Electronic Restoration >> Tools For Restoring Flood Damaged Electronics

Other manufacturer-supplied test data shed light on the typical behavior of tantalum polymer capacitors after exposure to various environmental stresses. Sanyo provided data that describe the typical response of their capacitors to dry heat, moisture, solder reflow conditions, and Tools For Restoring Flood Damaged Electronics DC lifetest at rated voltage and 125oC.

Data for two device types appear in Figure 24. The data of Figure 24 demonstrate that tantalum polymer capacitors absorb some moisture from the ambient environment and Tools For Restoring Flood Damaged Electronics this moisture affects the capacitors’ electrical behavior. Moisture absorption is also common MnO2-based tantalum capacitors that have plastic-encapsulation. 

The effect of the moistureis inferred by detecting parametric electrical shifts after drying the capacitors at 125oC for an unspecifiedperiod. Removing the moisture by drying at 125oC causes the capacitance and dissipation factor to dropslightly and Tools For Restoring Flood Damaged Electronics causes the DC leakage current to increases moderately. ESR is largely unaffected.

After this dry-out step, the capacitors are exposed to 60oC, 60%RH moisture conditions for 120 hours to Tools For Restoring Flood Damaged Electronics simulate controlled exposure to atmospheric moisture. Not surprisingly, this moisture exposure increasesthe capacitance and dissipation factor while reducing the DC leakage current. Again, ESR remains stable.

It is significant to note that DC leakage current typically decreases with moderate exposure to moisture and Tools For Restoring Flood Damaged Electronics increases as the capacitor dries out. The precise cause of this DC leakage current behavior is unknown andis typically opposite of the response for MnO2-based tantalum capacitors. However, the effect oncapacitance, DF, and ESR is similar to what is expected for MnO2-based tantalum capacitors. 

After moisture exposure, Tools For Restoring Flood Damaged Electronics the capacitors are exposed to two reflow profiles having peak temperature of250oC. The high peak temperature is sufficient to drive off substantial internal moisture as is reflected inthe reduction of capacitance and dissipation factor to levels similar to those observed in the pre-test state.

DC leakage current appears to decline slightly or stay the same. ESR increases slightly, Tools For Restoring Flood Damaged Electronics almost certainlydue to thermo-mechanical stresses on the interfaces among the cathode layers. These preliminary stressesare followed by a steady-state DC lifetest at full rated voltage and 125oC. All parameters remainsubstantially stable for 1000 hours.

KEMET provided 0, 500, and 1000 hour data for two tantalum polymer part types that were exposed to85oC, 85%RH humidity conditions with and Tools For Restoring Flood Damaged Electronics without rated voltage applied. The part were 470µF, 4V and47µF, 16V capacitors. Data for the humidity test with rated voltage applied appear in Table 1 while thepassive humidity data appear in Table 2. 

Performance for both humidity tests (with and without voltage applied) is comparable. As was observed inthe Sanyo data, Tools For Restoring Flood Damaged Electronics capacitance increased with exposure to humidity. For the 4V parts the increase was 12-13%, similar to the dry-to-wet capacitance change for the 4V Sanyo parts in Figure 24. The capacitance ofthe 16V KEMET parts increased about 8%.

Dissipation factor fell slightly after the initial measurements and then remained substantially stable. This is Tools For Restoring Flood Damaged Electronics different from the increased DF for the Sanyo capacitors after moisture exposure. The reason for thisdiscrepancy is unknown and may represent a measurement error during the zero-hour measurements. ESR was substantially stable for the 4V parts and increased slightly for the 16V parts. 

DC leakage wassubstantially stable. Although the test conditions were somewhat different for the Sanyo and KEMET tests, Tools For Restoring Flood Damaged Electronics overall performance was comparable. An important point must be made regarding the passive humidity test data of Table 2. The data indicate little degradation of performance after 1000 hours of exposure to 85oC, 85%RH. 

It must be noted thatthese data represent substantially superior performance of tantalum polymer capacitors versus MnO2-basedtantalum capacitors. MnO2-based tantalum capacitors typically perform poorly on passive humidity tests Tools For Restoring Flood Damaged Electronics and suffer substantially increased DC leakage after 1000 hours at 85oC, 85%RH. 

This DC leakage currentincrease is typically blamed on the migration of silver ions to the surface of the dielectric through porosityin the MnO2 cathode layer.The Tools For Restoring Flood Damaged Electronics reasons for the superior performance of tantalum polymer capacitors on this test are not known withcertainty. Some speculate that the polymer cathode coating is less porous than the MnO2 layer andphysically blocks migration of silver ions to the dielectric’s surface. 

Others think that the effective surfacecharge of the dielectric is altered by the polymerization process such that silver ions are repelled from the dielectric. Summary, Suitability for Space Applications, and Path ForwardThe primary objective of this FY05 task is to introduce the tantalum polymer capacitor technology and Tools For Restoring Flood Damaged Electronics contrast it with the well-established MnO2-based technology. 

As a foundation for this introduction, theorigins of the solid tantalum capacitor are reviewed Tools For Restoring Flood Damaged Electronics and the basics of its internal construction are described.The case for lower ESR is made and the original MnO2 cathode material of the solid tantalum capacitor isidentified as a major contributor to total device ESR. 

Various candidate materials are identified aspotential replacements for MnO2, and the conductive polymer PEDT is identified as the material of firstchoice, Tools For Restoring Flood Damaged Electronics while PPY is identified as a strong competitor. Processing options for creation of the conductivepolymer are identified and typical assembly steps for the resulting capacitors are photographically documented.

Typical electrical characteristics of tantalum polymer capacitors are then presented. It is emphasized thatthe only significant electrical performance differences versus MnO2–based capacitors are the desired 2 to 3times reduction in ESR and an undesired, Tools For Restoring Flood Damaged Electronics but manageable increase in DC leakage current. 

In spite of thehigher DC leakage current, Tools For Restoring Flood Damaged Electronics the general robustness of the dielectric of lower-voltage tantalum polymercapacitors is demonstrated to be significantly superior to that of MnO2-based capacitors and this robustnessreflects itself in superior long-term reliability on DC lifetests. This advantage does not hold for highervoltage devices. 

Finally, tantalum polymer capacitors are shown to react to environmental stresses in agenerally predictable fashion, Tools For Restoring Flood Damaged Electronics but appear to be more robust than MnO2-based capacitors to passivehumidity exposure.Suitability for Space Applications Some observations are in order regarding the suitability of tantalum polymer capacitors for use inspacecraft. The pertinent environmental factors are radiation and vacuum.


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