SMT Technology in The Development of Micro Assembly Technology

Micro-assembly technology is a high-density electronic assembly technology, which uses connection and packaging processes on high-density multilayer interconnection circuit boards to assemble micro-small electronic components into high-density, high-speed, and high-reliability three-dimensional electronic products. Its volume is much smaller than conventional electronic products, and the reliability is also improved to a considerable extent due to the greatly reduced solder joints.

1. Surface Mount Components

The key to SMT is surface mount components, including chip resistors, chip trimmer potentiometers, chip tantalum electrolytic capacitors, chip inductors, surface mount tactile switches, surface mount connectors, integrated circuits, hybrid integrated circuits, etc.

At present, the development direction of surface mount components is mainly: miniaturization and high performance, including high power, high precision, large capacity, high reliability, long life, high frequency. For example, the minimum lead center distance of the integrated circuit adopting the four-side pin flat package (QFP) has mostly reached 0.5mm, and the devices of 0.406mm and 0.305mm have been supplied in small quantities. Chip tantalum electrolytic capacitors use 60000CV/g tantalum powder, with a capacity of 33μF and 1000 cycles of temperature at -55～+125℃, and high temperature and humidity characteristics of 85℃, 90%RH. Surface-mounted integrated circuits have developed rapidly in recent years. There have been a variety of packaging forms, such as four-side pin flat package, thin small outline package, dual in-line package and so on.

2. Surface Mounting Process

With the development of high-density, high-function and high-speed circuit components, electronic components have also entered a higher level of micro-miniaturization and multi-pin narrow-pitch stage, which requires the SMT technology to be more refined and precise and with strictly controlled development. The surface mounting process mainly includes several aspects such as soldering technology, placement equipment, and the design and production of printed circuit boards.

Welding is a key process in circuit assembly technology that affects the reliability of circuit components and realizes narrow pitch technology. New welding processes developed in recent years include convection infrared reflow welding, vapor phase reflow welding, tool reflow welding, laser reflow welding, wash free soldering, vacuum reflow soldering, lead-free solder and corresponding soldering technology, etc.

The placement machine is the most difficult and most expensive key equipment in the surface mount production system. The mounting speed of large-scale placement machines suitable for mass production of consumer electronics products can reach 0.1-0.4 seconds per component, and the placement accuracy can reach ±0.01mm. After the CCD optical camera correction system is configured, the accuracy can reach ± 0.005mm.

The printed circuit board not only plays the role of carrying electronic components, interconnection lines, pads, etc., but also plays a role of heat dissipation. With the development of surface mounting of printed circuit boards, high density, high frequency, ultra-thin, and diversified trend. A series of changes have also appeared in the design and production technology of the corresponding printed circuit boards. For example, computer-aided design technology is widely used in automatic design, including from the network list of circuit diagrams to digital input operations, prepare to register the component database of component shapes, etc., pad inspection, and computer-aided manufacturing data output operations.

At present, computer integrated manufacturing system technology has been applied in surface mounting technology. Other key technologies related to printed circuit boards also include wiring and plate making, metallization technology, holeization technology, lamination technology and material technology. Screen printing machine is an important equipment in surface mounting technology.