Detailed analysis based on various capacitor failure reasons

Ceramic capacitor failure analysis:

The multi-layer chip ceramic capacitor consists of three materials: ceramic dielectric, terminal electrode and metal electrode. The failure mode is a stacking fault between the metal electrode and the ceramic dielectric. The electrical performance is affected by external force (such as gently bending the board or touching with a soldering iron). A) When the temperature is impacted (such as soldering iron), the capacitance is good or bad.

The specific defects of multi-layer chip ceramic capacitors can be divided into:

1, hot hit failure

2, twisted and broken failure

3, three major categories of raw materials failure

(1) Hot hit failure mode:

The principle of heat shock failure is that when manufacturing multilayer ceramic capacitors, the use of various compatible materials results in different thermal expansion coefficients and thermal conductivity of the internal tension. When the temperature conversion rate is too large, it is prone to breakage due to thermal shock. This cracking often occurs from the weakest structure and the most concentrated mechanical structure, generally at the interface close to the exposed termination and the central ceramic termination. Where the maximum mechanical tension is produced (generally at the hardest corners of the crystal), and hot shocks can cause a variety of phenomena:

The first is an obvious shape like a nail or a U-shaped crack.

The second is a tiny crack hidden inside.

The second type of crack also begins with the exposed central portion, or the lower portion of the ceramic/termination interface, and spreads as the temperature changes, or as the assembly progresses (see Figure 4).

The first type is a nail-like or U-shaped crack and the second type of tiny crack is hidden. The difference between the two is that the latter is less stressed and the crack is also slightly less. The first type of rupture is obvious, and it can generally be measured in the metallographic phase. The second type can only be measured after a certain degree of development.

(2) Twisted rupture failure

There are many possibilities for this kind of badness: there are two types according to the categories and performance:

The first case, the failure of the rupture caused by the SMT phase

When picking and placing parts, especially parts in the SMT stage, the centering claws of the pick and place are caused by wear, inaccurate alignment, tilting, and the like. The pressure concentrated by the centering claws causes a large pressure or cut rate, which in turn forms a break point.

These cracking phenomena are generally visible surface cracks, or internal cracks between 2 and 3 electrodes; surface cracks generally follow the direction of the strongest pressure line and ceramic displacement.

Damage or cracking caused by the vacuum pick-up head generally results in a round or half-moon indentation area on the surface of the chip with a non-smooth edge. In addition, this half-moon or round crack straight is also consistent with the tip.

Another damage caused by the tip, the crack caused by the tension, the crack will extend from one side of the assembly to the other side, these cracks may spread to the other side of the assembly, and its rough cracks may cause the capacitor The bottom of the broken.

Second, rupture failure caused by the production stage after SMT

Board cutting, testing, back assembly and connector mounting, and final assembly, if the solder assembly is distorted or the board is straightened after the soldering process, it can cause damage such as 'twisted cracks'.

When the sheet is bent and deformed by mechanical force, the range of motion of the ceramic is limited by the end position and the weld point, and the crack is formed at the interface of the ceramic. The crack starts from the formed position, from the 45° angle to the end. Then spread it.

(3) Raw material failure

Multilayer ceramic capacitors typically have two broad categories of fundamental visible internal defects that are sufficient to compromise product reliability:

The failure between the electrodes and the breaking of the bond line are broken.

These defects can cause excessive current, which damages the reliability of the components. The details are as follows:

1. The failure between the electrodes and the breaking of the bonding line are mainly caused by the high void of the ceramic or the gap existing between the dielectric layer and the opposite electrode, so that the dielectric layer is cracked between the electrodes, which becomes a latent leakage crisis;

2. The characteristics of combustion cracking are perpendicular to the electrodes and generally originate from the edge or terminal of the electrode. If the cracks are shown to be vertical, they should be caused by burning;

Remarks: The first type of failure in the raw material failure class is difficult to measure due to the separation degree of the internal structure of the parallel capacitor, and the third vertical structure metallography can ensure the measurement.

in conclusion:

The rupture caused by the heat shock will spread from the surface to the inside of the assembly, and the damage caused by the excessive mechanical tension can be formed by the surface or the inside of the component. These damages will spread in the direction of nearly 45° angle. If the material fails, it will cause cracks perpendicular or parallel to the internal electrodes.

In addition: the thermal shock rupture generally spreads from one termination to the other. The rupture caused by the pick-and-place machine causes multiple break points under the termination, and the damage caused by the distortion of the board is usually only one. Breaking point.

