DESIGNING OF PCB
Design of PCB is an important step in surface-mount technology. When mounting the surface-mount-type packages on a printed circuit board, the design of the mounting pads formed on the board and the choice of board material are important considerations. In particular, the soldering yield may be greatly influenced by the mounting pad design, and the board material may have a significant effect on the reliability after soldering. With regard to the reliability of solder joints, the thermal expansion coefficients of the mounted devices and the board must be considered. When mounting thin packages (such as TSOPs or CSPs), the silicon chip occupancy ratio is high in relation to the package size, and the apparent thermal expansion coefficient of the overall package is reduced. It is therefore necessary to select a board material that enables the package thermal expansion coefficient to be approached as closely as possible.

SOLDER PASTE
Solder paste composition is important for good solder joint. The main constituents of solder paste are solder powder and flux. Flux promotes soldering, consists of an adhesion-imparting agent, a solvent and an activator. The following criteria should be satisfied for the solder paste:
1. Quality remains stable with aging.
2. Good printability and applicability.
3. No hardening of the surface after solders paste manufacture.
4. Minimal occurrence of solder balls.
5. Maintenance of good clean ability, leaving no flux residue.

FOLLOW UP
As electronics products become smaller and lighter, the components used in them are also becoming smaller, with finer processes and higher mounting densities. It has thus become extremely difficult to continue the visual inspections previously carried out after soldering. From the standpoint of reducing the assembly costs of electronics products, also, automation of appearance inspections after soldering is becoming increasingly common. Reworking of defective places with a soldering iron has been the traditional method for soldering defects in mounted devices, but some of today’s electronics devices (IC packages such as CSPs and BGAs) cannot be reworked using a soldering iron.

THE EQUIPMENT
SMT requires use of screen-print equipment, placement equipment, soldering equipment, cleaning equipment, inspection equipment, and repair and rework equipment. The sub-products include manual screen-print equipment, semi-automatic screen-print equipment, automatic screen-print equipment, gantry robots, turrets, high-speed placement equipment, medium-speed placement equipment, low-speed placement equipment, reflow ovens, wave ovens, optical inspection equipment, X-ray inspection equipment, laser inspection equipment, manual repair and rework equipment, and semi-automatic repair and rework equipment.

One of the most significant developments in the electronics industry today is the drive to remove lead from the solder used in electronics products, and replace it with other metals, due to growing concerns about its impact on the environment which has put challenges for component and assembly equipment manufacturers, forcing them to experiment with various kinds of alloys in an attempt to find a suitable alternative to lead. As a result of the new defect types emerging from the adoption of lead-free solders, it is important to look for defects all the way from solder paste, placement, before and after reflow, after wave and then to test the product electrically through in-circuit test (ICT) and functional test equipment. SMT is used more but through-hole components are not becoming obsolete soon.