Ceramic Capacitor Failure Mode and Mechanism Analysis

Ⅰ. Ceramic Capacitor Failure Modes

There are three typical failure modes of ceramic capacitors to withstand voltage:

1. The first mode: electrode edge ceramic penetration (the breakdown point is at the edge of the silver surface)

(1) Possible reasons:

① Powder and its formulation issues

② Poor densification of plain edges

Figure. 1

(2) The specific performance of the failure mode in the process:

① Pinholes at the edge of the silver side

② There is a pinhole at the edge of the silver surface, and some ceramics at this position burst.

③ Cracks (pinholes first, then cracks, small black spots of ablation and carbonization on the surface of the element, the cracks are new traces.)

(3) Countermeasures:

Feedback information to the front-end process in a timely manner, requiring it to improve and improve the overall pressure resistance level of the ground.

2. The second mode: the ceramic chip is conducting along the edge or the edge of the ceramic chip is broken and damaged (the breakdown point is on the side of the element)

Figure. 2

(1) Possible reasons:

① There are stains on the surface of the plain ground, such as silver, flux, oil, solder slag, etc.

② There are conductive impurities in the paint

③ There are air bubbles in the paint

④ Poor paint density

⑤ Insufficient curing of the coating encapsulation layer 

(2) The specific performance of the failure mode in the process:

① Cross Arc

② Collapse

③ Side burst

(3) Countermeasures:

① Element appearance (diffusion, silver on the side) control;

② The flux level is moderately controlled, and the tile is immersed in the depth control;

③ Timely and thoroughly clean up the tin slag and other impurities in the tin bath;

④ Insulation quality certificate of the coating;

⑤ Quality assurance of coating encapsulation and curing process.

 3. The third mode: The ceramic chip in the electrode is penetrated (the breakdown point is in the center of the element (silver surface) and its surrounding position).

(1) Possible reasons:

① Very poor compactness 

② There are cracks, bubbles, conductive impurities, etc.

(2) The specific performance of the failure mode in the process:

① Pinholes in the center of the element and its surrounding

② Pinholes in the center of the element and its periphery. At the same time, some ceramics at this location burst.

③ Cracks (pinholes first and then cracks, there are small black spots of ablation and carbonization on the surface of the element, and the cracks are new traces.)

(3) Countermeasures:

① Element appearance (diffusion, silver on the side) control;

② The flux level is moderately controlled, and the tile is immersed in the depth control;

③ Timely and thoroughly clean up the tin slag and other impurities in the tin bath;

④ Insulation quality certificate of the coating;

⑤ Quality assurance of coating encapsulation and curing process.

Figure. 3

Ⅱ. The Seven Reasons for Failure

1. The influence of humidity on the deterioration of electrical parameters

The water film condenses on the surface of the capacitor shell when the humidity in the air is too high, lowering the surface insulation resistance of the capacitor. Moisture can also permeate the capacitor medium in semi-sealed capacitors, reducing the insulation resistance and insulation ability of the capacitor media. As a result, the impact of a high-temperature, high-humidity environment on the degradation of capacitor characteristics is enormous. The electrical properties of capacitors can be enhanced after drying and dehumidification, but the repercussions of water molecule electrolysis cannot be eliminated. For example, capacitors operate at high temperatures, and water molecules  are electrolyzed into hydrogen ions (H+) and hydroxide ions (OH-) by an electric field, resulting in electrochemical corrosion at the lead's root. The lead wire cannot be restored, even if it is dried and dehumidified.

2. Consequences of silver ion migration

Silver electrodes are used in the majority of inorganic dielectric capacitors. When semi-sealed capacitors are exposed to high temperatures, water molecules that permeate the capacitors cause electrolysis. The anode undergoes an oxidation reaction, in which silver ions interact with hydroxide ions to make silver hydroxide; the cathode undergoes a reduction reaction, in which silver hydroxide reacts with hydrogen ions to produce silver and water. The anode's silver ions are continually reduced to the cathode by the electrode reaction, forming discontinuous metallic silver particles that are connected by the water layer and extend to the anode in a tree-like fashion.  Silver ions migrate not only on the surface of the inorganic medium, but also diffuse into the interior, increasing the leakage current. A complete short circuit between the two silver electrodes can be employed in extreme instances, causing the capacitor to fail.

