PCB Assembly Process: A Guide Through The Complete Assembly Process

16 June, 2023 Posted by Sivaranjith Sivaraman

The electronic industry is flourishing like never before, fueled by automation, IoT adoption, and digitalization across all industries, including PCB fabrication. PCBs are the foundation of all electronic devices, and every advancement in PCB manufacturing impacts the electronic industry. Since the requirement is high and is further rising, the PCB assembly process has to be fast and precise. All designers and OEMs must know the complete assembly process to ensure their products are up to standard and that the product life cycle meets current trends.

What is the PCB assembly process?

The PCB assembly process turns the board into a functional device according to the design specifications. Component placement, soldering, inspection, and testing are all processes in the PCBA's production process. The process is defined according to the device standards, so it can be automated or manual. The process is done by following a set of steps:

A brief overview of PCB assembly

It all starts with schematics and Gerber files, which serve as blueprints for assembling PCBs. Designers utilize design tools to construct schematic layouts from the circuit and logic connections needed, adhering to established design guidelines. The schematic design thus created is approved only if it passes the quality check and is suitable for actual manufacturing. A new design after going through a quality check is called DFM (Design for Manufacturing) and involves input from designers, PCB fabricators, and assemblers.

The finalized design goes to fabrication, which involves the processing of each layer of material, etching traces on each layer, & stacking of layers. Then the outer layers are processed for solder paste application, which creates the bond between the board and components. The solder pasting process used for SMT assembly is called solder printing. Following solder pasting, solder masking is carried out, which involves applying the skin to the PCB board.

There are PCB manufacturing processes that occur before the PCB assembly process. You will learn more about the real step-by-step PCB assembly procedure that follows PCB fabrication.

PCB assembly: Step-by-Step Guide

The PCB assembly process comprises two main techniques: SMT (Surface Mount Technology) and THT (Through-Hole Technology). These methods involve distinct processing steps and are selected based on factors such as the PCB's specifications, manufacturing speed, and level of automation. Following the assembly phase, a final inspection and functional testing are conducted to guarantee the PCB's quality and operational effectiveness.

Also read: Necessary Design Files to Quote Perfectly for PCB Assembly Service

Thru-Hole PCB Assembly:

It is the traditional method of assembly and involves the use of both automated and manual processes, the steps involved in the THT assembly process are:

Component placement:

In this step, trained engineers precisely choose the proper components and mount them on the PCB. Engineers mount each component precisely following the norms and operating requirements of the thru-hole mounting technique, depending on the client's PCB design files. It is critical to pay great attention to detail while developing high-quality final goods. Engineers must consider aspects such as component polarity, orientation, and standard compliance.

Inspection and Rework:

It's time to check the components to ensure they are in the correct order once they have been successfully assembled. For this, the board is positioned in a transparent frame of actual placement so that test administrators can see whether the parts are properly positioned or not. The board must be returned for rework if any misplacement is found.

Wave soldering:

Wave soldering is a soldering technology used in through-hole manufacturing. The PCB is passed through a wave of molten solder with the attached components. When the PCB passes through the wave, the exposed component leads, or terminals, make contact with the solder. Wave soldering is a fast and inexpensive way to solder large quantities of PCBs using through-hole components, including resistors, capacitors, and integrated circuits.

Surface Mount Technology (SMT) Assembly Process

The Surface Mount Technology (SMT) Assembly Process is an efficient and advanced assembly method, it performs component soldering and assembly using a fully automated technique. The steps in the SMT assembly process are listed below.

Solder paste printing:

Solder paste, a mixture of solder alloy and flux, is applied to the board using a solder paste printer. Either stencil or solder screen is utilized to deposit solder paste at the appropriate locations where the components should be placed. The solder paste melts and turns into a liquid condition as a result of regulated heating applied to the entire assembly. Reflowing liquid solder forms solid solder junctions between component leads and PCB pads. The solder hardens after cooling, holding the components in place and creating electrical connections.

Component mounting:

Component mounting is an automated process in surface-mount technology (SMT). The PCB board is transferred to a pick-and-place machine, where all the components are placed in the designated positions on the board by the automatic machine hands. The machine is programmed to place the components in the correct locations as per the PCB design fed.

Reflow soldering:

The PCB is put through a reflow oven at a temperature of 500°F as the last step in reflow soldering. The components and board form a solid bond as the solder paste melts.

Mixed PCB assembly process

The mixed assembly process combines surface mount technology (SMT) and through-hole technology (THT) to provide a wide range of options for component placement. The mixed approach enables the accommodation of various types and sizes of components in a PCB design. Certain PCB standards mandate manual component placement of specific components; on such boards, a combination of THT and SMT mounting techniques must be employed. By offering mixed PCB assembly services, manufacturers can offer flexibility in component selection and assembly procedures.

Assemblers employ the mixed assembly in different ways, such as single-side mixed mounting, where one side of the PCB utilizes SMT while the other side uses THT, or double-side mixed assembly, which incorporates both SMT and THT on both sides of the PCB.

Final Inspection and Functional Testing

The final inspection is carried out to ensure the circuitry works properly as per the design. The testing includes a visual inspection, continuity test, signal integrity test, burn-in test, compliance test, and certification test. These final functional tests help ensure the quality and reliability of the assembled PCB before it is deployed for its intended purpose. It is worth noting that the specific tests conducted may vary depending on the complexity of the PCB design, industry standards, and customer requirements.

Take away

PCB assembly services are a crucial process in electronic manufacturing since they define the end product's quality, reliability, and compliance. Expertise, attention to detail, and adherence to quality standards are essential for guaranteeing the manufacturing of reliable and functional electronic devices. As technology and manufacturing methods evolve, the assembly process improves in efficiency, precision, and scalability. It is also necessary to evolve, adapt, and accept new technologies, as electronic device production should be faster than ever before.