The well known epoxy moulded SMT model for P-P is a blessed component when space is limited on the boards. There are several manufacturers, a good uF and VDC range, and we should be happy. But some get trouble from the start. From a population of 1,000 caps, some individuals will pop as soon as voltage is switched on. Remaining caps may work well for thousands of hours. If you study the early life of tantalytes, slow increase of voltage on Curve Tracer, you'll see small transients when the cap is charging. Means minor breakthroughs of the dielectric. After some resting, you can increase voltage again, and also find that the leakage current is quite low and normal, i.e. microamps. The phenomenon is called 'selfhealing' and is well known, not to say the way a tantalyte works. However, if the inrush current is not limited by a suitable series resistance, the energy may overrun the selfhealing capacity, and you'll get a permanent damage. Gives you a unacceptable leakage or even !
a short, or in worst case, an explosion. Such is the situation. The caps have kind of Akilles heal, and most cap manufacturers tell us as users how to handle the small creeps. E.g. after long storage (not specified) the caps may have to be 'formed' with a kind charging, before they are ready to work. Another example is that you must use a series resistance, say some 1 Ohm/VDC, in order to limit inrush currents, that can damage the dielectric. And derate vs temp, give enough cooling, limit ripple etc... We know all this since years back, but also that many of us as designers do not follow the recommendation, but want to use the full charge/discharge capacity without to much series resistance. The result is that some caps will shorten.
This was the prelude. Maybe slightly in the minor key. Nothing new. What I find remarkable is that you never see a notation on TN about SMT dry tantalytes. One or two exploded caps may not be a trouble in most cases, but for those making HiRel parts, it's not very popular. We speak about 10-30 years life and few events per thousands of operate hours.
The cap manufacturer can feel safe. We don't discuss various brands offically. Instead, I would like to put it like this:
a) are there any more out there with early incident experience (poffs).
b) how do you avoid the problem (screening? series resistors? etc)
c) restricted use of tantalytes
Better suited for SMT forum or IMAPS, but I know there are more of old dogs on TN..myself being over 60....after all, why bother about capacitors...'d better take my Nikon D70, new Sigma 400 and stroll in the forest and focus on living things...sigh..
Ingemar Hernefjord
Ericsson Microwave Systems
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