Many electronic components are not yet available in SMD for surface mounting. Due to this reason, SMT must accommodate some through-hole components. Therefore the term “Surface Mount Assembly is incomplete”. Surface mount components, active and passive, when attached to the substrate, form three major types of SMT assembly – commonly referred to as Type I, Type II and Type III. The process sequences are different in each type, and all the three types need different equipments.
Type III SMT assembly contains only discrete surface mount components (resistors, capacitors, and transistors) glued to the bottom side.
The Type I assembly contains only surface mount components. The assembly can be either single-sided or double-sided.
The Type II assembly is a combination of Type III and Type I. It generally does not contain any active surface mount devices on the bottom side but may contain discrete surface mount devices on the bottom side.
The complexity of SMT assembly in electronics is increased if large fine pitch,
ultra fine pitch, QFP (Quad Flat Pack), TCP (Tape Carrier Package), or BGAs (Ball Grid Arrays) and very small chip components (0603 or 0402 or smaller) are used on these assemblies along with conventional (50-mil pitch) surface mount packages.
Process of all three Types of Surface Mounting include – adhesive, solder paste, placement, soldering and cleaning followed by inspection, test and repair.