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I'm designing a DC-DC buck converter and when looking at the inductor's datasheet, I noticed an image showing a restricted area underneath the inductor, between the pads.

https://www.we-online.com/catalog/datasheet/74439346047.pdf

enter image description here

Presumably the dashed line (that's approximately 6.5mm) is the restricted area, and that covers the perimeter of the inductor, not just a pair of brackets [ ] around the pads.

I would generally avoid putting feedback/compensation/current sense traces between the pads, at least on the same layer, to reduce the risk of inductive pick up, but for a shielded inductor such as this, I might be a bit more willing to utilise the space, especially other layers.

It just so happens that on my current design, which is very restrained mechanically, I could really do with putting some ground stitching vias between a mid-plane and bottom copper - smack-bang in the middle between the pads. Note they would be through vias, not blind.

I don't recall seeing this keep-out area on other shielded inductors from different manufacturers such as Bourns or Abracon.

Should I change my views on this from rule-of-thumb to hard rule and convince the mech-eng to give me more space, or go for it and hope for the best?

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  • \$\begingroup\$ I would say to trust the datasheet on this one. The restricted area would include the space between the pads, and since that is also labeled as solder resist which exists to prevent solder bridges from forming, a via placed in between the pads would essentially open you up to solder bridges from pad->via->pad which would be... bad. \$\endgroup\$
    – InBedded16
    Feb 24 at 22:14
  • \$\begingroup\$ That's quite a difficult question... Another thing to consider is that ferrite material can be slightly conductive. Are you able to do tented or plugged vias? (Solder mask on top of the via to insulate it) \$\endgroup\$ Feb 25 at 14:24
  • \$\begingroup\$ Yes the vias would be tented. This inductor has 0.6mm tall pads (+/- 0.3) so shorts across the inductor's body, with respect to vias, should be unlikely. I doubt solder will wick across either. \$\endgroup\$
    – raaymaan
    Feb 26 at 16:22
  • \$\begingroup\$ Shielded inductors are not perfectly shielded. Placing anything directly under the inductor risks coupling to the inductor's magnetic field. I've learned by experience with this one. \$\endgroup\$
    – qrk
    Mar 3 at 0:26

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I would say there isn't one straightforward answer, but I try to give a few hints/considerations and my personal assessment.

There are three main types of inductors (simplified):

  • unshielded (coil exposed to surrounding air, magnetic field not shielded)

enter image description here

  • semi-shielded (coil usually covered with mixture of ferromagnetic material and epoxy resin)

enter image description here

  • shielded (coil fully encapsulated with ferromagnetic material)

enter image description here

Starting from the first type the shielding effect obviously increases accordingly. And also, the better the shielding, the better the magnetic flux surrounding the coil is held in place.

From a magnetic flux point of view it is better to have no copper traces/planes around the inductor. Thus, the magnetic field lines that form around the inductor are not significantly disturbed and the data sheet values can be better maintained. However, for shielded (semi-shielded) inductors the magnetic field is far (a bit) more concentrated around the coil and adding a GND copper plane underneath it will have way (a bit) less impact on the inductors properties.

From an EMI point of view I would say it is generally better to have a GND copper plane underneath it - this plane acts as a shield and protects nearby conductors (or inner layers) from picking up radiation. But keep in mind that the magnetic field gets disturbed and inductors properties are degraded (i.e. reduced inductance, ...). However, for shielded (semi-shielded) inductors this degradation is again way (a bit) less impacted.

Conclusion: If you are digging deep it gets complex but shielded type inductors are generally a little less critical to handle, which doesn't mean you don't have to think about what you're doing, but if it were my design, I'd try it with the GND vias mentioned underneath the inductor and have a good time working together with your colleagues from the mechanical engineering department ;).

I also see no risk with vias placed below regarding solder bridges with this type of inductor.

As a side note: Don't forget about the dot indicator (winding start) you'll find on many inductors. It tells you were the winding starts and you should connect this side close to the switching node (high dU/dt >> long traces would favor the antenna effect...)

Further reading:

PCB design for low-EMI DC/DC converters

The Behavior of Electro-Magnetic Radiation of Power Inductors in Power Management

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