Shelf life of 80's era axial tantalum capacitors? Compared to dipped tantalums? - Page 1
Forum section: electronics / repair

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 16, 2025, 01:28:26 am
content: My question is this - if stored in a benign environment unused (at least since I've had them here), are these capacitors considered usable/suitable for repairs?(background)Lately I've been on a tear repairing 70s and 80s era lab bench RF electronics (sweeper, spectrum analyzer, network analyzer, frequency counter, power meter, bench power supply, and such). None of the equipment was owned since new; it was all salvage rescue type things dating back 20 or 30 years (back when shipping costs weren't as onerous), and time has come to fix or dispose (fortunately, "fix" has been the outcome.) More often than not detailed technical manuals (down to the component level) are in hand, so these repair tasks have been very good learning experience.A principal failure mode is shorted tantalum capacitors (usually across a power supply rail, but sometimes elsewhere.)In my parts bin (assembled since the 1990s) I have some batches of axial tantalum capacitors (new); where there are date codes, 1980s seem to be the main timeframe. Sprague is one of the manufacturers represented (scanned catalog pages from the 60s or 70s shows the part numbers.) The parts I have range from 6V to 50V voltage and from 0.22 to 22 uF capacitance (usually the range needed for parts replacement.)It seems dipped tantalum caps are more readily available nowadays, however, the axial lead package was sometimes (but not always) used when these machines were manufactured. Axial lead parts seem scarce (and expensive) among US contemporary distributors, and auction site purchases seem would result in no different result than I now have (parts here vs. in some storage somewhere, but of same age range.)My question is this - if stored in a benign environment unused (at least since I've had them here), are these capacitors considered usable/suitable for repairs?

author: garrettm
author_rank: Frequent Contributor
timestamp: February 16, 2025, 04:25:05 am
content: So there are generally three types of packaging for tantalum caps: plastic encapsulation (modern parts), epoxy dipped (old/modern cheap parts) and hermetic cans (old expensive parts).To see if your tantalums are still good, first check their ESR @ 100kHz with an LCR meter, if they pass, then do a "burn-in test": hook up your tants to a power supply with 80% of their max voltage rating for 24 to 72 hours. (You can also check them for high DC leakage relative to the data sheet spec.) If they pass both tests, you are probably good to go (but they can still fail due to in circuit conditions that stress the SOA). To ensure they have long life, never allow reverse voltage greater than -1V and derate to 50% of the max DC voltage. If placing them in series, always use a balancing resistor across them (prevents sustained over voltage from charge imbalances).A while back, I bought a lot of 100 metal can type tantalums manufactured around 1980 and only 2 or 3 had "high" ESR (100 miliohm higher, relative to the rest) and all passed my burn-in testing. None of these parts have failed in the projects or equipment I've used them in.Some technical notes:https://www.kyocera-avx.com/docs/techinfo/Tantalum-NiobiumCapacitors/revtant.pdfhttps://www.kyocera-avx.com/docs/techinfo/VoltageDeratingRulesforSolidTantalumandNiobiumCapacitors.pdfhttps://www.vishay.com/docs/49268/tn0003-dcleakfailmode.pdfhttps://www.vishay.com/docs/40292/deratantcapsdeponcathsys.pdf

author: ME
author_rank: Frequent Contributor
timestamp: February 16, 2025, 10:50:06 am
content: The solid can type tants seem eye wateringly expensive, i saw some for plessey rt-320 hf mill backpack radio synth replacement, dry tants advertised for £75 each!.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 16, 2025, 10:51:32 pm
content: garrettm, thank you VERY much for the highly detailed and informative reply! Very very helpful indeed, your time is greatly appreciated!Yes, eye-watering expensive. There's no requirement to repair with identical parts (this is a hobby and the various units were saved from likely fate in landfill/metal recycling), however, axial lead Tantalum parts aren't all that common, so before I mechanically adapt to a contemporary package, I'm exploring what the issues might be, and for that I've received more/better than I asked for!Thanks so much again,Dave