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.
- 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...)