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One effect of the growing complexity of electronic systems has been the need for greater packing densities in printed circuit assemblies. Conventional PCB assemblies that use leaded components inserted One effect of the growing complexity of electronic systems has been the need for greater packing densities in printed circuit assemblies. Conventional PCB assemblies that use leaded components inserted into through holes have certain limitations in terms of packing density.

These limitations are (1) components can be mounted on one side of the PCB assembly only, and (2) center to center distances between the lead pins of the leaded components can be no less than 0.04” (1 mm) and are usually manufactured at 0.10” (2.5 mm). Surface Mount Technology (SMT) uses an assembly process in which the components are soldered to lands on the surface of the PCB board, rather than inserted into holes running through the board.

By eliminating the need for leads inserted into through holes in the board, several advantages accrue: (1) smaller components can be made with leads closer together, (2) packing densities can be increased, (3) components can be mounted on both sides of the board, (4) smaller PCBs can be used for the same electronic systems, (5) drilling of the many through holes during board fabrication is eliminated, but via holes to interconnect layers are still required, and (6) undesirable electrical effects are reduced such as spurious capacitances and inductances. Typical areas on the PCB board surface taken by SMT components range between 20% and 60% compared to through-hole components.

Despite these advantages, the electronics industry has not fully adopted SMT to the exclusion of Pin-in-Hole (PIH) technology. There are several reasons: (1) owing to their smaller size, surface mount components are more difficult to handle and assemble by humans; (2) SMT components are generally more expensive than leaded components, although this disadvantage may change soon as SMT component production techniques are improved; (3) inspection, testing and rework of the circuit assemblies are moredifficult in SMT because of the smaller scale involved; and (4) certain types of components are not available in surface PCB mount form. This final limitation results in some electronic assemblies that contain both surface mount and leaded components.