The Bathtub Curve
With the advancements in aluminum electrolytic capacitor technology, capacitors can be used in equipment that must have very long life or must operate under severe conditions. For example, the capacitors used in telecommunication applications have an expected life of over twenty years. Other aluminum electrolytic capacitors have run continuously at ~130&C for over one year. The careful selection of a capacitor for a particular application and proper installation in the circuit will assure good service life.
All components will eventually fail. Usually this occurs by a slow, steady drift of parameters called wear-out. Sometimes there is a sharp change in capacitor properties which is called a catastrophic failure.
The failure rate of aluminum electrolytic capacitors follows a bathtub curve with time as shown in Figure 1.
Figure 1. Bathtub Curve
(a) Initial Failure Period
This is the period during which failures are caused by deficiencies in design, structure, manufacturing processes or severe misapplications. Such failures occur soon after the components are exposed to circuit conditions. In aluminum electrolytic capacitors, these failures are either corrected through aging or found during the 100% inspection process and thus do not reach the field. The initial failures due to capacitor misapplication such as inappropriate ambient conditions, over voltage, reverse voltage or excessive ripple current can be avoided with proper circuit design and installation.
(b) Random Failure Period
This is a period during which the failure rate is the lowest. These failures are not related to operating time but to application conditions. Aluminum electrolytic capacitors feature fewer catastrophic failures during this period than semiconductors and solid tantalum capacitors.
(c) Wear-Out Failure Period
This is a period during which the properties of a component gradually deteriorate, and the failure rate increases with time. Aluminum electrolytic capacitors end their useful life during this period. The criteria for judging failures varies with application design factors.
The two types of failures are classified as catastrophic failures or wear-out failures and are defined as follows:
(a) Catastrophic Failures
This is a failure mode which destroys the performance of the capacitor. Short circuit and open circuit are examples of this failure mode.
(b) Wear-Out Failures
This is a failure mode caused by the gradual deterioration of the capacitor’s electrical parameters. The criteria for judging failures varies with application and design factors. Table 1 shows the failure criteria specified in JIS C 5141.
Table 1: Failure Judgment Criteria (JIS C 5141)
|Leakage Current||Not to exceed initial specified value|
|Capacitance||Not less than 85% of initial measured value||Not less than 80% of initial measured value|
|Tan d||Not to exceed 175% of initial specified value||Not to exceed 200% of initial specified value|
|Appearance||Show no remarkable abnormalities|
Capacitance decrease and tan d increase are caused by the loss of electrolyte in the wear-out failure period. This is due primarily to loss of electrolyte by diffusion (as vapor) through the sealing material. Gas molecules can diffuse out through the material of the end seal. If the electrolyte vapor pressure within the capacitor is in-creased, by high temperatures for example, the diffusion rate is increased. Swelling of the seal material by the electrolyte vapor pressure may also occur at elevated temperatures. This swelling may further enhance diffusion and mechanically weaken the seal.
Factors that can increase the capacitor temperature, such as ambient temperature and ripple current, can accelerate capacitor wear-out.
Wear-out can also be increased by high internal pressure caused by gas generation from excessive leakage current or attack of the cathode foil by electrolyte. These factors are not present in well-designed capacitors and are generally not a problem.