A zero-ohm link or zero-ohm resistor is a wire link packaged in the same physical package format as a resistor. It is used to connect traces on a printed circuit board. This format allows it to be placed on the circuit board using the same automated equipment used to place other resistors, instead of requiring a separate machine to install a jumper or other wire. Zero-ohm resistors may be packaged like cylindrical resistors, or like surface-mount resistors. One use is to allow traces on the same side of a PCB to cross: one trace has a zero-ohm resistor while the second trace runs in between the leads of the resistor, avoiding contact with the first trace. The resistance is only approximately zero; only a maximum is specified. A percentage tolerance would not make sense, as it would be specified as a percentage of the ideal value of zero ohms, so it is not specified. An axial-leadthrough-hole zero-ohm resistor is generally marked with a single black band, the symbol for "0" in the resistor color code. Surface-mount resistors are generally marked with a single "0" or "000".
Design considerations
In practice, zero-ohm resistors can be useful as configuration jumpers, but caution is exercised for PCB designs which may use zero-ohm resistors to select between options which require larger trace currents in the design. For these situations, it is better design practice to specify a low-ohm resistance, such as a 0.001-ohm to 0.003-ohm resistor, rather than the generic "zero-ohm" resistor in which the actual resistance may be higher, and tolerance is not given. The low-ohm resistors are easily obtained with 5% or 1% tolerances on a maximum specified resistance and can be safely utilized to pass much higher currents. For example, a surface-mounted 0805 size resistor of 0.003 ohms, rated at 1/2 watt can, in theory, safely pass up to 12.9 amperes of current. In practice, when approaching the power limit for a given package, it is good practice to either use an even lower-ohm product or go up one package size larger. The "cost" associated with taking up more board space for the larger package may also be a consideration. In this example, for 12 amperes to pass through the jumper, a lower resistance or a biggersurface-mount package are usually specified. In contrast, a "worst-case zero-ohm" real-world jumper with 0.05-ohm impedance in a similar 0805 package could only pass 3.1 amperes maximum. The use of specific tolerance resistances is a much safer design practice for higher currents than the "zero-ohm" option, although the bill-of-materials cost can be higher for low-ohm devices.
