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| Created: 07/27/2004 |
Product: Ceramic; Tantalum |
| Updated: 11/09/2005 |
Category: Failure
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What are the typical failure modes of your military capacitors? Can they ignite or burn? |
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Description :
What are the typical failure modes of your military capacitors? Can they ignite or burn?
Answer :
The typical failure mode of ceramic and tantalum capacitors is the short circuit.
Tantalum: Certain types of electrolytic capacitors, such as some tantalum types, use metallic slugs or foils, which can ignite in the presence of high temperature (as from a fault current in overstress) and in the presence of oxygen (typically supplied by overheated counter-electrode material, such as MnO2). However, also offers a surface mount tantalum (our organic “KO”) that is quite resistant to such ignitions. These parts use an organic polymer as the counter-electrode, which reduces the fault current and also does not release oxygen. Additionally, KEMET offers a line of SMT tantalums which incorporate an integral fuse, which changes the failure mode from short circuit to open circuit.
Ceramic MLCC: Ceramic MLCC’s do not ignite or burn in the traditional sense, since the ceramic chip does not contain materials capable of igniting in use. However, an MLCC can be damaged (flex cracks, thermal shock cracks, mechanical impact cracks, dielectric flaws), and this can allow the passage of large current levels, particularly in a low impedance, moderate voltage application. Under such circumstances, localized fault current can flow, causing joule heating at the fault site. This focused current can cause high temperatures at the fault site, and this can result in fracture, along with melted ceramic and metal components. While the capacitor chip does not ignite, the heat can ignite the underlying glue dot in cases of SMT mounting. In extreme cases, the capacitor may explosively rupture. In the case of selected SMT chips, it is possible to reduce the probability of the most common failure cause - flex cracks - by using a specially designed "flex-open" chip, which changes the characteristic short-circuit flex failure mode to an open-circuit. Once again, you can contact KEMET Sales to learn more about these parts.
Ceramic Leaded: These products can suffer a short through the failure mechanism described under “Ceramic MLCC”. In addition, they may experience dendritic growth if board assembly techniques promote the ingress of flux and other ionic products. Dendrites are encouraged by the presence of moisture, ionic materials, voltage, and time. The most common root cause is a radial leaded part inserted with the conformal coating down into the hole. In such cases, multiple factors can drive the board assembly flux up into the capacitor assembly where it promote the formation of dendrites. The factors include the type of flux (chemistry of activator, wetting ability/surface tension), the amount of conformal coating down in the hole (where the flux can wick up if the epoxy is cracked or damaged), and most importantly - the wash method (where high pressure or high temperature, coupled with contamination of the wash solution, can result in flux ingress.)
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