Electrical parts are 'glued' on the PCB using solder between the part's electrical terminals and copper pads on the PCB. The solder is usually the only way the part is mechanically affixed to the PCB.
TPH Pads Soldered (glued) to PCB |
SMT Pads Soldered (glued) to PCB |
Pad are copper areas on the top/bottom or both sides of the PCB.
TPH Pad (PCB invisible) |
SMT Pad (PCB invisible) |
If pads are on both sides then there is a hole drilled trough both the top and the bottom side of PCB and is usually plated with copper to provide an electrical connection between both sides.