Electrolytic Capacitor Paper: Density, Thickness & Dielectric Strength Guide for Capacitor Manufacturers

We choose electrolytic capacitor paper thicknesses of 15-70 μm and densities of 0.35-1.00 g/cm³ to balance space, absorption and electrical protection. Heming grades reach breakdown voltage results up to 1,795 V, but that number alone cannot guide our choice. Equivalent series resistance (ESR) represents resistive losses within a capacitor.

Choosing only the highest breakdown value is a beginner’s mistake. We must match impedance targets to manufacturing needs to ensure reliable power performance.

Key Takeaways

• Set the winding space budget.

• Account for electrolyte access.

• Compare grade-specific test results.

• Treat replacement codes as candidates.

• Require finished-device trial approval.

A useful starting point is a written priority: smaller winding size, lower losses or stronger separation.

How Electrolytic Capacitor Paper Works Between the Foils

Inside a liquid-electrolyte aluminum capacitor, the paper keeps the metal foils apart while the electrolyte reaches the oxide-covered anode surface. We wind the anode foil, cathode foil and cellulose separator together, then impregnate the element with electrolyte. Electrolytic capacitor paper prevents direct foil contact and holds liquid; it is not the main dielectric.

Paper Capacitor vs. Electrolytic Separator

A paper capacitor uses paper as its dielectric, while an aluminum electrolytic capacitor uses an aluminum oxide dielectric. The electrolyte functions as the cathode, and the cathode foil provides electrical contact. We choose the separator to support this liquid-bearing system rather than replace the oxide layer.

A film capacitor uses a polymer-film dielectric, so its construction differs from both paper capacitors and liquid-electrolyte designs. Polypropylene offers low dielectric absorption, meaning little stored charge returns after discharge. It also offers low dissipation factor, high breakdown strength and high insulation resistance.

Electrolytic Capacitor Symbol and Polarity

A polarized electrolytic capacitor symbol identifies terminal polarity; a plus sign marks the positive terminal. We follow the device’s polarity markings when connecting the anode side to the positive circuit node. Reversed polarity can damage the oxide layer and cause failure.

Separator paper does not make a polarized device nonpolarized, even though the sheet separates both metal surfaces. We cannot infer a safe connection from the paper’s appearance or the winding direction. The terminal markings govern assembly.

Temperature affects liquid conductivity, so cold operation can raise losses within the finished device. We assess this behavior as part of the liquid-electrolyte construction, not as a paper-only trait. A schematic symbol identifies the finished device, not a separate electrical connection to its paper separator.

Choosing Thickness, Density and Breakdown Specifications

The available specification envelope is broad enough that we need to select a grade, not simply order a thickness. Heming capacitor paper spans 15-70 μm and 0.35-1.00 g/cm³, with breakdown voltage results of 360-1,795 V. Thickness tolerances range from ±1.2 to ±5.6 μm by grade, so nominal size alone cannot define winding fit.

Thickness and Density for Compact or Low-Impedance Designs

Thinner paper saves winding space, while thicker sheets provide more physical separation between foil surfaces. Density affects impregnation and impedance: lower density favors low impedance, while higher density favors breakdown resistance. We balance those traits rather than push either value to its limit.

For prototypes, 15-25 μm is a starting band for ultra-low-ESR surface-mount device (SMD) designs. The 40-70 μm band is a starting point for high-voltage canisters, not a universal design rule. MJ2 combines 0.35-0.50 g/cm³ with 40-50 μm for low-impedance, high-frequency applications.

MTX spans 0.60-0.65 g/cm³ and 30-60 μm, with a listed operating temperature range extending to −55°C. Low-temperature performance also depends on the electrolyte, whose conductivity falls as it cools. ESR includes foil, liquid, lead and connection resistance.

Breakdown Voltage Is Not the Finished Capacitor Rating

Breakdown voltage is a test result in volts; dielectric strength expresses breakdown voltage per unit thickness. Neither value sets the finished aluminum electrolytic capacitor’s voltage rating. EX spans 0.75-0.90 g/cm³ and 20-30 μm, with results of 500-800 V.

XWF75-100 spans 0.75-1.00 g/cm³ and 30-60 μm, with results of 1,000-1,795 V. These are grade-specific results, not interchangeable ratings for capacitors. ASTM D149-20 addresses breakdown voltage and dielectric strength; IEC 60243-1:2013 addresses dielectric strength of solid insulating materials.

These standards are test-method references, not proof that either named grade underwent those tests. Humidity, impurities and thickness affect results. Compare breakdown results only after matching conditioning, electrode geometry and the electrical test procedure.

