Steps for PCB Assembly

PCB assembly is a lengthy process that involves several automated and manual steps. Each of these steps must be executed correctly by paying maximum attention to detail. Minor errors in any step of the assembly process will result in the final assembly failure. This article aims to familiarize you with the various processes involved in PCB assembly. For those who are new to the PCB manufacturing industry, this position will be very useful.

Some basic knowledge you must know

In order to better understand the PCB assembly process, you must have a clear understanding of the most basic unit of PCB – the base. The bottom of a PCB is composed of several layers like this.

• Substrates: These are specialized materials that can conduct electricity to the minimum extent possible. The commonly used substrates for insulation between two conductive copper layers are fluorine series resins, PPO or PPE resins, modified epoxy resins, fluorine series dielectric substrates, PTFE, etc.
• Copper: A thin layer of copper foil has been added to improve the thermal resistance and current carrying capacity of the PCB.
• Solder resist: Usually a green solder resist used to insulate copper wires from other conductive materials.
• Screen printing: The final layer of the PCB is screen printed, which helps provide text indicators for the components. The silk screen layer helps to identify test points, part numbers, warning symbols, logos, and manufacturing trademarks.

For all types of PCBs, the basic layers mentioned above are almost identical. The only difference between rigid, flexible, metal core, surface mount, or through-hole PCBs is the use of substrates. The manufacturer chose the substrate after considering the application.

DFM inspection design

Before the actual assembly process, the manufacturer should thoroughly inspect the PCB design documents to check their functionality and manufacturability. This stage is called DFM, which checks the design specifications of the PCB and analyzes any missing, redundant, or potential issues with the functionality. This stage helps detect design errors and allows designers to immediately remove all defects, leading to successful production.

Start the actual PCB assembly process

Now start the actual PCB assembly process as described below.

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 the PCB assembly process – 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.

Although the PCB assembly process steps are almost identical for all types of circuit boards, there are still some changes in THT assembly, SMT assembly, and hybrid technology. Let’s end the discussion by listing the assembly steps for THT, SMT, and hybrid boards.

Assembly steps of through-hole technology (THT)

1. Component placement
2. Inspection and correction
3. Wave soldering

Steps in Surface Mount (SMT) Assembly

1. Solder paste printing
2. Component installation
3. Reflow soldering

Steps in Hybrid Assembly

• Single sided hybrid components

1. Solder paste printing
2. SMD parts placement
3. Reflow soldering
4. THT component placement
5. Wave soldering

• One side SMT and one side THT

1. Surface mounting adhesive
2. SMD parts placement
3. Solidification
4. Flip
5. THT component placement
6. Wave soldering

• Double sided mixed packaging

1. Solder paste printing
2. SMD parts placement
3. Reflow soldering
4. Flip
5. SMT adhesive
6. SMD Part Placement
7. Solidification
8. Flipping
9. THT component placement
10. Wave soldering

Now, you must have a thorough understanding of the basic steps involved in the PCB assembly process. However, depending on the complexity of the project, these steps may also change. For customized PCBs, modifications are more obvious.