Leadwires are attached to provide both electrical and mechanical
support, and must survive the component fabrication process before
they are assembled.
Glass/metal sealing for diodes and some kinds of resistors makes
Kovar and similar alloys (iron/nickel based) a natural choice
of lead material, because it adheres to glass and doesn't
cause stresses on cooling. Even when (for power diodes like 1N4001)
the leads are heavy copper for cooling, the button that seals against
the glass is a magnetic material welded to the copper.
Nickel, also ferromagnetic, is frequently employed on surface
mount components, because a thin layer of nickel will hold solder,
where a thin layer of copper might dissolve into the solder
Copper isn't suitable for thin items that must be fired at high
temperatures (it oxidizes), and has chemical incompatibility with
some materials (only a few high-tech ICs have copper near the silicon