Electronic Restoration >> Fix A Burned Circuit Board

Once the dielectric's surface is so contacted, the externally-exposed MnO2 is covered with a finely divided carbon layer which is then covered with a conductive metallic shell. This outer shell facilitates a low resistance negative connection to the external circuit via a metallic terminal. The positive connection is typically made by welding a metallic terminal to a stub of tantalum wire that is connected to the Fix A Burned Circuit Board metallic surface of the porous tantalum slug. 

These terminal connections were accomplished with wire leads in earlier tantalum capacitors, but have evolved into surface-mount-compatible terminations in recent years.The finished capacitor is then covered with a protective case. Not only was the new tantalum capacitor smaller, Fix A Burned Circuit Board but it also had better electrical performance and reliability than the existing wet electrolytic capacitors. 

The small size resulted from the pressed, Fix A Burned Circuit Board porous slug made of finely divided tantalum powder. The improved reliability was mainly achieved because theMnO2 electrolyte couldn't escape or "dry out."The first process for manufacturing commercially viable tantalum capacitor was patented in 1960 by Richard Millard of Sprague Electric Company. 

The patent application was filed in 1955, not long after the Fix A Burned Circuit Board device was described by Bell Labs. A significant improvement patented by Millard was the "reform" stepin which the dielectric of the capacitor was repaired after each dip-and-convert cycle of MnO2 deposition.This repair was accomplished by application of voltage while the capacitor element was immersed in asuitable electrolyte. 

This process improvement dramatically reduced the leakage current of the finishedcapacitors. Millard also described ranges of temperature Fix A Burned Circuit Board and time for the MnO2 deposition process thatimproved total achieved capacitance and lowered ESR.A copy of the Millard's drawing of the patented capacitor appears in Figure 1. 

Visible in Millard's drawingare the fundamental construction details described above: central porous slug with dielectric coating filledwith MnO2 (10 and 11), positive leadwire connected to the slug (12), Fix A Burned Circuit Board outer MnO2 coating (14), carbon coat(16), outer metallic electrode (18), negative leadwire (20), and protective case (22).

A schematic drawing of the tantalum capacitor element of Figure 1 appears in Figure 2. The tantalumpowder particles are simplified to spherical shape, magnified, Fix A Burned Circuit Board and arranged in simplistic fashion toexaggerate the porosity available for impregnation with MnO2. The particles are shown partially meltedtogether and to the tantalum wire to form a continuous metallic structure. 

Dielectric appears on theexposed surface area of the tantalum metal and the dielectric is covered with MnO2 both inside and outsideof the porous structure. The external surface of the MnO2 is covered with a layer of carbon and Fix A Burned Circuit Board the carbonis covered with a metallic coat. Sprayed copper was a preferred metallic coating in Millard's patent, but healso mentioned zinc, tin, silver, and gold as suitable alternatives. 

In contrast with the schematic drawingsof Figures 1 and 2, Fix A Burned Circuit Board a high-magnification picture of the actual internal structure of a tantalum capacitorelement appears in Figure 3.Conventional MnO2-based tantalum capacitors are constructed in substantially similar fashion today. 

The most notable differences involve today's use of highly pure, more finely divided tantalum powder forincreased surface area (higher capacitance) in a given volume, Fix A Burned Circuit Board use of silver-filled paint for the metalliccoating, and use of the "chip" packaging configuration which lacks conventional leadwires but instead hasshort surface-mount terminations. 

Some typical high-reliability military-style surface-mount tantalum chipcapacitors appear in Figure 4. A more detailed description of the materials and assembly of modern tantalum capacitors appears in a later section. The compelling characteristics of tantalum capacitors are small size, high reliability, Fix A Burned Circuit Board and good parametricperformance over broad ranges of frequency and temperature. 

As mentioned earlier, the small size oftantalum capacitors results from the porous pressed powder structure of the capacitor element. Moderntantalum capacitors are fabricated from Fix A Burned Circuit Board powders that can deliver very high electrical charge storage characteristics when compared to other competing capacitor technologies.

Volumetric EfficiencyTantalum powder "charge" is specified as a capacitance-voltage product called CV. A typical high-chargepowder might be advertised as having 50,000 CV/gram of charge. What this means is that under ideal circumstances, Fix A Burned Circuit Board one gram of this powder should be sufficient to produce a capacitor having a product ofcapacitance times dielectric formation voltage of roughly 50,000 µF-volts. 

The dielectric formationvoltage is the maximum voltage achieved across the dielectric when the dielectric is created on the exposedsurface of the tantalum metal during chemical anodization in a suitable electrolyte. During dielectricformation, tantalum ions and oxygen ions unite and "form" the dielectric tantalum pentoxide after they Fix A Burned Circuit Board travel to roughly the midpoint of the existing oxide layer under the influence of an applied electric field.

If such a 1-gram capacitor element made from 50,000 CV/g powder had dielectric formed to 50 volts, itshould ideally achieve a capacitance of 1000µF. Since the dielectric formation voltage is roughly 3-4 timesthe final rated voltage of the capacitor, Fix A Burned Circuit Board this would be roughly a 16V rated tantalum capacitor.

Unfortunately, it's difficult to achieve the advertised charge and still have a reliable capacitor, but it's notuncommon to achieve a substantial fraction of the advertised charge. Early tantalum capacitors were made from powders having fairly low charge, Fix A Burned Circuit Board roughly in the neighborhoodof 1000 CV/g. Such powder could be used for making capacitors with fairly high rated voltages (>50V).

Some modern powders claim to exceed 100,000 CV/g, but are limited to use at relatively low voltagesbecause the powder particles are so small that formation of the dielectric consumes the tantalum that Fix A Burned Circuit Board connects the individual particles together when the formation voltage exceeds a predictable value.

Nevertheless, low-voltage capacitors with remarkably high capacitance are now possible, Fix A Burned Circuit Board and developmentof even higher-charge powders and processing methodology continue. It was stated earlier that much of the reliability improvement possible with tantalum capacitors was due tothe use of solid electrolyte that couldn't escape from the capacitor. 

This is certainly true in comparisonwith wet-electrolyte aluminum capacitors which have predictable wearout based on the leak rate of thepackage seal. But there is another factor contributing to the good reliability of tantalum capacitors. This Fix A Burned Circuit Board factor is the inherent self-healing properties of the MnO2 dielectric coating.

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