Component Failure-Mode Cheat Sheet
How each part fails — and how to confirm it
▣Circuit Card Academy
| Component | How it fails | How to confirm |
|---|---|---|
| Aluminum electrolytic | Dries with age/heat: capacitance drops, ESR rises (ripple, instability). Can also short. The #1 failing class. | Bulged/vented top = dead, replace. ESR meter, or scope the rail ripple. Capacitance out of circuit. |
| Tantalum | Fails SHORT, sometimes ignites. A burnt crater on the board is often a tantalum. | Ohms / diode-test to ground reads near 0 Ω; visible char. Confirm by lifting a leg. |
| MLCC ceramic | Cracks from board flex or thermal shock → intermittent or short. Crack can be invisible. | Classic 'rail reads ~0.5 Ω to ground'. Find by mV-gradient / thermal hunt, then lift to confirm. |
| MOSFET | Fails drain-source SHORT typically. Gate oxide is ESD-fragile. | Diode-test: gate-to-anything should be open; D-S shows body diode one way. Near-0 Ω D-S = shorted. |
| TVS diode | Fails SHORT after absorbing a transient — it did its job and saved the board. | Diode-test reads short across the protected input/rail. Replace it AND find the transient source. |
| Via | Cracked plated barrel from thermal cycling → intermittent: works cold, fails hot (or inverse), flex-sensitive. | Continuity across the via while flexing / heating / freezing. Provoke and watch. |
| Connector | Bent/recessed pins, fretting corrosion, cracked solder tails from cable strain. #1 field-failure site. | Visual under magnification; continuity pin-to-solder-tail; mV-drop under load on suspect contacts. |
| Crystal / oscillator | Mechanically fragile — a dropped board with a dead clock is a prime suspect. | Scope the oscillator output for correct frequency, amplitude, clean edges. Flatline = dead. |
| Resistor | Mostly fails OPEN or drifts HIGH, usually from an overload elsewhere — often the victim of a short. | Reads higher than marked = suspect (parallel paths only lower readings). Find what killed it before re-powering. |
| Solder joint | Cold (gray, lumpy), cracked (annular ring around lead), insufficient/excess, tombstoned SMD. Most common assembly defect. | Inspect under magnification per IPC-A-610. mV-drop under load or reflow-and-retest the suspect joint. |
Generic engineering reference. Always confirm a suspect part out-of-circuit (lift a leg) or against a golden board before replacing it; the solder joint, via and trace are suspects of equal rank with the component.