PCB Assembly and Welding Technology

The PCB assembly and welding process turns a circuit board into a functional prototype. The PCB assembly (PCBA) stage includes device placement, welding, inspection, and finally testing. The PCBA process can be manual or automatic, as determined by the manufacturer at each stage. PCB design starts from the schematic diagram. Design a PCB layout based on the schematic diagram. PCB layout defines the electrical connection paths (called wiring) in a circuit and the placement of devices. Once the PCB layout design is approved, printing will proceed. The produced PCB is a glass fiber material sheet fixed together with epoxy resin. The wiring made of copper is laid on the circuit board. The device is fixed to the circuit board through the welding process. The welding process uses a material called solder to fix the device in a designated position. The soldered PCB of the device constitutes the assembled PCB. After attaching the device to the PCB, it can be tested. Product Assembly Services China

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1. Three technologies were used in the PCBA process:
(1) Through-hole technology (THT) PCBA process
In through-hole technology, electronic devices with pins or plugins are soldered onto a circuit board to form a circuit. The leads or terminals of the device are inserted through holes or pads on the PCB and soldered on the other side.
(2) Surface Mount Technology (SMT) PCBA Process
There are two types of pads: through-hole and surface mount. In PCBs that use surface mount pads, surface mount devices (SMDs) are soldered to form circuits. The welding surface is also the surface on which devices are placed using solder paste.
(3) Hybrid technology PCBA process
As circuit design becomes more complex, it is impossible to persist in using only one type of device in the circuit. In PCBs that implement complex circuits, there are both plugins and surface mount devices. This type of PCB using hybrid devices is called a hybrid technology circuit board, and its assembly process uses the hybrid technology PCBA process.

2. Steps for PCB assembly
The order of the PCBA process varies depending on the mounting technology used, and the steps for conventional PCB assembly are as follows:
(1) Application of solder paste: Firstly, apply solder paste (a small particle of solder paste mixed with flux) to the board. For this application, most PCB manufacturers use templates (with several sizes, shapes, and specifications that match the specifications), which can only correctly apply the correct amount of solder paste to certain parts of the board.
(2) Component placement: Unlike in the past, the PCB assembly process at this stage is now fully automated. The picking and placement of parts (such as surface mounted components) was once manually completed, and now is performed by robot picking and placement machines. These machines accurately place components in the pre planned area of the circuit board.
(3) Reflow: Now, the solder paste and all surface mount components are in place. What are you going to do next? Curing solder paste to the correct specifications is crucial for correctly adhering PCB components to it. This is the relevant part of PCB assembly – Reflow soldering. To achieve this, pass the components with solder paste and the components on them through the conveyor belt, which passes through the industrial grade reflux furnace. The heater in the oven melts the solder in the solder paste. Once the melting is complete, the component will move again in the conveyor belt and be exposed to a series of cooler heaters. The purpose of these coolers is to cool the melted solder and cure it.
(4) Inspection: After the reflow process, the PCB should be inspected to check its functionality. This stage can help identify poor quality connections, misplaced components, and short circuits caused by continuous movement of the circuit board during the reflow process. PCB manufacturers have adopted multiple inspection steps, such as manual inspection, automatic optical inspection, and X-ray inspection, to check the functionality of circuit boards, identify low-quality solder, and identify any potential hazards. After the inspection is completed, the assembly team will make crucial decisions. Circuit boards with several functional errors are usually scrapped. On the other hand, if there are minor errors, send the board again for rework.
(5) Through hole component insertion: Some types of PCBs require through hole components to be inserted together with conventional SMD components. This stage is dedicated to the insertion of such components. To achieve this, which PCB components are used to transmit signals from one side of the board to the other, in order to create plated through holes. PCB through-hole insertion is usually achieved through manual or wave soldering to obtain results.
(6) Final inspection: It is now time to conduct the second level inspection. Here, perform functional testing on the assembled board or thoroughly inspect the PCB to monitor its electrical characteristics, including voltage, current, or signal output. Today’s manufacturers use a variety of advanced testing equipment to help determine the success or failure of finished boards.
(7) Cleaning: Due to the welding process leaving a large amount of flux residue in the PCB, it is crucial to thoroughly clean the components before delivering the final circuit board to the customer. To this end, clean polychlorinated biphenyls in deionized water. After the cleaning process, use compressed air to thoroughly dry the board. Now the PCB components can be prepared for customer inspection and inspection.

3. Welding technique
Regardless of the mounting technology used, all PCBA processes involve the welding process. Many different types of welding techniques can be used to connect electrical components to PCBs, including:
(1) Wave soldering: the most commonly used technique in THT and SMT PCBA. In wave soldering, the PCB moves on a wave like liquid hot solder, which solidifies and fixes the device.
(2) Brazing: The highest temperature, the strongest welding effect. In brazing, metal devices are attached by heating. However, this technology melts the metal at the bottom to adapt to the filler metal.
(3) Reflow soldering. The reflow soldering process uses heated solder paste to attach the device to the circuit board. The solder paste in the molten state connects the solder pads and pins on the PCB.
(4) Soft soldering: a popular technology used to fix compact and fragile devices onto printed circuit boards. In soft soldering technology, a metal space filler made of tin lead alloy is heated using an electric soldering gun or gas to fix the device to the circuit board.
(5) Hard welding: The welding effect is stronger than soft welding. Hard soldering is used to bond metal components, such as copper, brass, silver, or gold, at a temperature of approximately 600 ° F.

During the PCB assembly and welding process, if there are Design for Manufacturing (DFM) errors in the PCB design, designers will have to return to the layout process to solve these problems. And this may occur multiple times, and after multiple iterations, it will lead to significant design delays and waste of time, money, and manpower. A good DFM helps to maximize the smoothness of the entire design process, avoid unexpected occurrences, and ensure the predictability of construction costs. Product Assembly Services China

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