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In researching whether or not it's common practice to run traces/place vias underneath electrolytic capacitors (specifically SMD electrolytics, didn't seem like it would be an issue to me), I came across this document which makes some statements about electrolytic capacitors that conflict with with other information I've found.

First: "For circuits where the polarity is occasionally reversed, use a bi-polar type of aluminum electrolytic capacitor. However, note that even bi-polar type capacitors must not be used for AC circuits."

I thought one purpose of using bipolar electrolytics is where high capacitances are required in AC circuits.

Then: "Do not print any copper trace under the seal (terminal) side of a capacitor. Copper traces should be 1 mm (preferably 2mm or more) spaced apart from the side of the capacitor body.

· In designing a double-sided PC board, do not locate any
through-hole via or unnecessary hole underneath a capacitor.

· In designing a double-sided PC board, do not print any
circuit pattern underneath a capacitor."

On every board I've seen with leaked electrolytic capacitors, the electrolyte leaked much further out than just 1mm from the capacitor body. I get that maybe in the ideal case it would be best not to route traces under caps in case they fail and leak, but is this practice commonly adhered to? Is it really a bad idea to route traces/via under caps?

What is your take on the above statements?

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  • \$\begingroup\$ These days, how much circuits are really repaired or designed to be repaired? \$\endgroup\$ – PlasmaHH May 20 '18 at 13:06
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My take is that caution applies primarily in situations with power filter capacitors run at mains-like voltages (160/320V bus), but they didn't bother to qualify it. Such caps are typically through-hole. Imagine the capacitor leaking and the board burning..

The electrolyte could allow more than enough conduction to cause a lot of havoc, but not enough current to cause the fuse to blow.

I don't think there is much to worry about in a 47uF coupling capacitor... nor even a 6.3V low-Z filter cap on a PC motherboard.


As to your other question.. bipolar electrolytic capacitors are not designed to have AC across them, as the application information indicates. There are some non-polar electrolytics that are designed for AC, for applications such as audio crossover networks.

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  • \$\begingroup\$ Regarding the bipolar caps, could you elaborate? According to this thread: electronics.stackexchange.com/questions/304445/…, bipolar and non-polar are the same thing. I have an application where I'm using the following caps: mouser.com/datasheet/2/293/e-uun-880108.pdf as blocking caps in the signal chain of an audio circuit, should I switch to a different type? \$\endgroup\$ – User7251 May 20 '18 at 8:56
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    \$\begingroup\$ The terminology is a bit sloppy and manufacturer-dependent, and mostly from Asian suppliers so I would depend on the specifications and application recommendations over the English naming- however they may be useful as search terms. The blocking cap doesn't have (significant) AC voltage across it, right? If you can predict the DC bias, a polarized capacitor is better (typically lower leakage, more available, smaller and cheaper). Polarized caps can also withstand a bit of reverse voltage (a fraction of a volt is okay, some manuals say 10% of rated). \$\endgroup\$ – Spehro Pefhany May 20 '18 at 9:14
  • \$\begingroup\$ That project used the bipolar electro cap above in series with an audio transformer as a simple isolated input circuit, and either side of the input could have been biased slightly. The large cap was needed to maintain the desired frequency response. The audio signal was much larger than the bias in that case (3Vrms), I figured based on the name and what I had read elsewhere that a bipolar electrolytic would fit the bill for an AC signal. It worked fine for the short period I had it running, at least. By what you're saying, I assume this was not a satisfactory solution? \$\endgroup\$ – User7251 May 20 '18 at 10:03
  • \$\begingroup\$ It's probably fine unless there is a lot of AC voltage across the capacitor. If the cap value is chosen to block DC and transmit (virtually) all the audio it should be fine. \$\endgroup\$ – Spehro Pefhany May 20 '18 at 10:08
  • \$\begingroup\$ From Wikipedia: Special bipolar aluminum electrolytic capacitors designed for bipolar operation are available, and usually referred to as "non-polarized" or "bipolar" types. In these, the capacitors have two anode foils with full-thickness oxide layers connected in reverse polarity. On the alternate halves of the AC cycles, one of the oxides on the foil acts as a blocking dielectric, preventing reverse current from damaging the electrolyte of the other one. But these bipolar electrolytic capacitors are not adaptable for main AC applications instead of power capacitors ... \$\endgroup\$ – User7251 May 20 '18 at 10:23

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