Reliability Issues with Polymer and MnO2 Tantalum Capacitors

for Space Applications

Alexander Teverovsky, Jacobs Engineering, Inc, NASA GSFC published a presentation that provides a comparative analysis of degradation processes, failure modes and mechanisms in MnO2 and polymer technology tantalum capacitors. Analyzed conditions include effects of vacuum and radiation, soldering (pop-corning), long-term storage, operation at high temperatures, stability at low and high temperatures, and anomalous transients. Screening and qualification procedures to assure the space-grade quality of conductive polymer tantalum capacitors (CPTCs) are suggested.

Effect of Moisture

• CPTCs are more sensitive to moisture compared to MnO2 caps.
• Capacitance variations can reach 40% and DCL >10^4times.

Failures after Soldering

• Pop-corning, due to moisture increases delamination, introduces cracks in the package and might damage Ta2O5 dielectric.
• Cracks in packages facilitate penetration of oxygen that increases the rate of ESR degradation in CPTCs.
• Damage to dielectric causes first power-on failures in MnO2 capacitors. The effect has not been observed yet in CPTCs.
• Damage caused by soldering is lot-related.
• Pop-corning issues can be resolved by baking.
• Requirements for MSL testing should include measurements of ESR and surge current testing.
• Decrease of C in CPTCs is greater than in MnO2 capacitors.
• Soldering increases ESR in most types of capacitors, but the level of variations is lot-related.
• Soldering results in drying off capacitors by 50 to 93%

Effect of Vacuum

Drying in a vacuum has a similar effect as drying in the air:

• Decreasing of capacitance and DF;
• A relatively small changes in ESR;
• Variations of C and DF with V;
• Increasing of transient leakage currents, especially at low T.

Life Testing of Tantalum Polymer Capacitors

• No catastrophic failures during life testing and SSLT in 23 lots.
• CPTCs can operate reliably at high T at steady-state conditions.
• Increasing leakage currents with time is similar to MnO2 caps.
• Post-test DCL measurements might fail the limit.
• Erratic behaviour of currents in some samples/lots.

Recommendation for Specification & Qualification

• Surge current testing. The existing MIL-PRF-55365 requirements limiting maximum current after 1 mseccan be used for CPTCs.
• Burning-in at 105 ºC 1.1VR for 40 hours.

• Life testing at 105 ºC, 1.1VR for 1000 hr.
• High-temperature storage test, 1000 hrat 125 ºC.
• Thermal shock, 100 cycles between -55 and +125 ºC.
• Testing after baking at 125 ºC for 168 hours:
  • Surge current test at -55 ºC, 25 ºC, and +85 ºC.
  • Stability at low and high temperatures (including DCL at low temperatures).
  • Power cycling 100 cycles at RT and 0.75VR (5 sec ON/OFF using a power supply capable of rising voltage is less than 1 msec).

Source: NEPP.NASA.GOV