Is This Joint Making Contact?#
Solder joints, crimp connections, header pins, socket contacts — any place two conductors are supposed to be electrically connected. A joint can look fine and still be open or resistive. This is the most basic electrical test, and it catches the most faults on newly built or aging boards.
Continuity Testing#
Continuity mode applies a small test current and beeps when resistance is below a threshold (typically < 30–50 Ω). Fast go/no-go for checking dozens of joints in sequence.
The beep threshold varies by meter — a 40 Ω cold solder joint might beep. Continuity mode tells “not open,” not “good joint.”
Resistance Measurement#
For quantitative assessment, measure in Ohm mode. A good joint reads near zero.
| Connection type | Expected resistance | Suspect if |
|---|---|---|
| Solder joint (through-hole) | < 0.1 Ω | > 1 Ω |
| Solder joint (SMD) | < 0.1 Ω | > 1 Ω |
| Crimp terminal | < 0.05 Ω | > 0.5 Ω |
| IC socket contact | < 0.1 Ω | > 1 Ω, or varies when wiggled |
| Header pin connection | < 0.1 Ω | > 0.5 Ω |
| PCB trace (short run) | < 0.5 Ω | Depends on trace width and length |
Visual + Electrical Assessment#
Combine magnification with electrical testing for suspect joints:
- Visual inspection under magnification — look for cold joints (grainy, dull surface), insufficient wetting (solder ball sitting on pad), cracks, voids, or bridged pins
- Measure resistance across the joint
- Compare to adjacent known-good joints
- Gently press on the joint while measuring — if resistance changes, the joint is cracked or cold
Tips#
- Use continuity mode for fast screening of many joints, then switch to resistance mode for suspect ones
- Null DMM lead resistance first (short probes together and note reading) — leads contribute 0.1–0.5 Ω
- For true milliohm measurements (trace resistance, contact resistance), use 4-wire (Kelvin) measurement
- Press or wiggle suspect joints while measuring — cracked and cold joints reveal themselves under mechanical stress
Caveats#
- The beep threshold varies by meter — most beep below 30–50 Ω, so a marginal joint may still beep
- Lead resistance matters below ~1 Ω — use relative/delta mode if available
- In-circuit measurements include parallel paths through components — isolate the joint if possible
- Capacitors in the circuit cause slowly rising resistance readings as the DMM’s test voltage charges them
- A joint can look good visually and still be bad electrically — internal voids, cold joints under smooth exteriors (common with lead-free)
- A joint can look ugly (rough, dull) and be electrically perfect — common with lead-free solder
- BGA and QFN joints are invisible — assess only electrically or with X-ray
In Practice#
- Beep that cuts out momentarily when pressing on a joint indicates a cracked or cold solder connection
- Resistance that varies when wiggling a component indicates the joint is not mechanically sound
- Resistance reading that slowly rises instead of staying stable indicates a capacitor in the measurement path, not a joint problem
- Joint measuring > 1 Ω when similar joints measure < 0.1 Ω has excessive resistance — reflow or rework needed
- IC socket contact that measures differently after reseating was making poor contact — oxidation or bent pin likely