At the most fundamental physics level, a capacitor is made of exactly three things: two conductive electrodes (plates) separated by a non-conductive insulator called a dielectric. While the schematic symbol looks identical for every type, the physical materials used for those plates and that dielectric dictate everything about how the component behaves in your circuit. The dielectric material determines the capacitance density, voltage rating, temperature stability, equivalent series resistance (ESR), and even whether the part will fail silently or catch fire.
If you are designing a high-frequency RF filter, a 12V DC motor snubber, or a precision analog-to-digital converter (ADC) reference, the answer to 'what are capacitors made of' changes entirely. Below is a bench-level breakdown of capacitor anatomy, a dielectric comparison matrix, and the practical rules for reading codes, diagnosing failures, and substituting parts when your exact BOM item is out of stock.
The Core Anatomy: Plates, Dielectrics, and Packaging
The capacitance value (measured in Farads) is governed by the physical geometry and the dielectric constant ($k$) of the insulating material. The formula is $C = (k \cdot \epsilon_0 \cdot A) / d$, where $A$ is the overlapping plate area and $d$ is the distance between them. To get high capacitance in a tiny package, manufacturers must maximize $A$ (by rolling or stacking), minimize $d$ (by making the dielectric microscopically thin), and use materials with a high $k$.
- Multilayer Ceramic Capacitors (MLCCs): Made of alternating layers of metallic paste (electrodes) and ceramic powder (dielectric), pressed and fired into a monolithic block. The ceramic is typically Barium Titanate doped with other oxides to achieve specific temperature coefficients.
- Aluminum Electrolytics: Made of two strips of etched aluminum foil separated by a paper spacer soaked in a liquid or polymer electrolyte. The actual dielectric is a microscopically thin layer of aluminum oxide ($Al_2O_3$) grown electrochemically on the anode foil. The etching increases the surface area ($A$) massively.
- Tantalum Capacitors: Made from a pellet of sintered tantalum metal powder (the anode), anodized to form a tantalum pentoxide ($Ta_2O_5$) dielectric layer, and coated with manganese dioxide or a conductive polymer (the cathode).
- Film Capacitors: Made of ultra-thin plastic films (polypropylene, polyester, or PPS) vacuum-metallized with zinc or aluminum, then wound into a cylinder and potted in epoxy.
Capacitor Dielectric Comparison Matrix
Selecting the right capacitor is rarely just about matching the microfarad ($\mu F$) value. The dielectric material defines the part's real-world limitations. Use this matrix to match the material to your specific circuit requirements.
| Type / Dielectric Material | Construction Method | Typical Tolerance | Tempco / Temp Stability | ESR Range | Best Application |
|---|---|---|---|---|---|
| MLCC (C0G / NP0) Class I Ceramic |
Stacked multilayer block | ±2% to ±5% | 0 ±30 ppm/°C (Ultra-stable) | < 5 mΩ | RF tuning, PLL loop filters, precision timing, Class-D audio. |
| MLCC (X7R / X5R) Class II Ceramic |
Stacked multilayer block | ±10% to ±20% | ±15% over -55°C to +125°C (X7R) | < 10 mΩ | General decoupling, bypass, I2C pull-up filtering, non-critical coupling. |
| Aluminum Electrolytic Aluminum Oxide |
Wound foil + liquid/polymer electrolyte | -10% to +50% | High drift, capacitance drops at -20°C | 20 mΩ to 2 Ω | Bulk energy storage, SMPS input filtering, audio coupling. |
| Tantalum (MnO2) Tantalum Pentoxide |
Sintered powder pellet | ±10% to ±20% | Moderate drift, better low-temp than Al | 50 mΩ to 500 mΩ | Space-constrained low-frequency filtering, medical/aerospace (high reliability). |
| Film (Polypropylene) Metallized PP |
Wound metallized film | ±1% to ±5% | -200 ppm/°C (Very stable) | < 5 mΩ | High-voltage snubbers, AC line filtering (X/Y safety), audio crossovers. |
For a deeper look into how ceramic formulations affect performance, review the KEMET MLCC technical guide, which details the doping processes used to create C0G versus X7R dielectrics.
Decoding the Markings: What the Codes Actually Mean
Capacitor markings are notoriously inconsistent across form factors. Here is how to read the physical part on your bench.
The 3-Digit Ceramic and Film Code
Most through-hole ceramics and film capacitors use a three-digit code representing the value in picofarads (pF). The first two digits are the significant figures, and the third digit is the multiplier (number of zeros).
- Example: '104' = 10 followed by 4 zeros = 100,000 pF = 100 nF = 0.1 $\mu F$.
- Example: '223' = 22 followed by 3 zeros = 22,000 pF = 22 nF.
