Resistors can be embedded in PCBs  either by use of special layers laminated, etched, connected and optionally buried in a multilayer PCBs, or by employing carbon screen printing as shown further down that also can be buried.

Ohmega-Ply is one of these materials

Ohmega-Ply basic  tolerances depend on pcb production equipment and process control with typical tolerance values around +/- 10%. But when resistance layers are produced with larger than required resistances followed by  computerized laser trimming equipment  tolerances better than 1% regardless of the material used can be  achieved at low costs.

More information here http://www.ohmega.com/technology/

The OhmegaPly RCM is available in a number of standards sheet resistivities expressed in ohms/Sq, 25 ohms/Sq, 50 ohms/Sq, 100 ohms/Sq and 250 ohms/Sq

Formula: Resistance (in ohms) = (Length/Width) x (Sheet Resistivity)

Calculation example:

Sheet Resistivity = 100 ohms/Sq

Trace length       = 2 mm  (80 mil)

Trace width        = 0.4 mm  (16 mil)

Resistance: 100 x (2/0.4) = 500 ohm = 100 x (80/16) x 500

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