A picture teaches you to analyze the failure analysis of electrolytic capacitors

Can't see the picture, you can click on the picture, zoom in and view:

Tantalum capacitor:

Advantages: small size, large capacitance, diverse shape, long life, high reliability, wide operating temperature range

Disadvantages: small capacity, high price, weak voltage and current capability

Applications: military communications, aerospace, industrial control, video equipment, communications instruments

1. It is also a kind of electrolytic capacitor. It uses metal crucible as the medium. It does not use the electrolyte like ordinary electrolytic capacitors. Tantalum capacitors do not need to be wound with a capacitor film coated with aluminum film like ordinary electrolytic capacitors. But this also limits its capacity. ——We are in a large capacity, but need a low ESL scene, we use tantalum capacitors.

2. Since there is no electrolyte inside the tantalum capacitor, it is suitable for working at high temperatures. - Some temperature ranges require a wide range of scenes.

3. The working medium of tantalum capacitor is a very thin layer of tantalum pentoxide film formed on the surface of base metal. This layer of oxide film. The medium is combined with the one end of the constituent capacitor and cannot be present alone. Therefore, there is a very high working electric field strength per unit volume, and the electric capacity is particularly large, that is, the specific capacity is very high, so that it is particularly suitable for miniaturization. ——Scenarios with relatively high integration, the area occupied by aluminum electrolytic capacitors is relatively large, and the capacity of ceramic capacitors is insufficient.

4. The performance of tantalum capacitor is excellent. It is a small capacitor and can reach a large capacity. It has few competitors in power supply filtering and AC bypass. Tantalum electrolytic capacitors have the ability to store electricity, charge and discharge, etc., mainly used for filtering, energy storage and conversion, mark bypass, coupling and decoupling, and time constant components. Pay attention to its performance characteristics in the application. Proper use will help to fully utilize its functions, such as considering the working environment of the product and its heating temperature, and taking measures such as derating, if it is used improperly, it will affect the working life of the product. —— For example, if the USB interface output needs to be derated, the withstand voltage meets 5V, and the integration is relatively high. If the ceramic capacitor does not meet the high withstand voltage and large capacity, we have to choose tantalum capacitor. The energy storage effect of ceramic capacitors cannot be equivalent according to the parallel capacitance values, and the cost required to achieve the same effect is also very large.

5. The temperature stability of the capacitance of the tantalum capacitor is better. In some coupled and filtered scenarios, if the phase and the filtered frequency characteristics require a relatively high scene, and the space accuracy requirements are relatively high, a non-polar tantalum capacitor is used. Audio circuit design such as high sound quality requirements.

We need to consider the accuracy and consistency of the capacitors at different temperatures.

The temperature characteristics of ceramic capacitors are clearly not stable enough.

6. During the operation of the tantalum capacitor, it has the property of automatically repairing or isolating the defects in the oxide film, so that the oxide film medium can be strengthened and restored to its proper insulation capacity without continuous cumulative damage. This unique self-healing performance guarantees its long life and reliability advantages. ——The aluminum electrolytic capacitor can not meet the life expectancy due to dryness.

First, the mode of tantalum capacitor failure is terrible, but it burns smoke and smoke.

The principle of the failure mode of the "tantalum capacitor" is not described here.

Through this failure phenomenon, we know that if the capacitor fails, only the short circuit causes the circuit to be inoperable, or the work is unstable, which is a small problem, and it is a big deal. However, if a fire occurs on the customer's site, it is necessary to compensate the other party's personnel and property losses. Then it is a big problem.

This is an important reason why we should not use tantalum capacitors.

Second, the cost of tantalum capacitors is high.

Look at our Taobao to know the price difference between 100uF tantalum capacitor and 100uF ceramic capacitor. The price of tantalum capacitor is 10 times that of ceramic capacitor.

If the capacity requirement of the capacitor is below 100uF, most of us now, if the withstand voltage is satisfied, we generally need to use ceramic capacitors.

If you have a large capacity, or a high withstand voltage, the ceramic capacitor package is greater than 1206, try to choose carefully.

