At its most basic, a capacitor consists of two conductive plates separated by a non-conductive dielectric material. But asking what is in a capacitor on a workbench requires a much more specific answer. The exact internal materials—whether it is etched aluminum foil soaked in a borate electrolyte, sintered tantalum powder, or alternating layers of barium titanate ceramic—dictate its capacitance density, equivalent series resistance (ESR), temperature coefficient, and how it will eventually fail. Understanding the physical anatomy of these components is the difference between a power supply that runs cool for a decade and one that vents crusty electrolyte across your PCB in six months.
The Physical Anatomy: What Is Actually in a Capacitor?
The construction of a capacitor determines its electrical personality. While the schematic symbol is identical for all types, the physical reality inside the casing falls into three primary architectures:
1. Multilayer Ceramic Capacitors (MLCC)
Inside a tiny surface-mount MLCC, you will not find a single pair of plates. Instead, there are dozens to hundreds of alternating layers of metal electrodes (usually nickel or copper) and ceramic dielectric material (like barium titanate). These layers are stacked, pressed, and fired into a single monolithic block. This 'lasagna' structure is what allows a 0603-sized component to achieve capacitances up to 100µF, rivaling electrolytics from a decade ago.
2. Aluminum Electrolytic Capacitors
Crack open a radial through-hole electrolytic, and you will find two long strips of aluminum foil separated by a porous paper spacer, all rolled into a tight cylinder and stuffed into an aluminum can. The anode foil is electrochemically etched to create a microscopic, sponge-like surface area, then anodized to form a thin layer of aluminum oxide (Al2O3)—the actual dielectric. The paper spacer is soaked in a liquid or gel electrolyte (often an ethylene glycol and borate mixture), which acts as the true cathode and makes electrical contact with the rough oxide surface.
3. Film Capacitors
Film capacitors contain incredibly thin sheets of plastic polymer—typically polypropylene, polyester (Mylar), or PTFE. In 'metallized' film caps, a microscopic layer of zinc or aluminum is vacuum-deposited directly onto the plastic film. Two layers of this metallized film are wound together. The plastic film is the dielectric, and the metal vapor layer is the electrode.
Dielectric Materials and Performance Trade-offs
The dielectric is the heart of the component. It determines how much energy can be stored, how the capacitance drifts with temperature, and how much dielectric absorption (memory effect) occurs. Below is a data-dense breakdown of the most common dielectrics you will encounter in modern electronics.
| Capacitor Type | Dielectric Material | Typical Tolerance | Tempco / Temp Stability | ESR Range | Primary Use Case |
|---|---|---|---|---|---|
| MLCC (Class 1) | C0G / NP0 (Titanate) | ±1% to ±5% | ±30 ppm/°C (Ultra-stable) | Very Low (<50mΩ) | RF filters, oscillator timing, precision analog |
| MLCC (Class 2) | X7R / X5R (Barium Titanate) | ±10% to ±20% | ±15% over -55°C to 125°C | Low (<100mΩ) | Decoupling, bypass, general-purpose SMD |
| Aluminum Electrolytic | Aluminum Oxide (Al2O3) | ±20% | Poor (Cap drops heavily below -20°C) | High (100mΩ to >1Ω) | Bulk power filtering, audio coupling, energy storage |
| Tantalum (Solid) | Tantalum Pentoxide (Ta2O5) | ±10% to ±20% | Moderate (±10% over -55°C to 125°C) | Moderate (50mΩ to 500mΩ) | Space-constrained bulk decoupling, medical/defense |
| Metallized Film | Polypropylene (PP) | ±1% to ±5% | Excellent (-200 ppm/°C) | Very Low (<20mΩ) | AC line filtering (X/Y safety), motor run, snubbers |
Notice the trade-off: Barium titanate (X7R) gives you massive capacitance in a tiny package, but it suffers from severe DC bias effects. A 10µF X7R capacitor rated for 16V might only provide 2µF of actual capacitance when 10V DC is applied across it. C0G/NP0 ceramics do not suffer from DC bias, but they are physically limited to lower capacitance values (usually under 100nF).
Decoding the Markings: What the Codes Mean
Reading the value printed on a physical part is a mandatory bench skill. Manufacturers use different coding schemes depending on the physical size and dielectric type.
Ceramic and Film 3-Digit Codes
Small through-hole ceramics and radial film capacitors use a three-digit EIA code measured in picofarads (pF). The first two digits are the significant figures, and the third digit is the multiplier (number of zeros).
- 104: 10 followed by 4 zeros = 100,000 pF = 100 nF = 0.1 µF.
- 473: 47 followed by 3 zeros = 47,000 pF = 47 nF.
- 221: 22 followed by 1 zero = 220 pF.
This number is usually followed by a letter indicating tolerance: J = ±5%, K = ±10%, M = ±20%. A marking of 104K is a 0.1µF capacitor with a 10% tolerance.
Electrolytic and Tantalum Markings
Because they are larger, aluminum electrolytics usually print the exact value and voltage (e.g., 470µF 25V). The critical marking here is the negative stripe. On an aluminum electrolytic, the stripe with minus signs indicates the cathode (negative) lead.
