The first PCBs used through-hole technology,mounting electronic components by leads inserted
through holes on one side of the board and soldered onto copper traces on the other
side.Boards may be single-sided, with an unplated component side, or more compact double-
sided boards, with components soldered on both sides. Horizontal installation of through-
hole parts with two axial leads(e.g., resistors,capacitors, and diodes) is done by bending
the leads 90 degrees in the same direction, inserting the part in the boar(often bending
leads located on the back of the board in opposite directions to improve the part's
mechnical strengh), soldering the leads, and trimming off the ends.Leads may be soldered
either manually or by a wave soldering machine.
Through-hole PCB Technology almost completely replaced earlier electronic assembly
techniques such as point-to-point construction.From the second generation of computers in
the 1950s until surface-mount technology bacame popular in the late 1980s, every component
on a typical PCB was a through-hole component.
Trough-hole manufacture adds to board cost by requiring many holes to be drilled
accurately,and limits the available routing area for signal traces on layers immediately
below the top layer on multilayer boards since the holes must pass through all layers to the
opposite side. Once surface-mounting came into use,small-sized SMD components were used
where possible, with through-hole mounting only of components unsuitably large for surface-
mounting due to power requirements or mechanical limitations, or subject to mechanical
stress which might damage the PCB.