Process Flow of Through-Hole PCB Assembly Technology

As the electronics manufacturing industry evolves, an increasing number of manufacturers are outsourcing their PCBA business to professional processing plants. However, with numerous processing plants claiming to provide top-notch services, it can be challenging for customers to evaluate them effectively. One crucial aspect to consider is whether their manufacturing process is compliant with industry standards.

A typical PCBA project often involves two essential processes: SMT assembly and through-hole assembly. These processes aim to precisely install components onto the bare circuit board, enabling the desired functionality. In this article, FS Technology focuses on providing detailed insights into the through-hole assembly process. By understanding this process, customers can make more informed decisions when selecting manufacturers and gain a better understanding of the overall PCBA workflow.

Step 1: Assessing Requirements and Checking Documents

Before initiating a project with FS Technology, it is essential to provide us with the necessary design files, Bill of Materials (BoM), and other relevant documents. These files serve as detailed records of the project and enable us to conduct a thorough review. During this review process, our team carefully examines the provided files and offers upgrade suggestions or component replacements, if required.

To ensure accurate documentation, we meticulously record the quantity, item number, size, DIP component silk screen number, value, and other pertinent details of all the parts you provide based on the files. Additionally, we pay particular attention to the package shape and pinout diagram, as these specifications play a crucial role in the assembly process.

Step 2: Component Plug-in

Manual Plug-in

In PCBA projects, particularly in prototypes where turnaround time is crucial, customers often prioritize fast production to evaluate the feasibility and performance of their designs before moving to mass production or making any necessary modifications. To expedite the process, manual plug-in methods are commonly employed for THT process in smaller batch projects, eliminating the time-consuming programming required for automated assembly.

During the manual insertion process, our experienced engineers meticulously handle the plug material, carefully aligning it with the corresponding pins on the PCB board. Attention is given to the strength and precision of the plug insertion and removal to prevent any damage to the circuit board or nearby components. Additionally, we ensure consistency in the orientation, position, and height of the PCB components throughout the manual plug-in process.

Automatic Plug-in

For large-scale PCB assembly projects, FS Technology utilizes automated through hole assembly processes, particularly for consumer electronics and other electronic activities. This involves using an advanced AI plug-in machine, and the specific workflow is as follows:

Program Programming: Our skilled engineers edit the program of the plug-in machine based on the design file and BoM file, enabling the machine to automatically execute the plug-in process.
Loading and Positioning: The cleaned circuit boards are carefully positioned within the designated working area of the plug-in machine, ensuring precise alignment for component insertion.
Automatic Recognition: Automated assembly systems incorporate sophisticated vision systems and other sensors to accurately identify the slot or socket locations on the circuit board.
Component Supply: Components are typically supplied in tape and reel or other suitable forms, and they are delivered to the assembly line through automated feeding systems.
Insertion: Using robotic arms or other mechanical structures, the AI chip or module is retrieved from the feeding system and precisely inserted into the designated slot on the circuit board.

Step 3: Soldering

Manual Soldering Process

Part-to-part spacing

Manual soldering is a widely employed technique suitable for low-volume PCB assembly, lab assembly work, rework/repair tasks, and adding additional components to machine-soldered assemblies. This traditional method involves manually soldering leaded components to the PCB while ensuring appropriate distances between components, typically ranging from 50 to 100 mil pad-to-pad spacing.

In manual soldering, solder in the form of flux cored wire is commonly used. The operator has direct visibility of the soldering process, allowing for adaptability in creating reliable connections based on different circumstances and weldability requirements. Soldering remains a preferred choice for achieving the highest level of reliability in electrical connections. However, for batch soldering scenarios, where larger quantities need to be assembled, relying solely on high-quality hand soldering becomes economically impractical.

Use of Solder Iron

In manual soldering, the key focus is on achieving proficiency in handling the soldering iron. At FS Technology, we prioritize the safety of our technicians and recommend the implementation of protective measures to prevent burns during the soldering process.

The soldering iron utilizes electric current to heat its tip, which is a metal piece with a tin-plated surface specifically designed for soldering purposes. Due to the generally weak heat conduction from the tip to the joint, it is crucial to ensure that the tip temperature exceeds the soldering temperature.

To ensure the production of high-quality connections, it is essential to use a soldering iron with a properly tinned tip. It is recommended to clean the tip using a damp cloth or sponge, avoiding the use of files or steel brushes, as these can damage the tip surface. Commercially available soldering iron tips are available in various shapes and sizes, including specialized hollow suction desoldering tips. While there have been efforts to standardize tip designs, a universally accepted standardization has yet to be achieved in the industry.