A trailing letter indicates the tolerance. J = ±5%, K = ±10%, M = ±20%. Therefore, a capacitor marked 104K is a 100 nF part with a ±10% tolerance.
Polarity Stripes: The Bench Trap
Polarized capacitors must be installed in the correct orientation, but the marking conventions are opposites depending on the chemistry:
- Aluminum Electrolytic (Through-hole & SMD): The painted stripe with minus signs indicates the NEGATIVE (Cathode) lead. The long lead on a fresh through-hole part is positive.
- Tantalum (SMD & Through-hole): The painted stripe or bar indicates the POSITIVE (Anode) lead.
Failure Modes and Visual Diagnostics
Capacitors do not all fail the same way. Understanding the visual and electrical symptoms of dielectric breakdown saves hours of troubleshooting. For comprehensive failure analysis data, the Electronics Notes capacitor tutorial provides excellent baseline chemistry behaviors.
Aluminum Electrolytic: Electrolyte Boil-Out
The Cause: High ripple current, excessive ambient heat, or reverse voltage causes the internal liquid electrolyte to vaporize, building pressure inside the aluminum can.
Visual Symptoms: The cross-shaped vent on the top of the can bulges outward or pops open. You will often find a brown, crystalline crust (dried electrolyte) on the PCB directly beneath the component.
Electrical Symptom: Capacitance drops significantly, and Equivalent Series Resistance (ESR) spikes. A standard multimeter capacitance test might still show a 'good' value, but an ESR meter will reveal the part is dead. Always test electrolytics in-circuit with an ESR meter.
MLCC (Ceramic): Flex Cracking
The Cause: Ceramic is brittle. If the PCB bends during depanelization, connector insertion, or mechanical mounting, the board flex transfers stress to the rigid MLCC.
Visual Symptoms: Often invisible to the naked eye. Under a 10x loupe, you will see a hairline fracture at a 45-degree angle starting from the inner edge of the solder fillet and propagating up into the ceramic body.
Electrical Symptom: Usually manifests as a dead short or a severe drop in insulation resistance, causing the power rail to sag or a battery to drain rapidly.
Film Capacitors: Dielectric Absorption and Corona
The Cause: In high-voltage AC applications (like motor run caps), microscopic voids in the film can ionize (corona discharge), slowly eating away the metallization.
Visual Symptoms: The outer epoxy casing may appear melted, discolored, or split. In AC motor applications, the motor will hum loudly and fail to start due to lost phase-shift capacitance.
Electrical Symptom: Gradual loss of capacitance. Unlike electrolytics, film caps rarely fail short; they fail open or drop in value.
The Substitution Playbook: Swapping Parts Safely
When your exact BOM part is on a 40-week backorder, you need to substitute safely. Follow these four rules to avoid redesigning the board or introducing latent field failures.
1. Voltage Rating: Go Up, Never Down
You can always substitute a 50V capacitor for a 25V capacitor, provided it fits the physical footprint. However, be aware that in some MLCC Class II dielectrics (like X7R), a higher voltage rating in the same physical case size might mean a thicker dielectric layer, which can actually slightly reduce the effective capacitance under DC bias. Always check the manufacturer's DC bias curve.
2. Temperature Coefficient: Match the Circuit Function
Never substitute an X7R (Class II) for a C0G (Class I) in a precision timing circuit, an active filter, or an RF oscillator. X7R capacitors exhibit a severe voltage coefficient (they lose up to 50% of their capacitance when DC voltage is applied) and are piezoelectric (they generate noise when vibrated and act as microphones). C0G does not suffer from these effects. If the circuit relies on exact frequency stability, C0G is mandatory.
3. ESR Requirements in Switching Regulators
If you are repairing or building a high-frequency buck/boost converter (e.g., 500 kHz to 2 MHz), the output capacitor must have ultra-low ESR to handle the high RMS ripple current without overheating. Do not substitute a standard aluminum electrolytic (like a generic 105°C Nichicon) where the datasheet specifies a Polymer Aluminum or Low-ESR ceramic. The standard electrolytic will overheat internally and vent within weeks. For deep SMPS design criteria, refer to the Analog Devices guide on SMPS capacitor selection.
4. Tantalum to Ceramic Swaps
Many legacy designs use Tantalum capacitors for bulk decoupling simply because they offered the highest volumetric efficiency in the 1990s and 2000s. Today, you can almost always substitute a Tantalum with a parallel array of high-capacitance X5R/X7R MLCCs (e.g., three 22 $\mu F$ 0805 ceramics in parallel to replace one 68 $\mu F$ Tantalum). This eliminates the fire risk of tantalum, drops the ESR to near zero, and improves high-frequency bypassing. Just ensure the total MLCC capacitance doesn't exceed the regulator's stability limits, as ultra-low ESR can cause control loop oscillation in older linear regulators.