The most important failure mode of the chip ceramic capacitor is broken (the larger the package, the more likely it is to fail): the common failure of the chip ceramic capacitor is the fracture, which is determined by the brittleness of the dielectric of the chip ceramic capacitor. Since the chip ceramic capacitor is directly soldered The circuit board directly withstands various mechanical stresses from the board, while leaded ceramic capacitors can absorb mechanical stress from the board through the pins. Therefore, for patch ceramic capacitors, due to different thermal expansion coefficients or circuits The mechanical stress caused by the bending of the plate will be the most important factor in the fracture of the chip ceramic capacitor.

Third, the tantalum capacitor will be exhausted in the future, you can't buy it with money.

As early as 2007, the US Defense Logistics Agency (DLA) had stored a large amount of antimony minerals for more than a decade. To fulfill the decision of the US Congress, the organization will exhaust the last 140,000 pounds of plutonium it owns. Buyers who purchase antimony ore from the US Defense Logistics Agency include HC Starck, DM Chemi-Met, ABS Alloys, Umicore, Ulba Metallurgical, and Mitsui Mining, which represent the processing of these tantalum ore into capacitor grade powders and tantalum products. A large number of companies that wear parts or cutting tools. Bidders who purchase these antimony ore from the US Defense Logistics Agency have traditionally been consistent year after year, so when the supply of antimony ore becomes tight, some companies have to snatch new sources of ore because the US Defense Logistics Agency is running out of supplies. .

Why is this a very important development direction?

If the U.S. Defense Logistics Agency’s supply of antimony ore is lost, it is estimated that the plutonium ore supply market will leave a gap of 150,000 pounds in 2007, with a gap of 350,000 pounds in 2008. The timing of this incident is out of date, because the current supply capacity is distressing. For example, the second largest hard-and-selling ore seller in Australia, the Guarria subsidiary, has cut its ore production by 25% in the fourth quarter (half the production of the Green Bush mine) so that the company can complete its management in Australia. In the same vein, the production of antimony ore in Brazil's Metallurgical/CIF and Paranapanema companies declined in 2006 as they turned their interest to mining more profitable metals. In Africa, the main source of supply is the Democratic Republic of the Congo (DRC), which has not yet reached the capacity limit due to pressure from the United Nations. However, we have heard reports of many investors trying to obtain Congo's stockpiles of ore in 2006, feeling that this is a shortage of antimony ore. Signs.

Tantalum capacitors give design engineers the highest possible capacity in the smallest physical size. The capacity ranges from 47μF to 1000μF, which is particularly advantageous. Therefore, in the case of high integration and high capacity, low ESR, tantalum electrolysis Capacitors have their own advantages.

Alternative to high-capacity, low-voltage tantalum capacitors: high molecular polymer solid aluminum electrolytic capacitors

Compared with the traditional electrolytic capacitor, the polymer polymer solid aluminum electrolytic capacitor uses a conductive polymer material with high conductivity and high stability as a solid electrolyte instead of the electrolyte in the conventional aluminum electrolytic capacitor. The high conductivity of the electrolyte, combined with its unique structural design, greatly improves the shortcomings of traditional liquid aluminum electrolytic capacitors and exhibits extremely excellent characteristics.

Ideal for high frequency and low impedance. High-polymer solid-state electrolytic capacitors have extremely low loss and excellent high-frequency and low-impedance characteristics, so they are widely used in circuits such as decoupling and filtering. The effect is buried, especially the high-frequency filtering effect is excellent.

Through an experiment, it is possible to more clearly and clearly see the significant difference in high frequency characteristics between the polymer polymer solid aluminum electrolytic capacitor and the ordinary electrolytic capacitor. The high-frequency interference signal of 1MHz (peak-to-peak voltage 8V) is superimposed on the smoothing circuit input, and filtered by a 47uF polymer solid electrolytic capacitor to reduce the noise to a peak-to-peak voltage of 30mV. To achieve the same filtering effect, it is necessary to connect four 1000uF ordinary liquid aluminum electrolytic capacitors in parallel or three 100UF tantalum electrolytic capacitors in parallel.

In addition, in the case of better high-frequency filtering effect, the volume of the polymer solid aluminum electrolytic capacitor is significantly smaller than that of the ordinary aluminum electrolytic capacitor.

As the process continues to improve, the advantages of high molecular polymer solid aluminum electrolytic capacitors gradually emerge. At the same time, prices need to be further optimized.

Failure Analysis of Aluminum Electrolytic Capacitors

Aluminum electrolytic capacitors are a very common type of capacitor. Aluminum electrolytic capacitors are widely used: filtering; bypassing; coupling; shock wave absorption; noise cancellation; phase shift; For aluminum electrolytic capacitors, common electrical performance tests include: capacitance, loss tangent, leakage current, rated operating voltage, impedance, and so on. In the failure analysis case, there are many failure cases about aluminum electrolytic capacitors. What are the failure mechanisms of common aluminum electrolytic capacitors?

