Seven reasons in parsing multilayer ceramic chip capacitor MLCC failure problem
1, moist to worsen the effects of electric parameters
When the humidity is too high in the air, water film condensed in the condenser shell surface, can make the condenser surface insulation resistance decreased.In addition, for half a sealing structure of capacitor, the water also can infiltrate the capacitor dielectric, the capacitor dielectric insulation insulation resistance ability.Therefore, the high temperature and high humidity environment impact on condenser parameters deteriorate extremely significant.After drying to wet the condenser's performance can be improved, but the consequences of electrolytic water molecules cannot be eradicated.Capacitor in high temperature conditions, for example, water molecules under electric fields of electrolytic hydrogen ions (H +) and hydroxyl ions (OH -), lead the roots produce electrochemical corrosion.Even if dry to wet, it is impossible to make lead recovery.
2, the consequences of silver ion migration
Inorganic capacitor are mostly USES the silver electrode, half seal capacitor on the condition of high temperature, infiltration to produce electrolytic capacitors within the water molecules.At the anode oxidation reaction, silver ions and hydroxyl ions with production hydroxide silver;In the cathodic reduction reaction, hydrogen react with hydrogen ions to produce silver silver oxide and water.Due to the electrode reaction, the silver ion to the cathode anode reduction into discontinuous metallic silver grains, rely on water film connection as tree shape to the anode.Silver ion migration is not only occurred in the surface of inorganic medium, can also spread to the inorganic medium, caused by leakage current increase, serious when can use full short circuit between the two silver electrode, lead to capacitor breakdown.
Ion migration can be severely damaged silver positive electrode surface layer, lead solder joints and silver electrode surface layer between the separated by silver oxide with characteristics of semiconductor, make no dielectric capacitor equivalent series resistance increases, the metal part of the loss, the loss tangent value of capacitor.
Due to the positive electrode active area is reduced, electric capacity of capacitor will decline.Surface insulation resistance by inorganic capacitor medium between the two electrodes on the surface of silver oxide semiconductor and reduced.Silver ion migration serious when, set up between the two electrodes of the dendritic silver bridge, the insulation resistance of the capacitor declined dramatically.
To sum up, the silver ion migration will not only make the seal inorganic capacitor performance deterioration, and may cause the dielectric breakdown voltage drop, resulting in capacitor breakdown.
It is worth mentioning: silver electrode low-frequency monolithic ceramic capacitor because of the failure phenomenon, caused the silver ion migration than other types of ceramic capacitor is much more serious, the reason is that this kind of capacitor a firing process with multiple layered structure.Silver electrode is a sintering process with ceramic media, silver participated in the solid phase reaction on the surface of the ceramic medium, entering into the porcelain - silver contact interface layer.If the ceramic medium dense enough, the water infiltration, silver ion migration can not only occurred in the surface of ceramic dielectric, possibly through ceramic dielectric layer.Cascade structure of aperture is more more, the electrode position not accurate, small amount of left side on the surface of the medium and laminated layer coated on both ends of the outer electrode silver slurry infiltration aperture, reduces the medium surface insulation resistance, and shortening the path between the electrodes, prone to short-circuit phenomenon when the silver ion migration.
3, the breakdown mechanism under the condition of high temperature ceramic capacitor
Half a sealed ceramic capacitor on the condition of high humidity environment, occurrence breakdown failure is a common serious problems.The breakdown phenomenon of approximately can be divided into interelectrode dielectric breakdown and surface flashover breakdown.Dielectric breakdown can be divided into the early in the morning and evening time breakdown two breakdown and aging, early breakdown exposed the capacitor dielectric materials and production process of defects, these defects in ceramic medium dielectric strength significantly reduced, so that in high humidity environment under the action of electric field, capacitor or work early in the process of pressure test, produce the electric breakdown.Ageing breakdown mostly belong to the category of electrochemical breakdown.Due to the migration of silver ceramic capacitors, ceramic capacitors, electrolytic ageing breakdown has become quite a common problem.Silver migration, formation of conductive branch increased leakage current local, can cause thermal breakdown, fracture or burned the capacitor.Thermal breakdown phenomenon occurred in tube or more wafer form small ceramic capacitor, because of the severe breakdown when local heat thinner wall or small porcelain body burned or break easily.
In addition, the ceramic medium is given priority to with titanium dioxide, load conditions may also produce titanium dioxide reduction reaction, titanium ion by tetravalent trivalent becomes.Ceramic dielectric aging significantly reduces the capacitor dielectric strength, can cause a capacitor breakdown.Therefore, this kind of ceramic capacitor electrolytic breakdown phenomenon than does not contain titanium dioxide ceramic capacitor is more serious.
