At the heart of every electronic device is the printed circuit board. Manufacturing printed circuit boards is a long process with many steps. We will see SMT vs through hole technology in the following content. In older technology, electronics assemblies are used through hole technology. As the name suggests, SMT (Surface Mount Technology) mounts the electronic components on the surface of the printed circuit board.
SMT and through hole technology started at different times. Surface mount devices were developed as early as the 1950s, they were called flat packs. They were used in military applications. Flat packs had several disadvantages, including a very high cost because of gold leads. After flat packs came the DIPs (dual in-line packages), which were easy to solder in bulk. Then came hybrid technology, where both SMT and through-hole technology were used. Hybrid technology has substantially contributed to the development of surface mount technology and through hole technology, even though there is a significant difference comparing surface mount technology vs through hole.
As SMT vs Through-hole PCB assembly methods are used commonly, which method is the best choice for your projects? In this blog, we will give you a comprehensive introduction to SMT vs Through-hole PCB assembly methods, which will help you to make the right decision between surface mount technology and through hole technology.
PCB assembly process consists of two primary steps: placing components on the board, and soldering. Usually, the following methods are used in circuit board assembly:
- Through-hole assembly technology
- Surface mount assembly technology
- Hybrid technology (a combination of the above two)
Introduction to Through-Hole Technology
Components fit into the holes on the board. A restricted number of components can fit the board. Design becomes bulky. Talking about the cost between SMT vs through hole technology, through-hole technology offers an advantage in terms of cost. But in terms of time and labor, through-hole requires less automation, and more manual labor, which leads to reduced costs and more jobs being generated. It is customary that the circuit board designs comply with industry standards. These standards recommend the hole sizes, trace width, length, and spacing. Repair and rework are different between SMT vs through-hole technology; it is easier in through-hole technology in the case of repair and rework.
Key Considerations for the Through-hole Assembly
- Component Lead Hole Sizes: A hole tolerance of about 0.07 mm to 0.015 mm must be there between the lead and the hole for proper soldering according to industry standards.
- Circuit Board Size: The impact of circuit board dimensions affects the quality of the soldering process. As the board thickness increases, the soldering becomes more difficult, as the amount of heat needed to reach the through-hole decreases.
- Component Density: The component density varies depending on the assembly technology used. The number of components between surface mount technology vs through-hole technology is different. For the through-hole process, the number of components that can be placed is obviously less.
- Pb-free Soldering: According to the RoHS directive, it is restricted to the use of certain hazardous chemicals like Pb (lead) in the electronics industry because of their toxicity and health concerns. It can cause environmental pollution during the disposal of electronic waste. It is mandatory to use lead-free solder paste to have RoHS compliance. Moreover, the lead-free alloys used for soldering have high melting temperatures, resulting in higher soldering times. Lead-free solder alloys have more surface tension, which results in slow wetting and capillary flow. Pb-free solder has poor solderability characteristics compared to Sn-Pb solder.
- Tools: The tools also affect the through-hole technology. Separate drills should be used for plated-through and non-plated-through holes.
Introduction to Surface Mount Technology
The advent of surface mount technology has greatly contributed to the miniaturization of the whole circuit board and enhanced its functionality. In this type of assembly process, components are mounted on the surface of the board by soldering the components onto the pads on the board. There may be single-sided or double-sided mounting of the components, depending on the requirement. When it comes to the benefits of surface mount technology and through hole technology, the benefits of the SMT assembly process are reduced component size and rich component density.
In terms of hole sizes of surface mount technology vs through hole technology, SMT has reduced hole size by replacing bigger holes with smaller visas for connection between internal layers. The number of defects is reduced in the SMT process. In terms of costs and labor when comparing SMT vs through hole technology, SMTs have reduced costs, and less manual labor is required because of full automation. Proper cleaning is also done after the full board is soldered and manufactured as a part of the SMT process. Repair and rework are not easy in the case of SMT technology.
Key Considerations for the SMT Assembly
- Solder-joint Configurations: There are different types of solder-joint configurations available for different package types.
- Board Density: The board density is very high in the case of SMT, leading to thousands of solder joints being fabricated at a time.
- Component Miniaturization: Rework, replacement, or repair work is a little difficult in the SMT process due to miniaturization.
- Layout of the Board: A double-sided board requires two reflow processes. The larger components are soldered first on one side of the board.
- Solder Paste Printing: Screen printing or stencil printing techniques are used for applying solder paste to the SMT board. It has certain drawbacks. It can be done in a single attempt. If there is a problem during the first attempt, the board must be thoroughly cleaned before the second trial. Generally, a Pb-free solder is applied. Solder paste should be applied properly; otherwise, there can be an open circuit due to less paste. If too much paste is applied, it can result in a short circuit with neighboring connections.
- Placing Components: In this process, the machine accurately and properly selects the specifically required component and places it carefully on the board so that the defects are minimal.
- Solder Reflow: Here, the circuit board with all its components is passed through a furnace, where heat melts the solder and forms the joint. The furnace may be batch or in-line. The batch furnace is good when the quantity of boards is less. In the in-line furnace, boards out as soldered, are a part of the assembly line. In-line furnaces are used when production quantities are high. Wave soldering is used for hybrid technology.
Introduction to Hybrid Technology
Hybrid technology is not a new PCB assembly method. It refers to the mixture of both surface mount technology and through hole technology on the same printed circuit board. First, the SMT process is done, and afterward, the THT process is completed in this mixed technology. Due to this hybrid technology, the assembly process is complicated.
Conclusion
In all the above technologies, the main difference is the level of automation in manufacturing. Let’s compare SMT vs through hole technology: SMT technology can be fully automatic, so the efficiency is very high and the cost is low for mass production. Through-hole technology is inevitable for some components, as it is the only available assembly technology for some components, especially high-power components like filters, switches, relays, transformers, and connectors. However, the assembly cost of PTH assembly is high. So we can choose one technology from surface mount technology or through hole technology according to production quantity: when the production quantity is less, THT process is preferred, and when the production quantity is greater, SMT technology is preferred.
