After a printed circuit board is completed, electronic components must be attached to form a functional printed circuit assembly. Components can be attached using through-hole or surface-mount construction. In through-hole construction, component leads are inserted into holes in the board, while in surface-mount construction components are placed on pads on the outer surfaces of the board. Component leads are then electrically and mechanically fixed to the board using solder. Assemblies often use both through-hole and surface-mount components. Once populated, the board can be tested in various ways while powered off or on to check functionality and ensure proper assembly.