Refers to a fusible metal alloy used for joining metal parts; also used historically for pewter. In modern contexts, almost always means solder.
I attached the wiring with solder.
In electronics work, you use solder to attach components to the board.
The solder has come off this wiring.
I melted the solder with a soldering iron.
半田 refers to the low-melting metal filler itself used in soldering, while 溶接 is welding, a process that melts the base metals together without a filler.
半田 is the soft, low-temperature filler metal used in ordinary soldering, whereas ろう付け (brazing) uses a similar but higher-melting filler and process.
Standard kanji spelling; commonly used in technical and everyday contexts.
Katakana spelling is often used in technical or product contexts.
Kana spelling is common in informal writing and when the kanji is not needed.
Rarely used kanji form.
The word はんだ is believed to derive from the Portuguese 'solda' (solder), which entered Japanese during the period of Portuguese trade. The kanji 半田 are ateji (phonetic equivalents) and do not contribute to the meaning.