NKK Cross-References, Purity and Purchasing Checks

Matching a replacement code is a screening step, not proof that the wound capacitor will behave the same way. Matching density alone cannot approve a substitute. We check ESR, leakage current, capacitance stability and endurance in the actual foil/electrolyte system.

OEMs can buy electrolytic capacitor paper from specialty separator manufacturers such as Zhejiang Heming Paper or qualified industrial distributors. Zhejiang Heming Paper is a subsidiary of Xianhe Corp., listed on the SSE as 603733. Its application formats include SMD, radial, snap-in and screw-terminal aluminum electrolytic capacitors.

Other manufacturer cross-reference families include W1/PE3-PE5, MJ2/MR5D, MJT2/MEG-MEX and XWF-XMF/EDH-EDHM. W2 is double-layer paper at 30-70 μm; WS2 targets switched-mode power supplies at 0.80-0.85 g/cm³. Proposed savings of 20-35% and lead times of 15-25 days depend on the specific quote and qualification.

• Lot-specific purity: water-soluble chloride ≤2.0 mg/kg and sulfate ions ≤15 mg/kg by ion chromatography; water-extract conductivity ≤1.1 mS/m under GB/T 7977-2007. Iron inspection: ≤5 particles measuring 0.08-0.1 mm² per 1,800 cm², none larger than 0.1 mm².

• Moisture and pH acceptance: 3.0%-7.0% moisture under GB/T 462; pH 6.0-8.0 under GB/T 1545-2008.

• Roll fit: widths 500 or 580 mm; diameters 260-320 mm; core outside diameter 96.2 mm; custom slitting tolerance ±1 mm.

• Process declarations: require carbon-free and deionized-water-washed process statements for the proposed grade.

• Finished-device qualification: define the SMD, radial, snap-in or screw-terminal construction and intended temperature conditions.

Chlorine and iron contamination can contribute to dielectric breakdown, so impurity control supports more than low resistance. We record acceptance results against the approved grade and lot number. Retain a sample from each approved lot so later process changes can be traced against the original acceptance sample.

Conclusion

We get a dependable separator choice when electrical design and production requirements share the same acceptance criteria. Choose for the complete capacitor system, not the strongest single paper specification. Treat substitution as an engineering change, not an administrative purchasing change.

Our next step is a trial-approval record that names the intended device, acceptance criteria, and responsible engineering sign-off. That record gives quality staff a clear basis for judging stability during production. It also defines who can accept a failed criterion or require another trial.

Before releasing the next trial order, have engineering and quality sign the same approval record.

FAQ

Can ordinary tissue paper replace electrolytic capacitor paper?

No, ordinary tissue paper is not an acceptable replacement for OEM production. It lacks controlled ionic cleanliness, consistent thickness, and defined defect acceptance. Softness or absorbency does not make it suitable for use inside a wound element. We need a separator grade with defined acceptance limits, not a household sheet that merely absorbs liquid.

Does thinner separator paper always increase capacitance?

No, thinner separator paper does not always increase capacitance because the aluminum oxide layer is the dielectric. A thinner sheet mainly changes winding geometry and physical separation. It may leave room for more active foil within a fixed case. We assess that possible gain against the actual element design rather than assume a direct capacitance increase.

Is dry-paper breakdown voltage the same as electrolyte sparking voltage?

No, dry-paper breakdown voltage and electrolyte sparking voltage describe different measurements in different test systems. One examines failure through a paper specimen; the other concerns the electrolyte test system. We cannot substitute one value for the other, even when both use volts. Each result belongs with its own test conditions and acceptance rule.

Can separator paper be dried until its moisture content reaches zero?

We should not set zero moisture as the default target for separator paper. Use the approved grade’s conditioning requirement and validate any drying process with the actual electrolyte. A drying step changes the material’s starting condition before impregnation. We need trial results showing that the chosen process preserves winding behavior and finished-device performance.

Does a high paper temperature rating establish capacitor service life?

No, a high paper temperature rating does not establish the service life of an assembled capacitor. Electrolyte, seals, ripple heating and operating conditions also affect life. A material rating cannot stand in for an endurance assessment of the complete device. We judge the intended duty through finished-device testing rather than assign life from the separator alone.

Can two thinner separator layers replace one thicker layer?

Two thinner separator layers cannot automatically replace one thicker layer, even when their total thickness matches. The added interface can change winding behavior and how electrolyte enters the sheets. We must validate impregnation and finished-device performance in the proposed build. Equal total thickness is a dimensional match, not proof of equal behavior.