THT assembly (in Through-Hole Technology) is one of the oldest and most established methods used in electronics manufacturing, involving the insertion of electronic component leads into holes in printed circuit boards (PCBs). Despite the growing popularity of SMT assembly in surface mount technology, THT assembly continues to play an important role, especially in applications requiring high mechanical strength and those operating with high voltages and large currents.
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What is THT assembly?
THT assembly, also known as through-hole mounting, involves placing electronic component leads into holes on the PCB. These leads are then soldered from the underside of the board, providing a robust and durable electrical connection. This mounting method is ideal for larger components that must withstand significant mechanical stress, such as connectors and terminals.
THT assembly is primarily used in the production of devices where connection stability and durability are crucial, such as medical equipment, automotive systems, and industrial devices. Additionally, THT assembly is recommended wherever high voltages and power levels are present. In such applications, THT components are favored due to their larger sizes and the spacing between leads, which reduces the risk of potential discharges (arcing).
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Types of THT components
THT assembly includes a variety of electronic components tailored for different applications, each providing distinct functions in electronic circuits. These components can be categorized into several groups, each with its unique characteristics and applications:
- Radial components – Radial components are elements where the leads are arranged on only one side of the component. They typically have a cylindrical or disc shape. Common examples include electrolytic capacitors, which are frequently used in applications requiring high capacitance in a compact form. Radial components are easy to place in PCB holes, making them convenient to assemble.
- Axial components – Axial components feature leads placed on both sides of the element (axial components). These components are often resistors, ceramic capacitors, and diodes. Axial components are very popular in THT applications but often require additional lead forming before being inserted into the PCB mounting holes.
- Odd-Form components – Odd-form components are those with non-standard shapes or sizes, requiring different mounting procedures compared to standard THT elements. Examples may include items such as connectors, terminal blocks, communication modules, coils, transformers, and relays, which often require a customized approach to assembly, mainly during automated placement, where specialized insertion machines are needed.
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THT Assembly production process
1️⃣ Preparing the PCB – The first step in the THT assembly process is preparing the printed circuit board. The PCBs must be thoroughly cleaned to remove any contaminants that may affect soldering quality. Various methods are employed, including ultrasonic cleaners, which effectively eliminate residues of chemicals, dust, and grease. It is crucial that the PCB has holes with appropriate diameters and spacing to facilitate mounting of THT components.
2️⃣ Placing THT components – The next step involves placing THT components into the holes of the PCB. In manual assembly, operators carefully position components into the corresponding PCB holes, ensuring correct orientation and polarity. For mass production, automatic insertion machines, known as Inserters, are often used to precisely forming and trim the leads of each THT component to the required length and insert them into the PCB holes.
Accurate placement of THT components is essential to ensure proper circuit functionality. Errors, such as incorrect positioning or wrong polarity, can lead to device failure.
3️⃣ THT soldering – After component placement, the soldering stage follows. Various methods are used in the THT soldering process, including:
- Manual soldering – Operators use electric soldering irons to precisely connect component leads to the THT pads on the PCB. This method suits smaller production runs, especially when revisions are needed or automated soldering methods are not suitable due to the specificity of the manufactured device.
- Wave soldering – In the wave soldering method, the PCB, after being fluxed and preheated, is submerged from its bottom side in a wave of molten solder. This technique is efficient and suitable for large production runs where time and effectiveness are critical.
- Dip soldering – This method, involves fluxing the PCB, preheating it, and then soldering by dipping the bottom side of the PCB into a solder bath. This enables quick, good quality solder connections and is used for medium production series.
- THR soldering (Through-Hole-Reflow) – This method pertains to Pin-in-Paste (PIP) / Paste-in-Hole (PIH) assembly, where solder paste is first applied to the THT holes of the PCB, typically using SMT stencil printing. THT components are then placed, and the PCB assemblies are soldered in a reflow oven. This allows THT and SMD components to be mounted in a single soldering cycle, greatly saving time and costs. This method is especially recommended for mixed assemby that include both SMD and THT components. However, the downside is the impact of high temperatures on the entire THT component, making it unsuitable for THT components that have heat-sensitive packages.
4️⃣ Quality control of THT assembly – After THT soldering, the next critical step is quality control. Each PCB should undergo testing to ensure all solder connections are made correctly. Various inspection methods can be applied during this phase:
- Visual inspection – Operators check that all components are properly positioned and that there are no visible defects.
- Automated optical inspection (AOI) – At AssemTec Europe, we utilize a modern 3D AOI system that inspects PCBs for discrepancies such as missing components, incorrect placements, poor connections, or mislabeled parts.
- Electrical testing – This stage involves measuring voltage, current, conductivity, and functional testing, where we try to replicate the natural operating conditions of the device and test all functionalities.
5️⃣ Finalization of THT production – The last step in the THT assembly process is finalizing production. This stage includes separating PCB panels into individual PCBs, cleaning the PCBs, conducting additional quality checks, packaging, and preparing for shipment. The final stage may also involve the final assembly into housings. Furthermore, labels or emblems may be added, or PCBs and housings marked by laser for enhanced traceability of the final product.
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Advantages of THT assembly
THT assembly offers many advantages that make it a preferred solution across various industries:
- Strength of connections – THT technology allows for strong and durable connections that are less susceptible to mechanical damage and perform well in applications exposed to high mechanical, thermal, or electrical loads.
- Resistance to high temperatures and voltages – Due to the larger spacing between leads, THT components are better suited to operate under high voltages compared to SMD components. Furthermore, THT components, due to their significantly larger sizes, can dissipate heat more effectively — a crucial factor in high-power configurations.
- Ease of maintenance – With larger component sizes, replacing or repairing THT components is significantly simpler and cheaper compared to SMD components.
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Applications of THT assembly
THT assembly finds extensive use in areas where reliability, durability, and resistance to high voltages are required, such as:
- Medical equipment – medical devices like X-ray machines require reliable connections designed for high voltage operation
- Automotive industry – in vehicles where safety and reliability are paramount, and operating conditions often expose components to significant vibrations, THT assembly is particularly popular
- Musical instruments – THT assembly in musical instruments ensures longevity and reliability while also simplifying future servicing of the devices.
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THT assembly at AssemTec Europe
THT assembly, despite the increasing popularity of SMT technology, continues to play a significant role in electronics manufacturing. Due to its advantages, such as high mechanical strength of connections, resistance to high voltages and currents, and ease of servicing, THT assembly is an ideal solution in many industries.
By investing in THT assembly services, clients can ensure robust, reliable, and high-quality products. Attention to detail throughout each step of the production process translates into user satisfaction and long-lasting device performance.