In the early stages of printed circuit board manufacturing/assembly, all components, including microprocessors, resistors, capacitors, connectors, etc., were inserted into holes on the board and then soldered. But as system design becomes increasingly complex and the demand for miniaturization and lightweight of electronic products increases, it is necessary to integrate more functions on PCBs of the same size. If you choose old through-hole components, it is difficult to place a larger quantity. So smaller SMD devices were developed, and instead of inserting the components into holes on the PCB, they were placed on the surface of the circuit board and soldered. This technology is abbreviated as SMT. SMT assembly is an alternative to the through-hole method in PCB and flex PCB manufacturing, and surface mount technology can be said to have emerged and been applied since the late 1960s to early 1970s. We will discuss the basic content related to SMT in this article.
What is SMT?
SMT(Surface Mount Technology) is a circuit assembly technology that places pinless or short pin surface mount components on the surface of a printed circuit board or other substrate, and assembles them through methods such as reflow soldering or immersion soldering. It is the most popular technology and process in the electronic assembly industry. SMT assembly has a wide range of applications. It is an alternative solution to PCB assembly through-hole method. This technology is known for its high mechanical strength due to its ability to make components compact and convenient.
Surface Mount Technology Process Flow
The basic process flow of SMT mainly includes the following steps:
1. Solder Paste Printing
This is the first step of SMT technology, which applies solder paste onto the solder pads of the circuit board through screen printing to prepare for the soldering of components. Solder paste is a paste made by mixing solder powder and flux, which serves to connect electronic components and circuit boards. The equipment used is a solder paste printing machine.
2. Glue dispensing (this process is not necessary, only required when using mixed assembly)
Glue dispensing is to apply red glue to the fixed position of the PCB, mainly to fix the components on the PCB. The equipment used is a dispensing machine, usually located after the solder paste printing machine.
3. Pick and Place
In this step, the automatic surface mount machine picks up electronic components from the tape and quickly and accurately places them onto the corresponding solder pads on the circuit board. SMT machine is the most critical equipment in SMT production line, with the characteristics of high speed and high precision, which can automatically complete the picking, identification and placement of components. It affects the efficiency and operational status of the entire production line.
4. Curing (This process is mainly aimed at dispensing technology)
This step is to cure the patch adhesive, so that the surface assembled components are firmly adhered to the PCB board. The equipment used is a curing oven, usually located after the surface mount machine.
5. Reflow Soldering
After the surface mount is completed, the circuit board goes through a reflow soldering furnace. The temperature inside the furnace gradually increases, causing the solder paste to melt and forming a strong solder connection between the electronic components and the circuit board pads. Subsequently, the circuit board gradually cooled and the solder joints solidified. This step is usually located after the placement machine or curing oven.
6. Cleaning (optional)
If solder paste containing flux is used, cleaning may be necessary to remove residual flux substances after soldering. However, nowadays many use non cleaning solder paste, so this step is optional in many processes.
7. Inspection
After welding is completed, quality inspection of the circuit board is required. Common testing methods include visual inspection, ICT testing, FCT testing, automatic optical inspection (AOI), and X-ray inspection to ensure product quality.
AOI inspection: After placing the components, the PCB undergoes an automatic optical inspection (AOI) process to ensure that the correct components are ultimately in the correct position and there are no missing or incorrectly placed components. AOI uses high-resolution cameras to capture images of PCBs and compares them with predefined images of PCBs to detect any differences. After the reflow soldering process is completed, the PCB also needs to undergo another round of AOI inspection to check the soldering quality. If there are any defects, the assembly worker will rework the PCB to correct the problem.
In some cases, the manufacturing process also includes X-ray evaluation, which is an additional quality control step to ensure the correct formation of solder joints. It can also detect hidden defects, such as solder bridges or voids, which are not visible during visual inspection. X-ray inspection is particularly useful when dealing with dense or complex PCBs, as it is difficult to visually inspect all solder joints.
Surface Mount Technology assembly type
SMT (Surface Mount Technology) is an electronic assembly technology in which electronic components are directly mounted or placed on the surface of a printed circuit board (PCB). According to different application requirements and process characteristics, SMT assembly can be divided into the following types:
1. Single sided SMT assembly
Perform component mounting and soldering on a single surface of the PCB. Mainly suitable for simple circuits with low production costs.
2. Double sided SMT assembly
Install and solder components on both sides of the PCB. It increases the flexibility of circuit design and is suitable for more complex circuits, but the process complexity and cost also increase accordingly.
