One-sentence summary of SMT’s development history: It is a history of evolution from nothing to something, from bulky to portable, from crude to precise, and from backward to advanced. Detailed explanations follow.
In the electronics industry, SMT stands for Surface Mounted Technology (where “Surface” means surface, “Mounted” means installed, and “Technology” means technology). It is currently one of the most popular techniques and processes in the electronic assembly industry. Hereinafter referred to as SMT, Surface Mounted Technology emerged from the production need of miniaturizing through-hole components, which drove the development of smaller and lighter products. SMT is a circuit assembly technology that eliminates the need to drill mounting holes in printed circuit boards, allowing surface-mount components to be directly attached and soldered onto specified locations on the surface of the printed circuit board.
The development of electronic component packaging technology
Surface mount technology is not a new concept; it originates from earlier processes such as flat pack and hybrid mounting. Initially, the assembly of electronic circuits employed point-to-point wiring methods and did not involve any substrate at all. The first semiconductor device packages featured radial leads that were inserted into through-holes on monolithic circuit boards already used for packaging resistors and capacitors. In the 1950s, flat pack surface-mounted components were applied in high-reliability military applications. In the 1960s, hybrid technology became widely used. In the 1970s, influenced by Japanese consumer electronics, passive components were widely adopted, and in the past decade, active components have been widely used.
The development of SMT components and assembly technology:
| 年代 | 1950 | 1960 | 1970 | 1980 | 1990 |
| Generations | First | Second | Third | Fourth | Fifth |
| Typical Products | Vacuum tube radios, electronic instruments | General instruments, black-and-white TVs | Portable thin instruments, color TVs | Small high-density instruments, VCRs | Ultra-small high-density instruments, integrated cameras |
| Product Characteristics | Bulky, thick, limited functionality, unstable | Lighter weight, low power consumption, multifunctional | Portable, thin, low power | Pocket-sized, lightweight, multifunctional, micro power consumption | Ultra-small, ultra-thin, intelligent, highly reliable |
| Typical Electronic Components | Vacuum tubes | Transistors | Integrated circuits | Large-scale integrated circuits | Very large-scale integrated circuits |
| Characteristics of Electronic Components | Long leads, large size, high voltage | Axial leads | Single, dual in-line package integrated circuits, taped lead components | Surface mount, irregular structures | Composite surface mount, three-dimensional circuit substrates |
| Printed Circuit Board | Metal chassis, terminal block riveted terminals | Single-sided phenolic paper laminate | Double-sided epoxy glass cloth laminate, flexible polyimide board | Ceramic substrate, metal core printed board, multilayer high-density printed board | Ceramic multilayer printed board, insulated metal substrate |
| Assembly Technology Characteristics | Bound wires, manual soldering iron welding | Semi-automatic insertion, dip soldering | Automatic insertion, dip soldering, wave soldering, fusion welding | Two-sided automatic surface mount, reflow or wave soldering | Multilayer, high density, high-speed installation, flip-chip soldering, special soldering |
Electronic components are the building blocks of electronic information devices, and board-level circuit assembly technology forms the foundation for manufacturing electronic equipment. The emergence of different types of electronic components has always triggered a revolution in board-level circuit assembly technology. From the table above, we can observe the rapid development of SMT (Surface Mounted Technology). Considering our current lifestyle, take Apple’s in-ear wireless earbuds as an example. Compared to current over-ear headphones, their volume has been reduced by over 96%. When compared to electronic products from decades ago, the development of SMT technology has brought tremendous changes to our lives.
The development trends in SMT include PBGA, TBGA, FBGA, CSP, and FC, which are the current trends in IC packaging. The following table shows the development trends of BGA and FC packaging.