The silver layer on the positive electrode's surface can be severely damaged by ion migration. There is a silver oxide  with semiconductor qualities between the lead solder connection and the silver layer on the surface of the electrode, which increases the equivalent series resistance of the non-dielectric capacitor, increases the loss of the metal component, and improves the capacitor's performance. The tangent value of the loss increases dramatically.

The capacitance of the capacitor reduces as the effective area of the positive electrode decreases. The presence of silver oxide semiconductors on the surface of the dielectric between the two electrodes of the inorganic dielectric capacitor reduces the surface insulation resistance. When silver ion migration is severe, a dendritic silver bridge is built between the two electrodes, dramatically lowering the capacitor's insulating resistance.

To summarize, silver ion migration will not only degrade the electrical properties of open inorganic dielectric capacitors, but it may also induce a reduction in the dielectric breakdown field strength, resulting in capacitor failure.

It's worth noting that silver electrode low-frequency ceramic monolithic capacitors fail substantially more frequently than other types of ceramic dielectric capacitors due to silver ion migration. Silver participates in the solid-phase reaction on the surface of the ceramic medium during the initial sintering process of the silver electrode and the ceramic medium and penetrates into the ceramic-silver contact to produce an interface layer. Silver ions can migrate not only on the surface of the ceramic medium but also through the ceramic medium layer after moisture infiltration if the ceramic medium is not dense enough. The multi-layer laminated construction has several gaps, electrode positioning is difficult, and the quantity of margin on the medium's surface is limited. When the outer electrodes are covered at both ends of the laminated layer, the silver paste penetrates the gap, reducing the medium surface's insulation resistance and forming a gap between the electrodes. When the silver ions migrate, the channel is shortened, and the short circuit occurrence is common.

3. Breakdown mechanism of ceramic capacitors under high-temperature conditions

Breakdown failure is a regular serious problem when semi-sealed ceramic capacitors are used in high humidity environments. The two types of breakdowns that occur are dielectric breakdown and surface arcing breakdown. According to the timing of occurrence, a dielectric breakdown can be classified as an early or aged breakdown. Early failure reveals flaws in capacitor dielectric materials and manufacturing techniques. Ceramic dielectrics' dielectric strength is significantly reduced as a result of these flaws. The capacitor will experience an electrical breakdown during the withstand voltage test or in the early stages of operation due to the action of the electric field in a high humidity environment. The electrochemical breakdown is the most common type of aging breakdown. Because of the migration of silver in ceramic capacitors, electrolytic age breakdown has become a fairly prevalent problem. The conductive dendrites generated by silver migration can increase the leakage current locally, leading to thermal breakdown and the capacitor breaking or burning out. Because local heating is high during the breakdown, and thinner tube walls or smaller ceramic bodies are prone to burn or break, thermal breakdown occurs most frequently in tubular or disc-shaped tiny ceramic dielectric capacitors.

Furthermore, in a ceramic medium mostly constituted of titanium dioxide, the titanium dioxide reduction reaction may occur under stress circumstances, causing the titanium ion to shift from tetravalent to trivalent. The dielectric strength of the capacitor is greatly reduced when the ceramic dielectric ages, potentially leading to capacitor failure. As a result, the electrolytic breakdown of these ceramic capacitors is more severe than that of ceramic dielectric capacitors that do not include titanium dioxide, 

The migration of silver ions distorts the electric field between the capacitor electrodes, and because of the condensed water film on the surface of the ceramic dielectric in a high humidity environment, the corona discharge voltage on the capacitor's edge surface drops dramatically, resulting in the surface arc phenomenon. It can lead to arcing breakdown between the electrodes on the capacitor's surface in extreme circumstances. The surface breakdown is influenced by parameters like capacitance structure, inter-electrode distance, load voltage, protective layer hydrophobicity, and moisture permeability. The main cause of arcing between the electrodes on the edge surface is that the quantity of edge remaining in the dielectric is tiny, and silver ion migration and the production of a surface water layer when working in a humid environment insulate the capacitor's edge surface. A period of time is required for the formation and development of silver ion migration. As a result, the main failure mode during the withstand voltage test is the dielectric breakdown; however, after 500 hours of testing, the only failure mode is the excessively arcing breakdown between the edge surfaces.

