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The Murata LQP03TN2N0C02D 2nH 0201 size inductor apparently has a polarity marking on the package:

enter image description here

I don't see anything on the data sheet that suggests there's a difference installing it in a circuit one way vs the other. Even if there are other magnetic fields nearby I expect this to behave the same regardless of polarity.

This is a single two terminal inductor.

What am I missing about this part that makes knowing the polarity and installing it one way vs the other important?

Adam Davis
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    Murata have a an "appnote" which should answer your question. Actually it's the first thing that came out from google. https://www.murata.com/products/emiconfun/inductor/2012/05/14/en-20120514-p1 – Mike Mar 01 '18 at 20:39
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    @Mike: That is the correct answer. Would you like to post it as such? I had just found the same link and was about to make an answer when I saw your comment. – JRE Mar 01 '18 at 20:42
  • You can go ahead and answer if you want to. (I might have something to add that I have just remembered, I'll comment one of the answers) – Mike Mar 01 '18 at 20:45
  • It does not matter if you have only one discrete inductor. But mutual coupling can cause stray effects (up to 6%) – Tony Stewart EE75 Mar 01 '18 at 20:57

2 Answers2

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Murata has a site specifically about that "polarization" mark.

It isn't so much polarization as that the chip its self is in some way asymmetrical so that it changes its electrical properties depending on how it is mounted.

Depending on how you place those parts, the effective value of the inductor may go up or down a bit. Presumably, you can place an inductor so as to get a bit more inductance out of it rather than purchasing a different part. Or less inductance, as needed.

Murata includes this drawing illustrating the inductor value changes depending on orientation:

enter image description here

It seems to me that something is missing from the Murata explanation.I almost want to say that there must be some other object nearby that influences the effect on the inductor, but Murata doesn't mention it.

This presentation about Murata inductors on the Digikey site (see pages 36 and 37) implies that it is more to enable consistent performance.

Mounting the inductors in various directions relative to other parts of your PCB will give (slightly) different results.

The "polarization" marker just lets you get them all consistently the same way if you find that it makes a difference to your application.

The parts referred to are made in a version that is accurate to +-0.1nH. Presumably, it matters much more with those parts than for the others in that range that have +-0.2nH or +-3% tolerance.

JRE
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  • In the case of a multi-layer inductor, one may want to connect the terminal which connects to the outer winding to a low dv/dt point such that it will act as a shield. I have encountered such issues with a buck converter's harmonics which were affected by the inductor orientation. – Mike Mar 01 '18 at 20:56
  • I must say I'm a bit puzzled. How can properties change depending on the mounting direction? And the direction in reference to what? The north? The mecca? Does that mean that when you rotate your PCB, it changes again? Where is the PCB supposed to be on the little drawings they made? – dim Mar 01 '18 at 21:00
  • I can understand it changing when the spot is up versus down. For the rest of it, I am also sort of puzzled. – JRE Mar 01 '18 at 21:01
  • I would imagine that capacitive coupling to the pads or traces/planes below the inductor may effect the inductance slightly. – Mike Mar 01 '18 at 21:03
  • @Mike Yes, that could explain that there is a difference between direction 1 and direction 3, for example (assuming the images are viewed from the side, not from the top). But how do you explain there could be a difference between 1 and 2? Or between 5 and 7? In those cases, it's just the chip rotated on the PCB, so what's the difference? – dim Mar 01 '18 at 21:05
  • I agree that something is missing, and the fact that isn't not explained suggest that the part shouldn't be used until understood. If it's affected by gravity, for instance (maybe it's a secret accelerometer...) and the inductance varies by as much as 5% depending on orientation in relation to the earth, well that might matter in a given circuit. – Adam Davis Mar 01 '18 at 21:06
  • That said, this specific part is 2nH with a variance of +/- 0.2nH, so even this orientation error is smaller than the device variance. – Adam Davis Mar 01 '18 at 21:07
  • @dim maybe the inductor structure is not symmetric and when the inductor is on it's side (direction 5 -8) there is a different amount of coupling to the PCB. This is just a guess. Regardless, this graph shows the "statistics" of only 5 inductors. How was this measured ? Did they solder the same inductor the the PCB 8 times ? How did they make sure it's placed in the same location and the same amount of solder is applied ? What about the effects of soldering the same component 8 times ? What about the effects to the PCB after resoldering 8x5 times ? I've got a feeling that Murata just heard.. – Mike Mar 01 '18 at 21:13
  • I guess if the inductor has only one or a few turns, then the way the PCB traces approach it could matter, as the traces could behave as a portion of a turn. Also if it is 2nH then the height above the ground plane matters (for example, the inductance of a chip resistor depends if it is soldered with the resistive film up or down). – bobflux Mar 01 '18 at 21:14
  • ... customers request the polarity indication (like we did) and Murata just tried to find technical reasons for it. – Mike Mar 01 '18 at 21:14
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    @Mike Actually, I think JRE gave the reason in his last edit: This is just to allow customers to make consistent boards, since the inductors have different performances depending on its surroundings. So if you want all your boards to have similar characteristics, the mark allows you to make sure you have them all mounted the same way. But I'm pretty sure you can't predict what the variation in inductance will be before mounting it and testing, and the Murata graph is just here as an example, but is not to be used as reference data. – dim Mar 01 '18 at 21:19
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This answer has been extensively edited

I don't see anything on the data sheet that suggests there's a difference installing it in a circuit one way vs the other. Even if there are other magnetic fields nearby I expect this to behave the same regardless of polarity.

Consider how the winding inside this tiny inductor might be orientated: -

enter image description here

I've added the red lines to indicate how internally this inductor may be formed. I'm not saying it represents reality for this device but, given it is much less than a mm in length I think there has to be a single or double winding in order to reach an inductance of 2 nH.

If two devices were soldered in the wrong plane with the "dot" facing sideways, there would be greater magnetic coupling between them if they were placed close. So, in my estimation, the dot is there to help avoid this situation and gives a visual indication that the devices are oriented correctly.

My previous answer focussed on this type of effect but it was incorrect and I needed to think about this a bit more.

Andy aka
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