3. Mixed assembly
On the same PCB, there are both through hole insertion technology (THT) components and SMT components. Suitable for circuit boards that require the installation of both small components and larger, high-power plug-in components.
4. BGA (Ball Grid Array) assembly
Surface mount with BGA packaged chips, with solder balls located at the bottom of the chip as the pins. High pin density, suitable for high-density circuit design, good thermal management effect, but difficult to repair.
5. CSP (Chip Size Packaging) assembly
CSP is a smaller BGA package that is almost the same size as the chip. Very suitable for miniaturized, high-density electronic products such as smartphones and tablets.
6. QFN (Leadless Square Flat Package) Assembly
The QFN package does not have exposed pins, and there are solder pads at the bottom of the package. This type of packaging has a small volume, suitable for high-frequency and high-performance applications, and also has good thermal performance.
7. PoP (Stacked Package) Assembly
PoP is the process of packaging multiple chips together and assembling them through stacking. Used to further reduce circuit board area, especially suitable for applications where memory and processor are stacked and packaged.
Different types of SMT assembly technologies are suitable for different application scenarios. The choice of which type of SMT assembly to use depends on the specific requirements, design complexity, cost control, and production capacity of the product.
Surface Mount Technology Advantages
Some advantages of SMT over through-hole technology.
1. Stable performance
Due to the absence or short pins of the components, not only can signal transmission delay be reduced and good high-frequency performance be maintained, but they can also withstand vibrations and impacts during transportation and use.
2. Affordable price
The most obvious feature of SMT assembly is the reduction in manufacturing costs. Compared with through-hole technology, SMT assembly technology only requires drilling a small number of holes on the circuit board, reducing processing costs and labor costs in manual soldering.
3. Compact design
The volume and weight of SMT surface mount components are only about 1/10 of traditional plug-in components. Generally, after using SMT, the volume of electronic products is reduced by 40% to 60%, and the weight is reduced by 60% to 80%. SMT assembly can more effectively utilize circuit board space, accommodate more components, and achieve smaller PCB designs, making the design more compact. Widely used in HDI PCB circuit board assembly.
4. Efficiency
The SMT production line has a high degree of automation, greatly improving production efficiency. From the perspective of increasing production capacity, through-hole assembly may take 1-2 hours to complete, while SMT assembly only takes 10-15 minutes.
5. Simple
In through-hole assembly, leads are passed through holes to connect components. Due to the direct soldering of SMT components onto PCBs or flex PCB, the overall composition is much simpler.
6. Not prone to errors
SMT assembly relies heavily on machines, not humans. SMT can reduce the resistance and inductance of connections, minimize the adverse effects of RF signals, and provide better high-frequency performance.
Disadvantages of Surface Mount Technology
Difficult to manufacture and repair
SMT technology has high precision and requires precise equipment and technical support, which may be a challenge for small and medium-sized enterprises. In addition, SMT components are small in size and have dense solder joints, making it difficult to repair and replace these components.
Welding reliability issues
The solder joints of SMT components are small and easily affected by mechanical stress, which may lead to problems such as solder joint cracking and virtual soldering, especially in high vibration or thermal shock environments.
Not suitable for components with high power and mechanical strength requirements
Some high-power components or components that require significant mechanical stress, such as large inductors, electrolytic capacitors, etc., usually still need to be implemented through hole insertion technology because they have higher requirements for welding strength and heat dissipation.
Testing complexity
The testing of SMT circuit boards is more complex than THT boards because the solder joints are dense and hidden at the bottom of the components (such as BGA packaging), which may require higher cost testing equipment and complex testing procedures.
Environmental sensitivity
SMT technology has high environmental requirements, such as temperature, humidity, and other factors that may affect the quality of soldering and the long-term reliability of components.
By combining SMT with through-hole technology to gain the advantages of both, this problem can be alleviated. That means you can use both on the same circuit board. The obvious and simple way to solve any potential defects in SMT assembly is to ensure that you work with qualified and reliable partners. FLEX PLUS firmly believes that it can be your excellent partner.
When do I need to use SMT?
Generally speaking, SMT should be considered in the following situations:
1. You need to accommodate high-density components.
2. Requires compact or small products.
Although the component density is high, your final product still needs to be stylish and lightweight.
4. Request to specify the high-speed/frequency function of the device.
5. You need to use automation technology for large-scale production.
6. Minimize product noise as much as possible (if any)
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