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A cold solder joint is a solder connection that never fully melted and flowed, leaving a weak, unreliable bond between a component lead and its pad or terminal. Instead of a smooth, shiny fillet, a cold joint looks grayish and porous — a sign the solder failed to wet the metal surfaces properly.
Cold solder joints are usually caused by insufficient heat, a contaminated surface, or excessive impurities in the solder. The connection is mechanically and electrically weak, so it can cause intermittent faults, high resistance, or open circuits over time — which is why workmanship standards such as IPC J-STD-001 or NASA-STD-8739.3 classify it as a rejectable defect.
