The Development Process of Surface Assembly Technology

Background of the emergence of surface assembly technology

Over the past decade or so, the rapid development of electronic application technology has shown three significant characteristics.

(1) Intelligentization: Convert signals from analog quantities to digital quantities and process them with computers.

(2)  Multimedia: The transformation and development from text information exchange to sound and image information exchange, making electronic devices more humane and deeper into people's lives and work.

(3)  Networking: Use network technology to connect independent systems. High-speed and high-frequency information transmission enables resource sharing across units, regions, countries and even the world. The requirements of this development trend and market demand for circuit assembly technology are:

(4)  High density: The amount of information processed by electronic products per unit volume increases.

(5)  Speed: The amount of information processed per unit time is increased.

(6)  Standardization: Users' diversified demand for electronic products has transformed mass production of a small number of varieties into a production system of multiple varieties and small batches, which will inevitably put forward higher standardization requirements for components and assembly methods. These requirements force a revolution in the process of inserting electronic components on through-hole substrate PCBs, and the assembly technology of electronic products will inevitably shift to SMT in an all-round way.

2. A brief history of the development of surface assembly technology

A brief history of the development of surface assembly technology Surface assembly technology was developed from the manufacturing technology of component circuits.From the 1970s to the present, the development of SMT has gone through three stages:

The first stage (1970-1975): The main technical goal is to apply miniaturized chip components to hybrid circuits (called thick film circuits in our country) From this perspective, SMT has made a significant contribution to the manufacturing process and technology development of integrated circuits; at the same time, SMT has begun to be widely used in civilian quartz electronic watches and electronic calculators and other products.

The second stage (1976-1985): Electronic products were rapidly miniaturized and multi-functional, and began to be widely used in cameras, headphone radios, electronic cameras and other products; at the same time, a large number of automated equipment for surface assembly were developed , the assembly process and support materials of chip components have also matured, laying the foundation for the rapid development of SMT.

The third stage (1986 to present): The main goal is to reduce costs and further improve the performance-price ratio of electronic products. With the maturity of SMT technology and the improvement of process reliability, electronic products used in military and investment fields (automobiles, computers, industrial equipment) have developed rapidly. At the same time, a large number of automated surface assembly equipment and process methods have emerged, making chip components The rapid growth in usage on PCBs has accelerated the decline in the total cost of electronic products. One of the important foundations of surface assembly technology is surface assembly components. Its development needs and degree of development are also mainly restricted by the development level of surface assembly components SMC/SMD. For this reason, the development history of SMT and the development history of SMC/SMD are basically synchronized.

In the 1960s, the European Philips Company developed surface-mountable button-shaped micro devices for use in the watch industry. This device has developed into the current surface-mounted small-outline integrated circuit (SOIC).Its leads are distributed on both sides of the device in a gull-wing shape. The center distance of the leads is 1.27 mm, and the number of leads can be up to 28 or more pins.

In the early 1970s, Japan began to use quad flat package (QFP) integrated circuits to manufacture calculators. The leads of QFP are distributed on the four sides of the device in a gull-wing shape. The minimum center distance of the leads is only 0.65mm or less, and the number of leads can reach hundreds of pins. The plastic-encapsulated leaded chip carrier (PLCC) device developed in the United States has the leads distributed on the four sides of the device. The center distance of the leads is generally 1.27 mm, and the leads are in a "J" shape. PLCC occupies a small assembly area and the leads are not easily deformed. In the 1970s, the leadless ceramic chip carrier (LCCC) fully sealed device was developed, which replaced leads with metallized pads distributed on the four sides of the device. At the beginning of this stage, the level of SMT was marked by SMC/SMD with a lead center distance of 1.27 mm. In the 1980s, it gradually progressed to the stage where SMC/SMD with fine lead pitches of 0.65 mm and 0.3 mm could be assembled.After entering the 1990s, the assembly technology and assembly equipment of 0.3 mm fine lead pitch SMC/SMD became mature. In the early 1990s, CSP stood out because its chip area and packaging area were nearly equal, it could be processed and tested the same as conventional packaged ICs, it could perform aging screening, and it had low manufacturing costs.

