Copper is an excellent target material, with conductivity that is 95% of silver. But it is heavier and weaker than aluminum, so from a mechanical perspective it is often not an optimum target material. Note that some copper alloys may have lower conductivity than aluminum, leading to the unexpected result of a weaker response than an Al target.

Constructing a target as a copper region on a PCB is a technique that we have utilized for many applications. Commonly available PCB fabrication can reliably produce features finer than 5 mils (0.125 mm) with 1-oz. copper on FR4. The common plating thickness of 1-oz. copper is 37 µm thick, which is one skin depth for a sensor frequency of 3.1 MHz. While operation below one skin depth is still effective, to obtain a larger response, the sensor frequency should be above 6 MHz with 1-oz. copper. Thinner copper platings perform better with a correspondingly higher sensor frequency.

Duplicating the target design onto several layers will provide some improvement in response with lower frequency sensors.

FR4 is an excellent substrate for the PCB target – it is strong, dimensionally stable, light, and has a low loss tangent at the frequencies used by LDC sensors. In addition, the FR4 has a temperature coefficient of expansion which closely matches the copper traces on the board.

If the sensor is a spiral trace on a PCB, then the thermal expansion of the PCB target has the added benefit of matching the thermal expansion of the sensor.