SMT – Surface Mount Technology assembly: what is it?
SMT or Surface mount technology is a frequently used assembly process in the electronics industry. In this technology, electronic components are assembled with automated machines that place components on the surface of a board. This board is called the printed circuit board, PCB. In contrast to the procedures involved in the conventional through-hole technology (THT), SMT components are placed directly on the surface of a PCB instead of being soldered to a wire lead.
Background of Surface Mount Assembly
Though SMT was developed as early as 1960s, it has been popular since 1980s. These days majority of electronic products are assembled through the application of SMT. Components to be used in assembly are smaller in size and they can be mounted on both sides of a board. Hence SMT performs better on high-density and miniaturized products. Since the future will lean more towards low weight and miniature products in electronics, SMT might be increasingly accepted by the industry.
Steps in SMT technology
SMT assembly comprises placing and soldering components to the PCB, but also uses the following production steps:
- Applying soldering paste to the PCB
- Placing SMT components to the soldering paste on the PCB
- Soldering the boards with a re-flow process.
Each of these steps is explained below:
- Applying Soldering Paste
This is one of the first steps in the SMT assembly process. Soldering paste is printed on the boards using the silk-screen method.
- Component Placement and Assembly
The electronic components arriving in trays are loaded into the SMT machine. Pre-programmed X-Y coordinates are marked on the tray and intelligent software systems are used to manage the loading process.
- Component Soldering
Two different methods are used. For series production orders, the Reflow-soldering process is used where boards are put in a nitrogen atmosphere and heated. This results in the melting of the soldering paste that fuses the components to the PCB. The boards are then cooled off and the components become permanently affixed to the board .The SMT assembly process is then completed.
For highly sensitive components, a specialized vapor-phase soldering process is used. In this process, boards are heated until the specific melting point of the soldering paste is reached. This allows soldering at lower temperatures or soldering different SMT components at different temperatures depending on their individual soldering temperature profiles.
AOI and Visual Check
In order to ensure the quality of the assembled boards, AOI visual inspections are performed. The AOI visual check ensures consistency and accuracy in the SMT assembly production process. Using several cameras, the AOI system automatically checks each board and compares the appearance of each board with the pre-defined reference image. If there are any deviations, the operator corrects the mistake or pulls the board from the machine for further inspection.
Through Hole Technology THT versus SMT
Through hole technology (THT) assembly is applicable on large components, and maintains connectivity in the face of high power and high temperatures. Surface mount technology (SMT) is used for smaller and lighter components and also provides for more complex trace routing than THT and is more cost-effective.