I'm taking apart a Qi wireless power transfer receiver. The coil is etched into the pcb. The entire coil area (approximately 3.75 x 4.5 cm) is covered by a rectangular piece of hard, brittle material.

The material:

  • has a flat, matte grey finish.
  • is about 10 mil (0.254 mm) thick
  • is very brittle, and snaps easily
  • Appears similar to graphite "pencil lead" although it doesn't mark paper
  • Is somewhat electrically conductive: a 1cm x 1cm square is about 70 kOhm corner-to-corner.

I didn't think to measure the coil inductance before I removed the material, so I don't have a before/after comparison.

It was affixed to the PCB using a contact adhesive, and was on the "non-receiving" side of the PCB. In other words, when this PCB was positioned next to its matching power supply coil, the material was on the far side (not sandwiched in between).

What is this material? What might be its purpose?

(I was going to attach a picture, but it only shows a uniform grey piece of broken material...)

up vote 3 down vote accepted

Ferrite sheet, used for flux management in the wireless charging circuit.

Ferrite has a high permeability, and therefore can concentrate magnetic fields in the vicinity. It improves charging efficiency by "focusing" the magnetic fields, constraining them to the transmitter and receiver coil vicinity. It also serves to protect other circuitry and other devices from the magnetic fields.

I found a nice document here with a more in-depth explanation, and some nice figures which I will include here:

Magnetic flux density (simulation), no shielding: Without ferrite sheet shield

Same setup with added ferrite sheet shielding: With ferrite sheet shield

The effect is quite apparent: increased flux through the receiver, and basically zero flux "leaking" out on either side.

If you search "ferrite sheet for wireless charging" you'll find loads more information.

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