Resistance to GND
measuring continuity faults on signal nets

What it is

Resistance to GND is a fundamental diagnostic measurement: the ohmic path from any signal net, power rail, or component pin directly to ground plane/chassis ground. During offline board testing, you measure this value with a multimeter in resistance (ohm) mode, probing from the target node to a ground reference point (typically a test pad, via, or GND plane).

Low or zero resistance indicates a short circuit or direct connection to ground; infinitely high resistance (open) suggests a broken trace or lifted pad. Expected values depend entirely on circuit topology—some nets should have high resistance (signal lines, data buses), while others (power rails with load) should show moderate resistances reflecting their pull-downs and load paths.

This measurement is performed with power disconnected to avoid damaging the meter, the board, or ICs. It forms the foundation of offline board diagnostics and precedes any powered testing.

In practice

You encounter resistance-to-GND checks in three main scenarios during board-level repair:

Initial Triage

A board fails to power on. Before applying power, measure R_GND on critical nets: PPBUS, PP3V3, PP5V. If any rail shows near-zero ohms with power disconnected, you have a shorted capacitor, failed FET, or bridged pad. This protects your power supply from inrush current.

Locating Shorts

A decoupling capacitor or power inductor fails internally, creating a hard short. Measure R_GND at the cap leads or inductor pads. Zero ohms confirms the short. Now measure resistance across series protection elements—MOSFETs, sense resistors, inrush limiters—to isolate which component failed.

Tracking Broken Traces

A high-value signal net (e.g., a digital GPIO, I2C data line, or clock) loses connection to ground pull-down resistors or bias networks. Measuring R_GND at the source will show correct resistance, but mid-trace it jumps to megohms or infinity, pinpointing the break location under magnification.

Typical values

Note: Values vary by board design. Always reference the schematic to understand expected load paths. Some rails intentionally have bulk capacitance to ground, reducing measured resistance. Use context: if PPBUS measures 2 Ω and the schematic shows 1000 µF of capacitance, that is normal; if it reads 0.5 Ω, investigate.

Diagnostic strategy

Multi-point Mapping

Don't measure R_GND at only the main supply connector. Probe at component pads, bulk caps, inductor outputs, and distribution paths. A short may be localized: the main input reads clean, but a decoupling cap near a CPU reads zero.

Asymmetric Shorts

If R_GND on PP3V3 is extremely low but the PPBUS input is normal, the short is downstream. Use series elements (sense resistors, FET drain resistances) to binary-search the fault—measure across each in sequence.

Temperature Sensitivity

Some shorts (like electromigration on a trace or a cold joint) are temperature-dependent. If R_GND is marginal (e.g., 10 Ω when you expect 200), it may only fail under load. Heat the suspect area with a hair dryer and re-measure; watch for resistance drop as the short becomes active.

Meter Selection

Use a quality digital multimeter with a low-ohm mode or true RMS reading. Cheap analog meters have high internal resistance (10 Ω–100 Ω) and will give false high readings on low-impedance rails. Auto-ranging meters may not hold precise values on very-low-resistance paths.

See also

Related terms and techniques in this glossary:

• Pad Lifting — PCB pad damage and recovery — intermittent R_GND spikes under mechanical stress