Development Trends of BGA and FC Packaging:
| Year | 2000 | 2005 | 2010 | 2015 |
| BGA Ball Pitch | ||||
| Low-end Products | 1.27 | 1.00 | 1.00 | 0.80 |
| Portable Products | 1.27 | 1.00 | 0.80 | 0.65 |
| Mid-performance Products | 1.27 | 1.00 | 0.80 | 0.65 |
| High-performance Products | 0.80 | 0.65 | 0.65 | 0,50 |
| BGA Terminal Count | ||||
| Low-end Products | 312 | 512 | 684 | 968 |
| Portable Products | 420 | 684 | 800 | 1200 |
| Mid-performance Products | 840 | 1658 | 2112 | 3612 |
| High-performance Products | 1860 | 3280 | 3612 | 8448 |
| FC Chip Terminal Pitch | um | |||
| Portable Products | 165 | 100 | 70 | 35 |
| Mid-performance Products | 200 | 150 | 150 | 150 |
Development of Electronic Assembly Technology
The development of SMT is largely constrained by assembly processes. Without advanced assembly processes, it would be difficult to promote and apply advanced packaging technologies. Therefore, the emergence of advanced packaging inevitably places new demands on assembly processes. Generally speaking, BGA, CSP, and MCM can fully utilize standard surface mount technology (SMT) equipment and processes for assembly. However, due to the miniaturization of the terminal arrays in these packages, stricter requirements are imposed on the assembly processes, which in turn drives the development of electronic assembly equipment and processes. The trend of electronic assembly technology is towards agility, flexibility, integration, intelligence, and environmental protection. Future sections will specifically discuss SMT production lines.
Multi-cantilever machines have replaced turret machines and have become the mainstream trend for the future development of high-speed placement machines. Based on single-cantilever placement machines, dual-cantilever placement machines have been developed. Currently, the mainstream high-speed placement machines on the market are quad-cantilever machines derived from dual-cantilever machines, such as Siemens’ HS60, Universal’s GC120, Panasonic’s CM602, and Hitachi’s GHX-1. To enhance adaptability and improve efficiency, new placement machines are moving towards flexible and modular structures. Japanese company Fj divides placement machines into control hosts and functional module machines. Modules have different functions and can place components with varying precision and speed based on the requirements for different components, achieving higher efficiency. When users have new requirements, new functional module machines can be added as needed. As chip integration increases, the pitch of chip interconnects and the diameter of solder balls continue to decrease, placing higher demands on the alignment and positioning accuracy of placement equipment. It is necessary to research new motion design and control methods to achieve smooth, rapid, and precise positioning. The concept of a green production line refers to considering environmental protection requirements from the onset of M (Manufacturing) production. After years of research and development, lead-free soldering technology and no-clean soldering technology will enter a stage of comprehensive practical application.
Since the 1980s, high-density circuit assembly technology, also known as microelectronic assembly technology, has rapidly developed. During this development, three notable trends have emerged, which have not only significantly increased the assembly density of component-level C packaging and board-level circuit assembly but also blurred the distinctions between different levels of electronic circuit assembly. This has led to the disappearance of boundaries between electronic circuit assembly levels and facilitated technical integration between C device packaging and board-level circuit assembly, the two circuit assembly levels.
Several typical microelectronic assembly technologies:
| Name | Manufacturing Technology | Characteristics |
| Flip Chip (FC) | Processed using methods similar to SMT. I/O terminals (bumps) are arranged in a face array on the chip. During soldering, the chip is flipped and placed onto the PCB, aligning the bumps with the solder pads on the PCB. Heating then achieves interconnection between the FC and the PCB. | Features high-density assembly without the need for bonding pins or packaging. FC processing requires very strict control but is much faster than any other COB (Chip On Board) process. Specialized equipment is required, including high-precision mounting systems, underfill dispensing systems, and X-ray inspection systems. |
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Multi-Chip Module (MCM) |
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MCM technology is mainly divided into three categories: MCM-L (Laminated Multi-Chip Module), MCM-C (Ceramic Multi-Chip Module), and MCM-D (Deposited Multi-Chip Module). MCM technology is primarily used in ultra-high-speed computers and outer space electronic technology. |
| Three-Dimensional (3D) Assembly Technology | IC chips (MCM chips, WSI – Wafer Scale Integration chips) are stacked, and their side edges and vertical directions are used for interconnection, thereby transforming horizontal assembly into vertical, three-dimensional assembly. |
There are roughly three approaches to 3D assembly:
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