Silver ion migration serious distortion in the capacitor electrode edge electric field, and because of high humidity environment ceramic dielectric surface condensation water film, a significant reduction in the edge of the surface corona discharge the capacitor voltage, working conditions to produce surface electrode arc phenomenon.Serious when flying capacitor electrode surface arc breakdown.Surface breakdown and capacitor structure, polar distance, load voltage, protective layer of hydrophobic associated with factors such as moisture permeability.Edge on the surface of electrode arc breakdown is the main reason, medium and small amount, when you are working in a damp environment of water film formation on the surface of silver ion migration and edge surface of the capacitor insulation due to the generation of silver ion migration and development needs a period of time, so early in the withstand voltage test, the failure mode is given priority to with dielectric breakdown, until the test after 500 h, as long as the failure mode is excessive to the edge of the surface electrode arc breakdown.
4, the improvement of electrode materials
Ceramic capacitors have been using silver electrode.Silver ion migration and thus caused as a result of the accelerated aging of containing titanium ceramic dielectric ceramic capacitor is the main cause of failure.Some manufacturer to produce ceramic capacitor has no silver electrode, and use nickel electrode, use chemical nickel plating technology on ceramic substrate.Due to the chemical stability is better than silver nickel, electric mobility is low, to improve the performance and reliability of ceramic capacitor.
And as to silver electrode of monolithic ceramic capacitor at low frequency, due to the silver electrode and porcelain porcelain owe to burn when the next sintered at 900 ℃ can't obtain the density of ceramic dielectric, larger porosity;Besides silver electrode used cosolvent barium oxide can infiltrate into the porcelain body internal, rely on barium oxide at high temperature and silver, good ability of infiltrating "mutual fusion" between electrode and medium internal appear thermal diffusion phenomenon, namely macro to see "porcelain silver" phenomenon.Silver with barium oxide into the porcelain body, greatly reducing the effective thickness of the thin dielectric, the decrease of the product insulation resistance and reliability.In order to improve the reliability of monolithic capacitor, switch to silver - palladium electrode instead of the usually contains barium oxide electrodes, and the material formula by adding 1% of # 5 beads.Eliminates the next time in high temperature sintered metal electrode to the ceramic dielectric layer of thermal diffusion phenomenon, can cause porcelain sintering densification, makes the performance and reliability of the products are improved greatly, compared with the original technology and medium material, the reliability of the capacitor improved 1 ~ 2 orders of magnitude.
5, multilayer ceramic chip capacitor MLCC
MLCC is the most common failure fracture, this is amultilayer ceramic chip capacitor MLCC medium the brittleness of the decision itself.Because of laminated ceramic capacitor welding on the circuit board directly, take direct from a variety of mechanical stress, circuit boards and wire type ceramic capacitor will be pin to absorb the mechanical stress from the circuit board.Formultilayer ceramic chip capacitor MLCC, therefore, because of the different thermal expansion coefficient or PCB bending caused by mechanical stress will be the most important factor of laminated ceramic capacitor fracture.
6, multilayer ceramic chip capacitor MLCC
multilayer ceramic chip capacitor MLCC machinery after fracture, the fractures of electrode spacing will be lower than the breakdown voltage, insulation can lead to arc discharge between two or more electrodes and completely damaged multilayer ceramic chip capacitor MLCC.
multilayer ceramic chip capacitor MLCC mechanical fracture method to prevent as much as possible to reduce the PCB bending, reduce the stress of the ceramic tiles capacitance in circuit boards, decrease the laminated ceramic capacitor and the circuit board of the thermal expansion coefficient of the difference caused by mechanical stress.
How to reduce the stress of the multilayer ceramic chip capacitor MLCC in the circuit boards will otherwise described in the following, go here.Reduce the multilayer ceramic chip capacitor MLCC and the circuit board of the thermal expansion coefficient of the difference caused by mechanical stress, you can select package size of capacitor to slow down, such as aluminum PCB should as far as possible in less than 1810 encapsulation, if more than enough can use the method of parallel capacitance or adopt the method of lamination to solve, also can use a pin to encapsulate forms of ceramic capacitor is solved.
7,multilayer ceramic chip capacitor MLCC electrode end was melted
In wave soldering welding multilayer ceramic chip capacitor MLCC electrodes may appear when the end was solder melt away.The main reason is wave soldering laminated ceramic capacitor contact with high temperature soldering time is too long.Now in the market of laminated ceramic capacitor is divided into suitable for reflow soldering process and is suitable for wave soldering process, if will be suitable for reflow soldering process of laminated ceramic capacitor used in wave soldering and likely multilayer ceramic chip capacitor MLCC electrode end of melting phenomenon.Under different welding process of multilayer ceramic chip capacitor MLCC electrode end can withstand the high temperature of soldering time characteristic, in the back of the multilayer ceramic chip capacitor MLCC suitable precautions in detail, it is not in now.
Elimination method is simple, is in the use of wave soldering process, use as much as possible in wave soldering process of multilayer ceramic chip capacitor MLCC ;Or do not use wave soldering process as much as possible.