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Understanding the

Aluminum Electrolytic Capacitors

The rapid, ongoing growth in electronics has been aided by technological advances, leading to constant new challenges for manufacturers of these components.

Aluminum electrolytic capacitors are one such example. They have been traditionally used for filtering, timing networks, by-pass, coupling and other applications requiring a cost-effective, volumetrically efficient and highly reliable component.

View these technical notes for a more in-depth look at the fundamentals of aluminum electrolytic technology.

Guidelines for Using Aluminum Electrolytic Capacitors

Polarity

In DC applications, confirm the polarity. If the polarity is reversed, the circuit life will be shortened or the capacitor may be damaged. Generally, an intermittent reverse voltage of 1 volt DC is allowed. Capacitors used in circuits whose polarity is occasionally reversed or whose polarity is unknown requires the use of a bi-polar capacitor. Also note that an aluminum electrolytic capacitor cannot be used for AC applications.

 

Insulating Sleeving

General purpose aluminum electrolytic capacitors are covered with a sleeve made of polyvinyl chloride or similar material. In addition to the insulating properties, the sleeving is also used for marking.

 

Insulation From The Aluminum Can

The aluminum can is not insulated from the cathode, and when the internal element needs to be electrically

insulated from the can, capacitors specially designed for these insulation requirements should be used. Also, the dummy terminal is not insulated from the cathode and must not be connected electrically to the anode or cathode.

 

Operating Temperature

Choose a capacitor whose maximum specified temperature is greater than the operating temperature of the application. This will increase the life of the capacitor. However, if the temperature rating of the capacitor is less than the temperature of the application, the life of the capacitor will be severely decreased or the capacitor could fail catastrophically.

In general, for each 10 degree decrease in operating temperature the capacitor life will double and conversely it will be halved for each 10 degree increase in temperature as determined by the following life expectancy formula.

 

Where:

    LX = Lifetime at actual operating temperature TX
    LO = Lifetime at maximum rated operating temperature
    TO = Maximum rated operating temperature (°C)
    TX = Actual operating temperature (°C)

 

Ripple Current/Load Life

The life expectancy of an aluminum capacitor is not only determined by the ambient temperature, but also by the ripple current, and the ambient temperature plus the increase in temperature due to ripple current equals the operating temperature.

Do not apply a ripple current exceeding the rated maximum ripple current allowed for the capacitors as this will result in shortened capacitor life and may result in the capacitor venting or failing catastrophically.

In many cases capacitor heating due to ripple current is more severe than the ambient temperature stress, and an acceleration rate of approximately 2 for each 5-10°C temperature increase is realized. Following is the formula used to determine life expectancy.

 

Where:

    LX = Lifetime under actual ambient temperature and actual ripple current
    LO = Lifetime under maximum rated operating temperature and rated DC voltage with no ripple
    TO = Maximum rated operating temperature (°C)
    TX = Actual ambient temperature (°C)
    T = Inside temperature increase (°C) by actual ripple current
    K = Acceleration factor, varied from 5 to 10 by
    product and conditions

 

Rated Voltage

If the applied voltage exceeds the rated voltage of the capacitor, the capacitor may be damaged from an increase in leakage current. When using a capacitor with an AC voltage superimposed on a DC voltage, care must be exercised so that the peak value of the AC voltage plus the DC voltage does not exceed the rated voltage.

When capacitors are connected in series, the voltage distribution across the series may not be uniform. This is due to the normal DC leakage distribution and should be considered in the design process by using a higher rated voltage capacitor and/or using balancing resistors in parallel with each series capacitor.

 

Surge Voltage

The surge voltage rating is the maximum over-voltage including DC, peak AC and transients to which the capacitor may be subjected for short periods of time (not exceeding 30 seconds every 5 minutes). According to JIS C5141, the test shall be conducted for 1000 cycles at room temperature under test condition W of JIS C5141or at the maximum operating temperature under test condition B and C of JIS C5141. Under test, the capacitor shall have voltage applied through a current limiting resistor of 1000 ohms without discharge. After test, the electrical characteristics of the capacitor are specified in JIS C5141. Unless otherwise specified, the rated surge voltages are as follows:

 

Rated Voltage (V) 6.3 10 16 25 35 50 63 80 100 160
Rated Surge Voltage (V) 8 13 20 32 44 63 79 100 125 200

 

Rated Voltage (V) 200 250 315 350 400 450 500
Rated Surge Voltage (V) 250 300 365 400 450 500 550

 

Heavy Duty Charge/Discharge Applications

The standard aluminum electrolytic capacitor is not suitable for circuits in which there is a frequent charge and discharge cycle. If a standard capacitor is used in circuits in which the charge and discharge cycles are frequently repeated, the capacitance value may drop and the capacitor may be damaged. Please consult our engineering department for assistance in these applications.

