pick-and-place equipment can reliably handle MLCCs down to 0201 packages, which makes them ideal for dense layouts. Their small size allows designers to distribute capacitance exactly where it is needed, rather than relying on a few large components.
Long-Term Mechanical Traits
Mechanical considerations also include assembly yield and long-term reliability. Very small packages demand tighter placement tolerances and better process control, with ceramic capacitors being vulnerable to cracking from board flex and thermal cycling if layout and panelisation are poorly managed. Choosing a package size slightly larger than the minimum can often improve robustness with minimal impact on board area.
Fundamentally, mechanical and installation characteristics are all about the practicality of real-world designs. The goal is not to chase the smallest or cheapest component, but to select a capacitor that fits the physical realities of the product, the manufacturing process, and the operating environment. Ignoring these factors does not simplify the design, it will simply defer the consequences.
