Fundamentally, there are two PCB design systems - through hole and surface mount. In through-hole process, components have connecting pins that are pushed through plated holes (vias) in the PCB and protrude through to the bottom side of the board. Generally, this system employs hand or wave soldering; where the assembled PCB bottom side is passed across and through a wave of solder which connects the protruding component pins to the copper circuit board tracking. Through-hole assembly has been replaced in many applications by surface mount circuit boards and components.
Surface mount PCBs do not have holes for component leads, but are designed with connecting pads on the surface of the board. The pads and components are interconnected to each other via copper or gold tracks. Surface mount components have connections that are almost parallel with the component body allowing the component to sit on the board surface, with the component connecting leads coinciding with the appropriate PCB connecting pads. Surface mount board assemblies are soldered using reflow soldering. Components may be mounted on the top and bottom sides of the PCB (double sided assembly) increasing the number of components to a given area of PCB. This in turn facilitates miniaturisation and reduced product dimensions.
Printed circuit board materials, for both types of board, utilise several materials depending on the application. The vast majority of applications use a laminate material referred to as FR4, which means “Flame Retardant” and the 4, is woven glass-reinforced epoxy resin. FR4 laminate is characterised by a reasonable thermal, electrical and dimensional stability during the PCB assembly and soldering process. The expansion coefficient of FR4 in the X-Y plane is approximately 16 ppm per °C.