 

Vent

The safety vent needs adequate clearance to work properly. It is advisable to leave a minimum clearance above the vent of 2mm for the can diameters of 16mm and smaller, 3mm for the can diameters of 18-35mm, and 5mm for the can diameters of 40mm and larger.

 

Adhesive & Coating Materials

When an adhesive is used on the rubber seal of the capacitor to anchor it to the printed wiring board, the adhesive must not contain any halogenated hydrocarbon nor any chemical which could damage the rubber seal or PVC sleeve.

Also, after solvent cleaning and before using an adhesive or coating material on the capacitor, evaporate the solvent residue from the rubber seal of the capacitor for at least 10 minutes at 50-85°C by forced air.

 

Mechanical Stress on Lead Wires & Terminals

If excessive force is applied to the lead wires and terminals, they may be broken or their connections with the internal element may be affected. (For strength of terminals, refer to JIS C5102, C5141 and C5142.) The distance between the terminal holes on the circuit board should be the same as the spacing between the lead wires or terminals on the capacitor.

 

1. Axial and Radial Lead Types

Improper insertion of the lead wires into circuit boards may cause electrolyte leakage, lead wire breakage or impair the lead wire connections with the internal element. When the distance between the two terminal holes on the circuit board cannot be made the same as the distance between the lead wires, formed capacitor leads are recommended.

 

2. Snap-In Type

Improper insertion of the terminals into the circuit boards may break the terminals or impair their electrical connections with the internal elements. The blank terminal of a multi-terminal capacitor should be considered to be at the same potential as the electrolyte, or cathode, and should therefore be isolated from the circuit.

 

3. Screw Terminal Type

Too much torque applied in tightening the screws into the terminal will result in stripping the threads and possibly increasing the contact resistance. On the other hand, if the screws are not tightened enough, the high contact resistance will cause localized heating at the terminals resulting in early failure.

 

Soldering

Incorrect soldering may shrink or break the sleeving of the capacitor. Please read the following information carefully before soldering.

1. If the soldering iron comes in contact with the capacitor body during wiring, damage to the polyvinyl sleeve and/or case may result in defective insulation or improper protection of the capacitor element.

2. When soldering a printed circuit board, care must be taken so that the soldering temperature is not too high and the wave or soldering time is not too long. Otherwise, there will be adverse effects on the electrical characteristics and the insulating sleeve of aluminum electrolytic capacitors. In the case of miniature aluminum electrolytic capacitors, nothing abnormal will occur if the soldering process is performed at less than 260°C for less than 10 seconds.

3. During soldering, the sleeve may melt or break if it comes in contact with the circuit board traces. To avoid this problem, do not locate circuit board traces under the capacitor body.

4. The sleeving may be melted by solder which migrates up through the terminal holes in the circuit board. To avoid this problem, the same application as stated in paragraph 3 is recommended.

5. When soldering adjacent components to the capacitor, preheated lead wires or terminals may tear the capacitor sleeve if these terminals come in contact with the capacitor sleeve. Therefore, mount the capacitors carefully so that the adjacent components' terminals or lead wires do not come in contact with the sleeve, particularly when mounting on through-hole circuit boards.

For surface mounting capacitors, the reflow soldering conditions are specified in the Surface Mount section of United Chemi-Con's H7 catalog.

 

Cleaning

Aluminum can be aggressively attacked by halide ions, particularly by chloride ions. Even small amounts of chloride ions inside the capacitor will cause corrosion which contributes to rapid capacitance drop and vent-ing. Therefore, the prevention of chloride contamination is the most important check point for quality control in production.

Solvent-proof capacitors are required when chlorinated hydrocarbons are used for cleaning. If aluminum electrolytic capacitors without the solvent-proof construction are present on the circuit board, alcohol based solvents are recommended for cleaning.

The mechanism of corrosion in aluminum electrolytic capacitors by chloride ions can be explained as follows:

Chlorinated solvents are absorbed and diffuse through the polymer seal entering the capacitor. Various chemical reactions may occur depending upon the particular solvent and electrolyte, but the final result is the release of chloride ions.

Chloride ions can penetrate through imperfections and micro-cracks in the aluminum oxide dielectric layer reaching the underlying aluminum metal. At these points, the aluminum metal is attacked by soluble chloride as shown in the following anodic half-cell reaction:

Al + 3Cl- -> AlCl3 + 3e........(8)

There is always at least 1 to 2% water in the electrolyte and this is sufficient enough to hydrolyze the AlCl3:

AlCl3 + 3H2O -> Al (OH)3 + 3H+ + 3Cl-....(9)

This reaction releases the chloride ions to further attack the aluminum. The hydrogen ion increases the local acidity which causes the oxide dielectric to dissolve. Thus, localized corrosion occurs at an accelerated rate with the attack of both the metal and the dielectric.

