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- February 23, 2023
- PCB knowledge
- (0)
- 07 mins
SMT vs THT – what is the difference
The characteristics of SMT process technology can be reflected by the difference between it and traditional through-hole technology (THT). From the perspective of assembly process technology, the fundamental difference between SMT and THT is “sticking” and “inserting”. The difference between SMT vs THT is also reflected in the substrate, components, component form, solder joint form and assembly process.
THT uses leaded components, design the circuit connection wires and mounting holes on the printed board, insert the component leads into the pre-drilled through holes on the PCB, temporarily fix them on the other side of the substrate by wave soldering and other soft Brazing technology is used for welding to form reliable solder joints and establish long-term mechanical and electrical connections. The main body of components and solder joints are distributed on both sides of the substrate.
With this method, since the components have leads, when the circuit is dense to a certain extent, the problem of reducing the size cannot be solved. At the same time, it is difficult to eliminate the faults caused by the proximity of the leads and the interference caused by the length of the leads.
Surface assembly technology(SMT) refers to placing sheet-like components or miniaturized components suitable for surface assembly on the surface of the printed board according to the requirements of the circuit, and assembling them with soldering processes such as reflow soldering or wave soldering, constitute the assembly technology of electronic components with certain functions.
On a traditional THT PCB, components and solder joints are located on both sides of the board; while on an SMT PCB, solder joints and components are on the same side of the board. Therefore, on the SMT printed circuit board, the through holes are only used to connect the wires on both sides of the circuit board, the number of holes is much smaller, and the diameter of the holes is much smaller. In this way, the assembly density of the circuit board can be greatly improved. Let’s take a look at SMT vs THT in detail.
Surface Mount Technology(SMT)
The full name of SMT is the abbreviation of Surface Mounting Technology. It is currently the most popular technology and process in the electronic assembly industry.
It is a circuit assembly technology that mounts surface mount components (SMC/SMD for short) on the surface of a printed circuit board or other substrates, and solders and assembles them by reflow soldering or dip soldering.
Advantages of SMT
1. High packing density: Compared with traditional through-hole components, SMD components require less board surface space. In addition, the application of SMT components makes electronic products shrink by 60% in volume and 75% in weight.
2. High reliability: SMD is small in size, light in weight, with high reliability and impact resistance. The use of automated production enables soldering and placement with high reliability.
3. High frequency: Since SMD does not contain leads, both parasitic inductance and capacitance will decrease as the frequency increases.
4.Cost reduction: Due to the rapid development and wide application of chip components, the cost of chip components is also decreasing, and SMT assembly simplifies the entire manufacturing process and reduces manufacturing costs. As far as SMC is concerned, the leads do not need to be reorganized, bent or cut, thus shortening the entire production process and improving production efficiency.
SMT process
The basic process components of SMT production include: solder paste printing (or dispensing), placement (curing), reflow soldering, cleaning, testing, and repair.
1. Solder paste printing: Its function is to print solder paste or SMD glue on the pad of PCB to prepare for the welding of components. The equipment used is a solder paste printing machine, which is located at the front end of the SMT production line;
2. Glue dispensing: It is to drop glue onto the fixed position of the PCB, and its main function is to fix the components on the PCB. The equipment used is a glue dispenser, which is located at the front end of the SMT production line or behind the testing equipment;
3. SMD placement: Its function is to accurately install the SMD components to the fixed position of the PCB. The equipment used is a placement machine, which is located behind the screen printing machine in the SMT production line;
4. Curing: Its function is to melt the patch adhesive, so that the surface mount components and the PCB board are firmly bonded together. The equipment used is curing oven or reflow soldering, located behind the placement machine in the SMT production line;
5. Reflow soldering: Its function is to melt the solder paste, so that the SMD components and the PCB board are firmly bonded together. The equipment used is a reflow oven, located behind the placement machine in the SMT production line;
6.SMT cleaning: its function is to remove the welding residues harmful to human body on the mounted PCB board, such as flux. The equipment used is a washing machine, the location may not be fixed.
7. Inspection: Its function is to inspect the welding quality and assembly quality of the mounted PCB board. The equipment used includes magnifying glass, microscope, online tester (ICT), flying probe tester, automatic optical inspection (AOI), X-RAY inspection system, functional tester, etc. The location can be configured in a suitable place on the production line according to the needs of the inspection;
8.Rework: Its function is to rework the PCB board that has failed. The tools used are soldering iron, rework station, etc.
Through-hole technology(THT)
In the past, most of the components on the PCB were mounted in the THT method. It was not until the SMT became popular in the last few decades that the industry had another mainstream mounting method. THT is an assembly technology that solders and assembles electronic components to the position of the mounting welding holes specified on the circuit board through the leads of electronic components.
Through-hole technology or mixed technology circuit boards are still used in most products that do not require miniaturization. For example, TVs, home audio-visual equipment, switching power supply chargers, industrial, medical, and large-scale communication equipment, etc., are still using THT through-hole technology.
Advantages of THT
1. Firm assembly: Buried hole installation technology can firmly limit the assembled components;
2. Leads are more suitable for transmission: THT technology is also suitable for two-terminal current transmission in the schematic diagram of the given circuit board;
3. Easy disassembly and assembly: it is more convenient to manually remove and replace components on the PCB.
THT process
As the name implies, the through-hole technology process involves drilling holes in the PCB. The board house drills these holes where the components will go, according to the customer’s design. After drilling the holes, thread the leads through them. Consistently and correctly placing the leads in the holes is critical. Assemblers need to make sure they have the correct polarity and orientation when fabricating through-hole boards.
Next, assemblers must inspect the components and make any necessary adjustments.
Finally, the assembler solders the leads securely in place, possibly using wave soldering, where the PCB is passed slowly through liquid solder at high temperatures.
Initially, workers did all these steps manually. Today, automated insertion and placement machines and other equipment assist with many parts of the process. There are two main types of through-hole components—axial leads and radial leads. Axial leaded components have leads at both ends. They come out of the part in straight lines. Radial leaded components have two leads on one side. The leads run through the entire board, so the various layers of the PCB can be connected.
Conclusion
Although SMT is a newer assembly method, it cannot completely replace THT, and the two methods have their own advantages for users.
In general, SMT is more space and cost efficient; while THT is more advantageous in resisting mechanical stress, electrical stress and thermal stress, for example, components like connectors and transformers may need to withstand greater mechanical shock.
Currently on the market, these two different assembly methods are often used on the same circuit board at the same time. For example, components that need to be connected to other components can be assembled by THT, while other components on the circuit board can be assembled by SMT, so for PCBA manufacturers, SMT vs THT, both are indispensable.
