Basic SMT Process
Electronics assembly is the general term for joining electrical components to printed circuit boards. Surface mount technology (SMT) is the electronics assembly process where electrical component leads are joined to the PCB via individual pad connections located on the board surface.
The basic SMT process consists of the following steps:
1. Solder paste is applied to the PCB using a screen printer. Stencils designed with holes over individual pads control the solder application to the board.
2. Electronic components are positioned on the PCB using placement equipment (pick-and-place machines, chip shooters, etc.). Component leads are placed in direct contact with the solder-pasted pads.
3. The solder paste is heated until liquids (reflowed) then cooled until the solder hardens and creates permanent interconnection between the component leads and the PCB. This process is performed in a solder reflow oven.
4. After reflow, the assembled circuit board can be cleaned, tested or assembled into a final product.
High-volume SMT lines use automated equipment to perform these steps. These lines can typically produce a completed circuit board in less than 20 seconds, with placement machines that can position upwards of 40,000 components per hour on the PCB. However, once the solder paste is applied and the components are placed on the board, the only way to create a functioning circuit board is an effective solder reflow process.