Resistance is the opposition a path offers to current. A thick copper plane has almost none; a thin trace has a little; a resistor has exactly as much as its designer chose; an insulator has so much that practically no current flows. The unit is the ohm, written Ω.

The relationship is simple: for a given voltage, the higher the resistance, the lower the current. This is Ohm's law, and it is the basis for most measurements you take on a board.

Resistance on a laptop board

You meet resistance in three ways:

  • Deliberate resistance — resistors setting voltages, limiting current, pulling signals up or down and sensing current in the charger.
  • Unwanted resistance — corrosion on pads, cracked solder joints and damaged traces add resistance where there should be none, causing voltage drops and intermittent faults.
  • Resistance to ground — every net has some resistance to ground through the parts connected to it. A healthy rail usually reads from hundreds of ohms to many kilohms; a shorted rail reads a few ohms or less.

That last point is the basis of short hunting: measuring the resistance of a rail to ground with the board unpowered tells you quickly whether something on it has failed short.

How to measure it

Use a multimeter in resistance mode with the board unpowered and the battery disconnected. The meter injects a small current and calculates resistance from the voltage it sees.

  • Measuring a resistor in circuit often reads lower than its marked value, because other parts are connected in parallel. Lift one end for an exact reading.
  • Readings on large rails drift for a few seconds while capacitors charge; wait for them to settle.
  • For very low values — a shorted rail, a current-sense resistor — ordinary meters are imprecise below about one ohm. Use the meter's relative mode to subtract the probe resistance, or switch to voltage injection for locating shorts.

Resistance and temperature

The resistance of most materials changes with temperature. Copper resistance rises as it warms; thermistors are built to change resistance sharply with temperature and are used on boards to sense battery and component temperatures. When a fault comes and goes as the board warms up, a joint or trace whose resistance changes with heat is a likely cause.

Common mistakes

  • Measuring resistance on a powered board — the reading is meaningless and you risk the meter.
  • Trusting in-circuit resistor readings without allowing for parallel paths.
  • Calling a rail "shorted" from a reading of a few hundred ohms. That is often normal for a large, busy rail; compare with a known-good board.

More on reading these values in How to Find a Short Circuit on a Laptop Motherboard.