SMD tantalum capacitors flip this convention: the colored band or line on the epoxy case indicates the anode (positive) lead. Mixing up the polarity marking conventions between aluminum and tantalum is a common mistake that leads to catastrophic failure.
Failure Modes and Visual Symptoms
Capacitors do not just 'stop working'; they fail in specific, predictable ways based on their internal chemistry. Recognizing these visual symptoms saves hours of troubleshooting.
Aluminum Electrolytic: Venting and Drying Out
The liquid electrolyte inside an aluminum cap slowly evaporates through the rubber end seal over time, a process accelerated by heat (the Arrhenius equation dictates that lifespan halves for every 10°C rise in ambient temperature). As it dries, ESR increases and capacitance drops. Visual symptom: The top vent bulges outward, or the rubber seal at the bottom pushes out, leaking a crusty, brownish-black residue onto the PCB. If subjected to reverse voltage or massive overvoltage, the internal pressure builds instantly, causing the vent to rupture with a loud pop and a plume of acrid smoke.
MLCC (Ceramic): Flex Cracking
Ceramics are brittle. If a PCB is bent during depaneling, connector insertion, or drop-testing, the mechanical stress transfers to the MLCC. Visual symptom: Often invisible to the naked eye. A micro-crack forms internally, allowing moisture to ingress or creating a high-resistance short. The board may pass initial testing but fail in the field when humidity bridges the crack, resulting in a dead short that burns the component off the pad.
Solid tantalum capacitors use manganese dioxide as the cathode. If subjected to a voltage spike exceeding their rating, or if reverse-biased even slightly, the tantalum pentoxide dielectric breaks down. The resulting short circuit heats the manganese dioxide, which releases oxygen. This oxygen feeds the burning tantalum metal, resulting in a violent, self-sustaining chemical fire that cannot be blown out by standard airflow. Always derate tantalum capacitors by at least 50% (e.g., use a 20V rated part on a 10V rail) and never use them on high-surge input lines without series resistance. For modern designs, replace them with high-capacitance X5R/X7R MLCCs or polymer aluminum caps.
Film Capacitors: Self-Healing
Metallized film capacitors have a unique failure mode: they heal. If a voltage transient punctures the plastic film dielectric, the arc vaporizes the microscopic metal electrode immediately surrounding the puncture. Visual symptom: None externally. The capacitor clears its own short circuit. However, each 'healing' event removes a tiny amount of electrode area, causing a gradual, stepped drop in total capacitance over years of operation in snubber or AC-line applications.
Substitution Rules: Swapping Parts Safely
When the exact BOM part is out of stock, you must substitute safely. The golden rule is to understand which type for which job. Never swap a component based solely on the microfarad (µF) rating.
Rule 1: Voltage Ratings Can Go Up, Never Down
You can safely replace a 16V rated capacitor with a 25V or 50V part, provided it physically fits. However, you must account for dielectric derating. As mentioned, a 50V X7R ceramic might lose 50% of its capacitance at 25V DC. If you are substituting ceramics, check the manufacturer's DC bias curves. For electrolytics and film caps, voltage derating is less aggressive, but a higher voltage rating means a physically larger part, which can introduce unwanted parasitic inductance (ESL) in high-frequency switching regulators.
Rule 2: Match the ESR Profile for Regulators
Do not blindly swap a standard aluminum electrolytic for a 'Low-ESR' variant, or vice versa, in linear or switching regulator feedback loops. Older LDOs (like the classic LM1117) actually require a specific amount of ESR (typically 0.1Ω to 1Ω) on the output capacitor to maintain loop stability. If you substitute a zero-ESR MLCC or polymer cap, the regulator will oscillate, ringing wildly on the output. Conversely, in a high-frequency buck converter output filter, substituting a standard electrolytic for a Low-ESR type will result in massive output voltage ripple and overheating.
Rule 3: Capacitance Tolerance in Timing vs. Filtering
For bulk power supply filtering (smoothing rectified AC or buffering a DC rail), substituting a 470µF cap with a 680µF or 1000µF cap is generally beneficial, as it lowers ripple voltage. However, if the capacitor is part of an RC timing network, an active filter, or an oscillator (like a 555 timer circuit or a crystal load capacitor network), the exact value and tight tolerance (C0G/NP0 or 1% film) are mandatory. Swapping a ±5% film cap for a ±20% electrolytic in an audio crossover or a timing circuit will shift the cutoff frequency or oscillation rate entirely out of spec.
Rule 4: Temperature and Lifespan Equivalency
Check the temperature rating printed on the sleeve. Replacing a 105°C rated electrolytic with an 85°C rated part in a enclosed power supply will result in premature death. Furthermore, check the rated lifespan (e.g., 2,000 hours vs 10,000 hours at max temp). According to Cornell Dubilier's Aluminum Electrolytic Application Guide, selecting a part with a higher base lifespan rating at the same temperature provides an exponential increase in real-world field reliability due to the Arrhenius temperature scaling factor.
Knowing what is physically inside the casing—and how that chemistry reacts to voltage, heat, and frequency—transforms capacitor selection from a guessing game into a precise engineering decision. Always verify the dielectric class, check the DC bias curves for ceramics, and respect the polarity markings before applying power.
