Backwards Tantalums

We all are aware that the tantalum capacitor is a polar device and must be oriented properly to be functional.  However the question does arise from time to time concerning tantalum capacitors that are mistakenly assembled backwards.  There also can be questions about reliability if there is reverse voltage applied to the capacitor for a short period of time.  These questions do not have a straightforward and clear answer.
The tantalum capacitor begins by sintering a porous tantalum powder to form the anode of the capacitor.  The dielectric layer, Ta2O5 is then grown using an electrochemical process and the thickness depends on the electrolyte chemistry and the forming voltage.  A higher forming voltage results in a thicker dielectric and a high rated voltage part.  The cathode has typically consisted of MnO2 although lately polymers are also used. For more detailed constructional details, consult reference 1 or similar articles.
If a tantalum capacitor is reversed biased there is an increased leakage current and the voltage/current curve resembles a diode curve.  There are various physical theories to account for this asymmetrical behavior.  John Prymak, Kemet, explains this as a reduction of the Ta2O5 resulting in Ta nodes extending into the dielectric layer (ref 2). Alexander Teverovsky, NASA, details asymmetric metal-insulator semiconductor structure to explain the experimental data (ref 4).  There are other theories as well. Whatever the correct theory, what does the experimental data say about reverse biased tantalum capacitors?  The electrical characteristics depend on the magnitude of the reverse bias voltage.  If the reverse voltage is small, then there is almost no leakage in leakage current and erratic changes in current are not seen. How small is small?  It has been measured (ref 4) than reverse voltages as low as 6% of the rated voltage can result in erratic changes in leakage current.  There is a two stage degradation process and this erratic change in leakage current is a precursor to an actual hard failure.  During the first state where there the leakage current increases, there is a period of stability which could extend for 100s of hours.   Interestingly, the effects of the reverse current at this state are reversible.  That is, applying a forward voltage (or even letting it sit unbiased) will reverse the degradation process if it hasn't progressed too far.  If the reverse bias is applied to long periods of time, the stage 2 degradation process occurs and the part is permanently damaged.  Studies have shown that there are variations is reverse bias characteristics across different parts and lots of parts.  Some lots could survive reverse bias voltages of 15% for many hours before entering into stage 2 failure.
So if a tantalum capacitor is installed backwards in an assembly, could it make it through the test process and get to the end customer?  The answer is yes, it is possible. Let’s say the circuit design uses a 50% derating so the reverse bias voltage would be is 50% of the rated voltage or below.  See reference 3 where studies were performed at both 50% and 25% reverse bias voltage. At 50% reverse voltage, most of the capacitors failed within 15 minutes of the test.  At only 25% reverse voltage most of the capacitors failed within 250 hours.  So then, depending on the percentage of reverse bias voltage and the length of the test process, a tantalum capacitor may or may not failure during product testing.
References
1. J. Gill, Basic Tantalum Capacitor Technology, AVX Technical Brochure
2. J. Prymak, A Theory of Reverse Voltage Failures in SMT Tantalum/Electrolytic Capacitors, 2002 CARTS
3. A. Teverovsky, Reverse Bias Behavior of Surface Mount Solid Tantalum Capacitors, CARTS 2002
4. A. Teverovsky, Effect of Reverse Bias Stress on Leakage Currents and Breakdown Voltages of Solid Tantalum Capacitors, 2011 CARTS

Hot Melt Conformal Coating?

The Ohio Chapter of the SMTA met on September 18th the meeting was conformal coatings: surface preparation, material selection, application and rework. Jim Stockhausen of Elantas PDG, Inc. presented on material selection. Jim briefly mentioned a class of materials that could be called “hot-melt” conformal coating. These are materials that could be applied either with a dispensing machine or with a hot melt gun. The reason I was interested in this material is that there are times that a spot coating is needed over a specific component or area of the assembly. Dispensing material with a hot melt gun over a specific component could be a better solution than either brushing on a traditional coating or sending the assembly through an in-line conformal coating process.

The hot melt conformal coatings are different that the common hot melt adhesives. One such material is Bectron MR 3406. This coating material is based on polyolefin chemistry in contrast to the conventional polyamide based materials used for adhesives. This results in the coating having better adhesion and absorbing less moisture compared to the adhesive material. The MR3406 is rated to 130C and has low humidity absorption. It is resistant to acids, solvents and fungus growth. It generally meets the IPC-CC-830 tests although it is not rated to this specification because there is no category defined for this material. Rework is easy because it can be softened and peeled away.

I do have a couple of concerns. First, since the material softens at 150C, the temperature at the nozzle must by about 190C-200C. Although the coating cools quickly, is there a danger of thermal shock damage to the component or PCB? Also the adhesion to PCB surfaces with flux residues would need to be tested. I don’t know of any immediate application for this material but it could potentially be used in spot coating applications.