Leakage

In normal use, leaks may occur after a period of sealing. Generally, temperature rise, vibration or sealing defects, etc., may accelerate the deterioration of the sealing performance. The result of the leakage is a decrease in the capacitance value, an increase in the equivalent series resistance, and a corresponding increase in power dissipation. Leakage reduces the working electrolyte and loses the ability to repair the anodized film medium, thereby losing its self-healing effect. In addition, because the electrolyte is acidic, the leaking electrolyte can contaminate and corrode other components and printed circuit boards around the capacitor.

2. Dielectric breakdown

The breakdown of the aluminum electrolytic capacitor is caused by the rupture of the anodized aluminum dielectric film, causing the electrolyte to directly contact the anode. The aluminum oxide film may be partially damaged due to various materials, processes or environmental conditions. Under the action of the external electric field, the oxygen ions provided by the working electrolyte can re-form the oxide film at the damaged part, so that the anodized film can be filled and repaired. . However, if there are impurity ions or other defects in the damaged part, the filling and repairing work cannot be perfected, and micropores may be left on the anodized film, and may even become a penetrating hole, so that the aluminum electrolytic capacitor is broken down. Process defects such as anodic oxide film are not dense and firm. When the subsequent riveting process is not good, the burr on the foil strip will cause the oxide film to be stabbed. The leakage current of these puncture parts is large, and local overheating causes thermal breakdown of the capacitor.

3. Open the way

An open circuit usually occurs when the connection performance inside the capacitor deteriorates or fails. The deterioration of the electrical performance connection may be the result of corrosion, vibration or mechanical stress. When the aluminum electrolytic capacitor is operated in a high temperature or hot air environment, the anode lead foil may be broken due to electrochemical corrosion. Poor contact between the anode lead foil and the anode foil also causes the capacitor to open intermittently.

4. Other

1) In the early stage of work, aluminum electrolytic capacitors will cause a decrease in capacitance due to the continuous repair and thickening of the anodized film (called the complementary effect) during the load operation.

2) In the later stage of use, due to the loss of the electrolyte, the solution becomes thicker and the resistivity increases, so that the equivalent series resistance of the electrolyte increases and the loss increases. At the same time, the viscosity of the solution is increased, and it is difficult to sufficiently contact the uneven oxide film layer on the surface of the aluminum foil, which reduces the effective plate area of ​​the electrolytic capacitor, resulting in a decrease in capacitance. In addition, when operating at a low temperature, the viscosity of the electrolyte also increases, resulting in an increase in electrolytic capacitor loss and a decrease in capacitance.

parameter	Aluminum electrolytic capacitors
capacitance	The industry can achieve 0.1uF ~ 3F (common capacity range 0.47uF ~ 6.8mF), working voltage from 5V ~ 500V. From 25 ° C to the high temperature limit, the capacity increase does not exceed 5% to 10%; for the -40 ° C limit capacitance, at -40 ° C, the capacity of the low-voltage capacitor will decrease by 20%, and the high-voltage capacitor will decrease by as much as 40%; In the temperature range of -20 ° C to -40 ° C, the capacity declines the fastest; for the -55 ° C limit of the capacitance, at -40 ° C, the drop usually does not exceed 10%; at -55 ° C, does not exceed 20%.
ESR	At 100 kHz/25 ° C, the ESR value is generally in the range of several tens of mΩ to 2.5 Ω, and the Low ESR model is generally tens of mΩ. The ESR value varies with temperature, generally from 25 ° C to the high temperature limit, ESR will drop by about 35% to 50%; and from 25 ° C to the low temperature limit, ESR will increase by 10 to 100 times.
ESL	The parasitic series inductance value ESL of aluminum electrolytic capacitor is stable and does not change with frequency and temperature. For general aluminum electrolytic capacitors, ESL will not exceed 100nH, such as SMT package, its value is in the range of 2nH~8nH; radial insertion Packing: 10nH~30nH; screw-terminal: 20nH~50nH; and axially inserted structure, the value can reach 200nH.
On-board operating frequency range	Mainly for low frequency filtering, no more than a few hundred KHz, but still has some effect within 1 MHz.
Weak reliability point and its avoidance	The reliable application of aluminum electrolytic capacitors is mainly concerned with temperature. Because the electrolyte of aluminum capacitors is liquid, the heating of the core will cause the electrolyte to volatilize, and eventually dry up and fail. When the capacitor is applied in the pulse AC circuit, the ripple current flows through the ESR. The resulting loss of heat will seriously affect the life of the device.
Recommendations	For high temperature applications greater than 75 °C, use small-sized aluminum electrolytic capacitors as little as possible. Try to use a larger capacity specification to take advantage of aluminum electrolytic capacitors. It is suitable for rectification smoothing of power frequency, switching power supply input filtering and output filtering of low frequency switching power supply. It is not recommended for output filtering of high frequency switching power supply.

