Steps in the PCB Assembly Process

Steps in the PCB Assembly Process

PCB assembly is a lengthy process involving several automated and manual steps. Each of these steps must be performed correctly with the greatest attention to detail. Minor mistakes in any step of the assembly process will cause the final assembly to fail. This post is designed to familiarize you with the various processes involved in PCB assembly. This position will be very useful for those who are new to the PCB manufacturing industry.

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

l Substrates: These are specialized materials that conduct electricity minimally. Common substrates used as insulating layers between two conductive copper layers are fluorine series resins, PPO or PPE resins and modified epoxy resins, fluorine series dielectric substrates, PTFE, etc.

l Copper: A thin layer of copper foil is added to improve the thermal resistance and current carrying capacity of the PCB.

l Solder resist: usually green solder resist, used to insulate copper wires from other conductive materials.

l Silkscreen: The final layer silkscreen of the PCB, which helps provide text indicators for the part. Silkscreen layers help identify test points, part numbers, warning symbols, logos and manufacturer markings.

The basic layers above are pretty much the same for all types of PCBs. The only thing that differs on rigid, flex, metal core, surface mount or through hole PCBs is the use of a substrate. The manufacturer chooses the substrate after considering the application.

Design for Manufacturability (DFM) Check

Before the actual assembly process, the manufacturer should thoroughly examine the PCB design file to check its functionality and manufacturability. This phase, called DFM, examines the design specifications of the PCB while analyzing any missing, redundant, or potentially problematic features. This stage helps detect design errors and allows designers to remove all defects immediately, leading to successful production.

Start the actual PCB assembly process

Now begins the actual PCB assembly process as described below.

1. Application of solder paste: First, apply solder paste (a small particle of solder paste mixed with flux) on the board. For this application, most PCB manufacturers use stencils (several sizes, shapes, and specs) that can only properly apply the correct amount of solder paste to certain parts of the board.

2. Placement of components: Unlike in the past, the PCB assembly process at this stage is now fully automated. The pick and place of parts, such as surface mount components, once done manually, is now performed by robotic pick and place machines. These machines precisely place components into pre-planned areas of the board.

3. Reflow: Now the solder paste and all surface mount components are in place. What are your next steps? Curing the solder paste to the correct specifications is critical to properly adhering the PCB assembly to it. This is this relevant part of the PCB assembly process - reflow soldering. To do this, the components with the solder paste and the components on it are passed through a conveyor belt that runs through an industrial-grade reflow oven. A heater in the oven melts the solder in the solder paste. Once the melting is complete, the components are moved through a conveyor belt again and exposed to a series of cooler heaters. The purpose of these coolers is to cool the molten solder and allow it to solidify.

4. Inspection: After the reflow process, the PCB should be inspected to check its functionality. This stage helps identify poor quality connections, misplaced components, and shorts due to the continuous movement of the board during reflow. PCB manufacturers employ multiple inspection steps, such as manual inspection, automated optical inspection, and X-ray inspection, to check the functionality of the board, identify lower-quality solder and pinpoint any potential pitfalls. After the inspection is complete, the assembly team will make a crucial decision. Boards with several functional errors are often scrapped. On the other hand, if there are minor errors, the board is sent again for rework.

5. Through-hole component insertion: Certain types of PCBs require through-hole components to be inserted along with regular SMD components. This stage is dedicated to the insertion of such components. To do this, plated-through holes are created by which PCB components pass signals from one side of the board to the other. PCB through-hole insertion is usually achieved using manual or wave soldering.

6. Final Inspection: Now is the time for the second level inspection. Here, an assembled board is functionally tested, or a PCB is thoroughly inspected to monitor its electrical characteristics, including voltage, current, or signal output. Today's manufacturers utilize a variety of advanced test equipment to help determine the success or failure of finished boards.

7. Cleaning: Since the soldering process leaves a large amount of flux residue in the PCB, it is critical to thoroughly clean the components before delivering the final board to the customer. To do this, wash the PCBs in deionized water. After the cleaning process, use compressed air to dry the board thoroughly. The PCB assembly is now ready for customer inspection and inspection.

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

Through Hole Technology (THT) Assembly Steps

1. Component placement

2. Check and correct

3. Wave soldering

Steps in Surface Mount (SMT) Assembly

1. Solder paste printing

2. Component installation

3. Reflow soldering

Steps in Mixed Assemblies

lSingle-sided hybrid components

1. Solder paste printing

2. SMD parts placement

3. Reflow soldering

4.THT component placement

5. Wave soldering

l One side SMT and one side THT

1. Surface Mount Adhesive

2. SMD parts placement

3. Solidification

4. Flip

5.THT component placement

6. Wave soldering

l Double-sided assortment

1. Solder paste printing

2. SMD parts placement

3. Reflow soldering

4. Flip

5. SMT glue

6. SMD parts placement

7. Solidification

8. Flip

9.THT component placement

10. Wave Soldering

Now, you must have a solid 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 custom PCBs, the modifications are more obvious.