In 1994, various Japanese manufacturing companies had proposed various CSP solutions, and since 1996, small batch products have appeared. In order to adapt to the increasing demand for the input/output number of the same SMD due to the increase in IC integration, that is, the number of leads, the grid array type SMD has also been formed since the 1990s by regularly distributing the leads on the entire mounting surface of the SMD. It began to develop in the 1990s and soon became popular and applied. Its typical product is the ball grid array (BGA) device. At this stage, SMT is compatible with the development of SMC/SMD. While developing and improving ultra-fine pitch assembly technology with lead spacing of 0.3mm and below, it is also developing and improving the assembly technology of new devices such as BGA and CSP. It can be seen that the continuous shrinkage and changes of surface-mounted components have promoted the continuous development of assembly technology. While the assembly technology increases the assembly density, it also puts forward new technical requirements and uniformity requirements for components. It can be said that the two are interdependent, promote each other and develop.MCM is an advanced hybrid integrated circuit that has developed rapidly since the 1990s. It assembles several IC chips on a circuit board to form a functional circuit block, called a multi-chip module (MCM). . Since MCM technology is to install multiple bare chips directly on the same substrate and package them in the same casing without packaging, compared with general SMT, its area is reduced by 3 to 6 times and the weight is reduced by more than 3 times.It can be said that MCM technology is an extension of SMT. The functions of a set of MCMs are equivalent to the functions of a subsystem. Usually the MCM substrate has more than 4 layers of wiring and more than 100 I/O pins, and connects CS, FC, and ASIC devices to them. It represents the essence of electronic assembly technology in the 1990s and is the crystallization of semiconductor integrated circuit technology, thick film/thin film hybrid microelectronics technology, and printed board circuit technology. MCM technology is mainly used in ultra-high-speed computers and outer space electronics technology. In order to adapt to the requirements of higher density, multi-layer interconnection and three-dimensional assembly, SMT is currently in a new stage known internationally as MPT (Microelectronic Packaging Technology, micro-assembly technology).MPT, with MCM and 3D as its core, uses micro-welding and packaging processes to assemble micro-components (mainly highly integrated ICs) through high-density assembly, three-dimensional assembly and other assembly methods on high-density, multi-layer interconnected PCBs. Assembling to form high-density, high-speed and high-reliability main structure microelectronic products (components, parts, subsystems or systems). This technology is an important part of today's microelectronics technology, especially in cutting-edge high-tech fields. It has very important application prospects in aerospace, aviation, radar, navigation, electronic jamming systems, anti-jamming systems, etc. As the fourth generation of electronic assembly technology, SMT has played an extremely important role in the development of modern electronic products, especially in the miniaturization, lightweight, high performance and high reliability of cutting-edge electronic equipment and military electronic equipment. .

3. Development trends of surface assembly technology

Since its advent in the 1960s, SMT technology has entered a fully mature stage after more than 40 years of development. It has not only become the mainstream of contemporary circuit assembly technology, but also continues to develop in depth. The general development trend of surface assembly technology is: components are getting smaller and smaller, assembly density is getting higher and higher, and assembly is becoming more and more difficult. Currently, SMT is making new technological progress in the following four aspects:

(1) The size of components is further miniaturized. Among the mass-produced micro electronic products, 0201 series components (outline size 0.6 mm × 0.3 mm), QFP with a narrow pin pitch of 0.3 mm, or large-scale integrated circuits in new packages such as BGA, CSP and FC have been produced in large quantities. use. Due to the further miniaturization of component volumes, higher accuracy and stability requirements have been put forward for the SMT surface assembly process level and the positioning system of SMT equipment.

(2)  Further improve the reliability of SMT products. Faced with the widespread use of micro-SMT components and the application of lead-free soldering technology, under extreme operating temperatures and harsh environmental conditions, the stress caused by the mismatch of linear expansion coefficients of component materials is eliminated to avoid this stress causing circuit damage. Board cracking or internal disconnection, and component welding damage have become issues that have to be considered.

(3)   Development of new production equipment. In the mass production process of SMT electronic products, solder paste printers, placement machines and reflow soldering equipment are indispensable. In recent years, various production equipment are developing towards high density, high speed, high precision and multi-function, and advanced technologies such as high-resolution laser positioning, optical visual recognition systems, and intelligent quality control have been promoted and applied.

(4)  Surface assembly technology of flexible PCB. With the widespread application of flexible PCBs in the assembly of electronic products, assembling SMC components on flexible PCBs has been overcome by the industry. The difficulty lies in how to achieve the accurate positioning requirements of rigid fixation for flexible PCBs.