parameter	Tantalum electrolytic capacitor
capacitance	Limited to the solid sintered process structure and materials, its CV value (capacitance and voltage product) is not large, the capacity and voltage have a certain range, generally from 0.1uF to 1000uF (common capacity range 1uF ~ 220uF); working voltage from 2V~ 50V (common pressure range is 6.3V~50V); the value of capacity decreases with increasing frequency. Due to the solid MnO2 electrolyte, its capacity temperature characteristics are relatively stable, even when the temperature is as low as -190 °C, the capacity is Only 10% reduction.
ESR	The temperature characteristics of ESR are relatively stable. The manufacturer gives a maximum ESR of 100KHz, which can be used as a reference for design, but the actual value is generally much smaller than the maximum value.
ESL	In the case of good wiring, it is generally about 2nH.
On-board operating frequency range	Medium and low frequency filtering, no more than a few MHz, mainly between several hundred KHz to several MHz.
Weakness of reliability and its avoidance	The reliable application of tantalum electrolytic capacitors mainly focuses on voltage derating and voltage change speed, and it is not possible to obtain sufficient voltage derating. At the same time, it is recommended to replace other capacitors with faster power-off. At the same time, the tantalum capacitor process of the edge specification is not mature enough, and it should be used with caution, especially in the case of high reliability requirements.
Recommendations	It is not recommended to use tantalum capacitors for filtering of DC voltage above 15V, especially at power input ports with faster power-on. For low voltage but faster power-on, it is recommended to start slowly. High temperatures increase the probability of tantalum capacitor failure, so voltage derating is required in high temperature applications.

parameter	Ceramic capacitors
capacitance	The first type (NPO or COG), low capacity, high stability; the most stable electrical performance, basically does not change with changes in temperature, voltage and time; the second type (X7R), large dielectric constant, the same volume of capacity It is 20 to 70 times larger than the first type, but when the temperature changes from -55 ° C to 125 ° C, the capacity change is generally ±10%, and the maximum is +15% to -25%, the third category (Z5U), Its dielectric constant is high, and it is commonly used in large-capacity capacitor products, but its capacity stability is worse than that of X7R; its capacity can be 5 times that of the second category. However, capacity and loss are sensitive to temperature and voltage, and its stability is very poor. When the temperature changes from -25 ° C to 85 ° C, the capacity changes from +20% to -65%.
ESR	The ESR is between a few mΩ and a few hundred mom, and the smaller the capacity, the larger the ESR. ESR is linear with temperature change. In X7R medium, ESR is 20% of room temperature at 125 °C, and more than 3 times of room temperature at -55 °C. NPO is more stable, and the coefficient of variation is about 1/3 of X7R.
ESL	The ESL varies with the package. In general, the ESL of the 0603 and 0805 packages is about 1nH in good wiring, and about 1.2nH in 1206 and 1210.
On-board operating frequency range	High-frequency filtering, many types, from several MHz up to hundreds of MHz, 1GHz can be.
Weakness of reliability and its avoidance	It is susceptible to cracks caused by temperature shock, mainly due to temperature shock during welding, especially wave soldering. Improper repair is also an important cause of temperature shock cracking. Multilayer ceramic capacitors are characterized by their ability to withstand large compressive stresses, but their resistance to bending is poor, and any operation that may cause bending deformation may cause cracking of the device.
Recommendations	Do not place the ceramic capacitor cloth in the stress area during the board layout, such as the edge of the board, near the fasteners, etc., to maximize the multilayer ceramic capacitor to avoid areas where large mechanical stress may occur during the process. In addition to the relatively stable NPO capacitance, the X7R capacitor and Z5U capacitor (or Y5V) have characteristics that vary with temperature and bias.