4. Improvement of electrode materials

Silver electrodes have long been utilized in ceramic capacitors. The main causes of ceramic capacitor failure are silver ion migration and the resulting accelerated aging of titanium-containing ceramic dielectrics. In the fabrication of ceramic capacitors, some producers have employed nickel electrodes instead of silver electrodes, and electroless nickel plating has been used on the ceramic substrate. Ceramic capacitor performance and reliability are increased because nickel has superior chemical stability to silver and has low electrical mobility.

The monolithic low-frequency ceramic dielectric capacitor with silver as the electrode, for example, has a large porosity because the silver electrode and the ceramic material are sintered at 900 °C for one time, the ceramic material cannot obtain a dense ceramic medium, and there is a large porosity; additionally, the silver electrode is widely used. The cosolvent barium oxide will penetrate into the interior of the porcelain body, relying on the good infiltration "mutual fusion" ability of barium oxide and silver at high temperatures, resulting in thermal diffusion inside the electrode and the medium, resulting in macroscopically visible "porcelain absorption." silver" occurrence. The effective thickness of the medium is considerably reduced after silver and barium oxide enters the porcelain body, resulting in a loss in insulation resistance and product reliability. Silver-palladium electrodes are utilized instead of electrodes that generally contain barium oxide, and 1% of 5# glass frit is added to the material formula to improve the dependability of monolithic capacitors. It prevents thermal migration of the metal electrode to the ceramic dielectric layer during the first sintering at high temperatures, allowing the ceramic material to sinter and densify more quickly, improving the product's performance and durability. Compared with the original process and dielectric materials, the capacitor's reliability is improved by 1~2 orders of magnitude.

5. Fracture of laminated ceramic capacitors

Fracture is the most prevalent failure mode of laminated ceramic capacitors, which is determined by the brittleness of the dielectric. Because the laminated ceramic capacitor is directly welded to the circuit board, it is immediately exposed to the circuit board's mechanical stresses, whereas the leaded ceramic capacitor may absorb the mechanical stress through the pins. As a result, the mechanical stress induced by various thermal expansion coefficients or circuit board bending will be the primary cause of laminated ceramic capacitor rupture.

6. Fracture analysis of laminated ceramic capacitors

The electrode insulation separation at the fracture will be smaller than the breakdown voltage once the laminated ceramic capacitor is mechanically cracked, resulting in arc discharge between two or more electrodes and complete failure of the laminated ceramic capacitor, 

Reduce the bending of the circuit board as much as possible, reduce the stress of the ceramic chip capacitor on the circuit board, and reduce the difference between the thermal expansion coefficient of the laminated ceramic capacitor and the circuit board, mechanical stress is the main methods to prevent mechanical fracture of laminated ceramic capacitors.

The steps for reducing the stress on laminated ceramic capacitors on the circuit board are detailed below and will not be duplicated here. Reduce the mechanical stress induced by the thermal expansion coefficient differential between the laminated ceramic capacitor and the circuit board by choosing a capacitor with small package size. The aluminum-based circuit board, for example, should employ a package that is as small as possible. It can be solved using several parallel connections or lamination, and it can also be solved using a ceramic capacitor in the form of a pin package.

7. The electrode terminals of the laminated ceramic capacitor are melted and showered

The electrode terminals may be melted off by the solder when wave soldering laminated ceramic capacitors. The fundamental explanation is that the wave soldering laminated ceramic capacitors have spent too much time in contact with the high-temperature solder. On the market today, laminated ceramic capacitors are classified into two types: those appropriate for reflow soldering and those suited for wave soldering,  The phenomena of extreme head melting. The time characteristics of high-temperature solder that laminated ceramic capacitor electrode terminals may bear under various welding procedures are detailed in the applicable precautions for laminated ceramic capacitors, and will not be duplicated here.

The solution is simple: utilize laminated ceramic capacitors that adhere to the wave soldering process as much as feasible when utilizing the wave soldering process, or avoid using the wave soldering process as much as possible.