An open circuit is the opposite of a short circuit: instead of an extra path, a needed path is missing. Current cannot flow through a break, so whatever lies beyond it loses its supply or its signal. A multimeter in resistance or continuity mode shows "OL" (overload, meaning infinite as far as the meter can tell) across an open.
Where opens happen on laptop boards
- Fuses — a blown fuse is an open by design, protecting the board from a short downstream.
- Inductors and ferrite beads — rarely open on their own, but they can crack or burn after a short.
- Solder joints — thermal cycling cracks joints under heavy parts and BGA chips; the break may open only when the board is warm or flexed.
- Traces and vias — corrosion after liquid damage eats through thin traces and via barrels, often near connectors and under chips. See corrosion.
- Connector pins — bent, burnt or corroded pins in battery, display and keyboard connectors.
- Lifted pads — pads torn off during rework or by impact.
- Components failed open — resistors that burnt out, MOSFETs that failed open after overheating.
Symptoms of an open
An open usually produces a clean absence rather than a collapse:
- a rail present on one side of a part and missing on the other;
- a signal that exists at its source but never arrives at its destination;
- a peripheral that is not detected at all;
- intermittent faults that come and go with temperature or pressure, typical of cracked joints.
Compare that with a short, where a rail is dragged down everywhere along its length.
How to find it
- Measure along the path with power applied. Find the point where the voltage or signal disappears. The open lies between the last good point and the first bad one.
- Confirm with the board unpowered. Use continuity mode or resistance mode across the suspected part or trace segment.
- Use the boardview to list every pad on the net and test continuity from the source to each one.
- Inspect under a microscope. Corrosion, cracks and lifted pads are often visible once you know where to look.
Repairing an open
- Replace the failed part — fuse, resistor, inductor — with the correct value and rating.
- Reflow a cracked joint with fresh flux.
- Bridge a broken trace or a failed via with fine enameled wire between the nearest solid pads on the same net, using the boardview to confirm the net.
- Replace damaged connectors rather than patching pins when the damage is widespread.
Common mistakes
- Replacing a blown fuse without finding what blew it. The new fuse will blow too, or worse, hold while something else burns.
- Testing continuity across a capacitor and calling it open — capacitors block DC by design.
- Trusting a beep test on a low-resistance path: continuity mode beeps below a threshold that may be higher than a healthy path should read.