Tā yòng làotiě bǎ diànxiàn hàn zài diànlùbǎn shàng.
He soldered the wire onto the circuit board with a soldering iron.
Tips
usage
The electric soldering iron used on circuit boards is 电烙铁. Before electric irons, clothes were pressed with a 烙铁 heated on the stove; today that job belongs to the 熨斗 'clothes iron'. In speech the second syllable is often unstressed: làotie.
memory
烙 'to burn a mark, to bake on a hot surface' also gives 烙印 'brand mark' and 烙饼 'griddle pancake'.