Tantalum caps: Still problematic? - Page 1
Forum section: electronics / projects

author: ubbut
author_rank: Regular Contributor
timestamp: October 11, 2019, 10:09:44 am
content: Hi,looks like we need to replace 0805 10µF ceramic caps with tantalum caps. (Reasons are outlined below). I'm am aware tantalum have the reputations of failing spectacularly*, but I wonder if this is still the case today? These are the parts we tested our circuit with and intend to use:https://www.rohm.com/products/tantalum-capacitors/conductive-polymer-capacitors/tcto/tctop/tctop1a106m8r-productthanks*and also of having negative impact on the environmentReasons:We have a voltage reference, but need half that voltage for bias purposes. This is achieved with a simple resistor divider, followed by a 10µF ceramic (X7R) cap. What could go wrong?The boards are already manufactured and ready to be assembled. And of course now we realized there is a problem.It is not the voltage dependency of the caps that causes problems, but their temperature stability.The board warms up a bit when powered, about 50°C, if I now blow some air on it to cool it down, the voltage at the divider changes for a few seconds and then goes back to its original value. This is of course because the capacitor changes its capacitance which in turn changes the voltage at the divider for a short time.Electrolytic caps do not show this problem. But these are not available in 0805 (as far as I know).Not wanting to re-spin the board I tried tantalums and they also do not show the temperature dependency (at least not measurably in this setup)

author: tggzzz
author_rank: Super Contributor
timestamp: October 11, 2019, 10:23:48 am
content: There are other possible explanations for what you observe.A decent diagnosis will involve removing adjectives (e.g. "bit") and replacing them with numbers, a schematic, a photo of the relevant part of the board, and preferably an infra-red image of the hot board.The quick test you should do is to remove the capacitor, and observe if the problem is still there. It may be necessary to use a different (external) source for the half reference bias voltage.BTW, a 10uF X7R is unlikely to be 10uF when there is a significant voltage across it. Some lose 80-90%(!) of their capacitance when at their rated voltage.

author: wraper
author_rank: Unknown
timestamp: October 11, 2019, 10:24:16 am
content: Tantalum capacitors (except tantallum polymer) need voltage derating. At least 50%, preferably 70% derating.QuoteYou probably don't need 10uF and much smaller capacitance (with less dependency) would suffice. Effect would be less noticeable also due to lower capacitance as such. Also you could simply reduce resistance of divider.

author: ubbut
author_rank: Regular Contributor
timestamp: October 11, 2019, 10:38:08 am
content: Quote from: tggzzz on October 11, 2019, 10:23:48 amThat has of course been done and the capicitor also replaced with an electrolytic one (as described above). The effect vanishes.Quote from: tggzzz on October 11, 2019, 10:23:48 amWell aware of this, but the absolute capacitance is not that relevant here. The dynamic capacitance is the problem. Also only 2.5V, so not as much an issue.Quote from: wraperI do, this is to reduce noise (of the resistors themselves).

author: Ian.M
author_rank: Super Contributor
timestamp: October 11, 2019, 10:45:13 am
content: @ubbut,Tantalums were always problematic*if run close to their rated voltage or from a low impedance high current power rail, or if passing high pulsed currents. Provided you've chosen one with a rated voltage of *AT* *LEAST* double (and preferably three times) your max working voltage, none of those conditions apply in your circuit.N.B. yourMouser.delinkabove iswasborked - it appears to contain a session ID and stripping that still wasn't valid.However searching for the part number found it on the manufacturer's website:https://www.rohm.com/products/tantalum-capacitors/conductive-polymer-capacitors/tcto/tctop/tctop1a106m8r-productand the datasheet and faq are particularly helpful.* with a significant risk of loss of magic smoke and unintentional illumination!

author: ubbut
author_rank: Regular Contributor
timestamp: October 11, 2019, 10:57:58 am
content: Thanks, the FAQ is great.So quick checklist:Voltage rating is 10V, actual voltage 2.5V. So 4 times --> OKImpedance is several kOhms --> OKQuote from: Ian.M on October 11, 2019, 10:45:13 amThanks, fixed the link in my first post

author: Gyro
author_rank: Super Contributor
timestamp: October 11, 2019, 11:57:40 am
content: Being on the output of a reasonably high resistance voltage divider sounds like a very benign environment for a Tant. They usually fail due to overvoltage or excessive ripple current being demanded of them. I'm sure 2x voltage rating would be fine in this situation and that mainly to minimize leakage current.