Automatic Soldering

Wave Soldering

In the current PCB assembly industry, wave soldering has become a widely adopted method for assembling and processing through-hole circuit boards. At our PCBA factory, we utilize wave soldering to ensure efficient and reliable production.

During the wave soldering process, a flux is applied to the printed circuit board in a seamless, in-line procedure. The board is then preheated to prepare it for the subsequent step of being submerged in molten solder. This controlled wave of solder plays a crucial role in separating the connecting bridge between the solder joint and the pin, effectively preventing soldering defects. It is important to note that the entire wave soldering process is automated. The specific steps involved in the wave soldering process are as follows:

The completed PCBA, which has undergone the through-hole insertion process, is transported to the soldering equipment through a docking station.
Within the soldering equipment, the PCBA is introduced into a separate tank where a thin layer of PCB flux is applied to the underlying surface of the board and its pins. This flux coating plays a vital role in facilitating the soldering process.
The PCBA then proceeds to a preheating zone, where the temperature is carefully controlled. During this stage, the flux on the board undergoes a process of solvent evaporation, preparing the board for optimal soldering.
As the preheated board moves forward, the active agents and resins present in the flux undergo decomposition and activation. This step ensures thorough preheating of both the board and its components.
The underside of the board encounters the first wave of molten solder, which is carefully regulated to achieve precise soldering. The solder adheres to the PCBA board surface with the assistance of this wave.
Subsequently, the second wave of hot solder comes into contact with the bottom side of the PCBA board, further enhancing the soldering quality and ensuring reliable connections.

Wave Soldering Machine

A soldering machine typically has the following four components:

Flux sprayer
Preheating pad
The pump creates the wave
Pan solder

A wave soldering machine has a solder tank that is first heated up and a specific temperature is kept on it for the soldering process.

Most of the time, a solder wave is inside the tank and the PCBA will go over the bottom of the board. This means that the board links up with the solder wave at that point.

Additionally, one needs to be careful when changing the wave’s height. By doing this, you would have to stop the wave from going over the front of the board. This is when the solder gets to where it needs to be.

The metal fingers on the conveyor make sure that the PCBA stays in place in a specific spot. Different temperatures won’t hurt the metal fingers as it is made of titanium. Plus, the solder doesn’t change it in any way.

Step 4: Washing

The surface of the PCBA will still have flux, ionic impurities, organic residues, and other substances after the soldering process is complete. The majority of circuit leakage may be attributed to the impurities that have attached themselves to the surface of the PCBA board. These compounds have the potential to cause harm to electronic components and disrupt circuits. Ionic contaminants on the board’s surface can cause it to malfunction if it is kept in a humid environment. This will lead to common PCBA problems or even circuit board shorts. Because of this, not only is it necessary for us to clean the PCB before soldering but also the cleaning that occurs after soldering is of utmost significance.

Even though water washing is one of the more affordable options, it is often used only after an in-depth investigation of technical procedures has been conducted. Water-washing fluxes must be powerful enough to leave behind corrosive residues on the PCB and its components if they are to be used. It’s essential to keep in mind that water won’t remove rosin-based or synthetic, water-insoluble fluxes.

Cleaning the PCBA board can be used as one of the criteria to measure the strength of a PCBA company. As far as I know, many small workshops do not wash the board after the PCB assembly is completed, which may eventually lead to a rather ordinary inspection of the product by a customer although problems may arise when it is put into electronic applications.

Step 5: Inspection and Functionality Test

During the through-hole assembly process, your PCBA board must undergo several checks and the inspection process includes visual inspections, AOI inspection, in-circuit testing (ICT) or flying probe tests, and even automated or semi-automatic bench tests. Functional testing determines if we will send a produced board to a customer after it has been made and left the factory. This testing is the most exhaustive testing regimen available. The objective is to uncover manufacturing faults that impair dependability, such as missing or wrong components, and solder connections that aren’t correctly connected.

This all means that the THT PCBA is subjected to incoming inspections that comply with the applicable standard and regulations. Things that need to be checked when inspecting components include whether the components’ performance, specifications, and packaging meet order requirements, product reliability requirements, assembly technology and equipment requirements, and storage requirements. In addition to the basic checks mentioned above, we also test the lead collinearity and lead coating layer thickness to verify that they can withstand heating and comply with the technology’s requirements.

Through-Hole PCB Assembly Process FAQ

Is through-hole assembly a must?

While Surface Mount Technology has significantly contributed to the miniaturization of electronic devices, the use of Through-Hole Technology remains essential in certain aspects of PCB assembly. THT is commonly employed for components packaged in Dual In-Line format.

Does FS Technology provide through-hole assembly service?

We proudly offer comprehensive turnkey through-hole assembly services, ensuring a seamless and efficient process from the manufacturing of through-hole PCB to the final assembly.