Recommended cleaning solvents therefore are those free of halogens. When halogenated solvents must be used, solvent-proof capacitors whose seal constructions are specially designed for this application are recommended. A terpene or petroleum base solvent swells and damages the rubber seal of the capacitor. An alkaline saponification detergent may damage the aluminum metal and marking. The cleaning solvents compatible with our products are as follows:

 

Non-Halogenated Solvent Cleaning

Solvents Higher Alcohol base:
    Pine Aplah ST-100S
    Clean Through 750H, 750K, 750L & 710M
    Technocard FRW-14 to 17
Cleaning Conditions 60°C max. within 10 minutes
Immersion (with or without ultrasonic)
Remarks
  1. The wash, rinse and drying process should be so arranged that other components and PC boards do not rub off the marking of the capacitor. Shower cleaning may affect the marking.
  2. For water rinse, control the conditions to avoid sleeve shrinkage.
  3. For alkaline solvents like Clean Through 750H, etc., do not leave residual alkaline on the capacitor after the cleaning process.

 

HCFC Solvent Cleaning

The following HCFC solvent cleaning is compatible with the solvent-proof type capacitors.

 

Solvents HCFC (AK225AES)
Cleaning Conditions 5 minutes max. (3 minutes max. for SREC and KRF; 2 minutes max. for KRE) with one immersion, ultrasonic or vapor cleaning. Immersion (with or without ultrasonic).
Applicable Series (Only solvent-proof products) Surface Mount MF, MFK, MV, MVK, MF-BP, MFK-BP, MV-BP, MVK-BP
Low Profile KRE, SRAC, KMA, SRG, KRG
Radial Lead SMEG(~250V), KMG, SME(~250V), KME(~250V), SME-BP, KME-BP, SXE, LXF, KMF(~100V), EX, LX, LXA, GXC, LLA, KRF, TXG
Axial Lead SME, KME(250V & less with diameter 18mm & smaller)
Remarks Where the capacitor seats on the PC board without any gaps between its end seal and the PC board, the solvent might remain there. The residual solvent should be sufficiently evaporated by forced air drying at least 10 minutes at 50-85°C immediately after the solvent cleaning.

Non-clean flux: Both ionic halogen and nonionic halogen damage the capacitor when they penetrate into the inside of the capacitor through the rubber seal. Some of the flux called non-halogenated flux contains less ionic halogen activator with a large amount of nonionic halogen added.

Some adhesives, dampproofing agents and dustproofing agents also contain halides and should be used with caution.

 

Electrolyte and Separator Paper

An aluminum electrolytic capacitor uses a flammable separator paper in the internal element which is impregnated with a flammable, electrically conductive electrolyte. If the electrolyte should leak out onto the PC board, it may short or erode away the copper traces and might catch on fire with a voltage applied. Be careful with the location of the vent, copper land and copper trace when the PC board is designed. Where a through-hole PC board is used, any copper traces and land should not be located under the capacitor. If it is, leave a space above the trace of at least 1-2 mm.

 

Storage

The electrical characteristics of aluminum electrolytic capacitors are dependent on temperature; the higher the ambient temperature, the faster the deterioration of the electrical characteristics (i.e., leakage current increase, tan d increase, capacitance drop, etc.). If an aluminum electrolytic capacitor is exposed to high temperatures such as direct sunlight, heating elements, etc., the life of the capacitor may be adversely affected. When capacitors are stored under humid conditions for long periods of time, the humidity will cause the lead wires/terminals to oxidize and thus impairing solderability. Therefore aluminum electrolytic capacitors should be stored at room temperature, in a dry place and out of direct sunlight.

A voltage treatment/reformation process to electrolytic capacitors may have to be applied after a capacitor has been stored for more than 2 or 3 years. If aluminum electrolytic capacitors are stored above room temperature for long periods of time, the anode foil may react with the electrolyte increasing the leakage current. After storage, the application of even normal voltages to these capacitors may result in higher than normal leakage currents.
In most cases the leakage current will return to normal
levels in a short period of time. However in extreme cases, the amount of gas generated may cause the safety vent to open.

Capacitors that are stored for long periods of time should be subjected to a voltage treatment/reforming process (Note 1) which will reform the dielectric and return the leakage current to the initial level. Leakage current increase during storage will vary with the working voltage of the capacitors, normally in this order:

Low voltage capacitors < Middle voltage capacitors < High voltage capacitors.

Note 1: In the reformation process the applied voltage is gradually increased up to the rated voltage without exceeding the initial specified leakage current of the capacitor. After reaching the rated voltage, continue applying this voltage for 30 to 60 minutes.

 

Figure 28. Effect of Storage Time and Temperature