PCB Assembly Process

What is the PCB assembly process?

Regarding the PCB assembly, it’s hard for people familiar with the electronic industry to agree that the PCB assembly process is not the most critical step in developing any electronic product. The assembly process’s accuracy will determine the final product’s performance, reliability, safety and cost-effectiveness. Suppose we want to ensure a successful outcome. In that case, it is essential to have an experienced team with knowledge of PCB design principles and an understanding of component placement techniques and soldering processes.

Additionally, some experienced in the PCB assembly process may share their views, “Being accessible to quality materials such as high-grade solder paste and fluxes can help optimize the PCB assembly process for maximum efficiency, right?” The answer is yes. With careful planning and attention to detail throughout each production stage, you can be sure that your finished product will meet or exceed industry standards for quality assurance.

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Most briefly understanding of PCB assembly process

For people who is not working in PCB assembly factory like us, they may think PCB assembly is quite easy: just buy PCBs and electronic parts first, and then assembly the parts on the PCB. It is correct if we ignore the detailed steps and many times of inspections and testings. Here we would like to introduce the most most briefly understanding of PCB assembly process:

PCB and components sources

Before PCB assembly, we need buy or producer printed circuit boards as the Gerber files, and source all the electronics components ac the BOM list. Of cause, DFM and DFA are always needed before production

SMT assembly & reflow soldering

When we get all the PCBs and components, we need to apply solder paste on the SMT pads according to the stencil, and then pick and place SMD components on the PADs by machine, and lastly run through reflow oven to establish the components on the PCBs. AOI and manual visual inspections are always needed after SMT assembly.

PTH assembly and wave soldering

If there are PTH components on the PCBs, we need to insert them in the relative place after SMT assembly. When we make sure the PTH components are correctly placed, the PCBs will tun through hot waving soldering machine, hot liquid tin will run up into the PTH holes, and the pins of the electronic parts will be soldered on the PCB after cooling. AOI and manual visual inspections are always needed after PTH assembly.

Final inspection and test

After SMT assembly and PTH assembly, we will do final inspections to make sure all components are soldered correctly and pefectly. And also do ICT test, FCT (functional test) if needed to make sure all PCB will be functionally well.

PCB Assembly process in details

After a brief introduction of PCB assembly process, here we would like to talk about the all the processes in PCB assembly.

Following these steps will ensure that the PCB assembly process runs smoothly and produces an end product of the highest quality.

1. PCB bare board manufacturing/purchasing

Viasion provides one-stop services and also offers PCB bare board manufacturing. We can supply very wide range of PCBs, such as normal FR4 PCBs, High TG FR4, halogen free FR4, High speed PCB, high frequency PCB, microwave PCB, HDI PCB, blind & buried via copper, heavy copper PCB, flex PCB, rigid-flex PCB, Aluminum PCBs, copper based PCB, PTFT material, IC substrate PCB., etc.

2. DFM inspection

According to Viasion’s experience, before carrying out the actual assembly process, we thoroughly check the documents of PCB design, having a sufficient understanding of its function and manufacturability. This stage is called DFM. Some familiar with it may wonder, “Can DFM inspection check PCB design specifications and analyze if there are any missing superfluity or potential problems?” Yes, it can do all that. This stage helps inspect design mistakes and allows the designer to clear all defects, leading to successful production.

3. Incoming quality control ( IQC )

Generally, during the PCB bare board production, we will arrange the components purchasing simultaneously. After all components arrive, and before SMT assembly, we need to test and verify all incoming components and deal with the quality problem if there are any. The IQC workers will check and test if the incoming components meet our strict requirements for quality.

According to the part number and quantity of the BOM list, check if the incoming components are correct and if the shape is normal ( if they have transshape, broken pin, oxidize etc. ), especially for IC chips or other complex and expansive components. Also, we need to use tools such as test fixtures, multimeters etc., to spot test or full test for the incoming components. If we find any defects or differences with the BOM list, we will return all those components to component suppliers or customers.

4. Machine data programming—Gerber/CAD transfer to Centroid/Placement/XY file

After receiving the bare PCB board and all components, the next step is setting and programming all machines needed during the assembly. SMT machine, AOI ( automatic optic inspection ) machine, and other machines need to set up a program. For example, an SMT mounter needs to be programmed before it can accurately mount components, this program is better generated by CAD data, but it usually can’t work. Gerber files are always used because this is the file required for manufacturing bare PCB.

Before starting programming, the BOM list provided by the customer needs to be sorted out in advance, and the customer usually needs to provide the electronic file in excel format. Also need to provide Pick & place file.
Check whether there is any omission or duplication after the combination of BOM and Pick & place file, and save it into the language format required by the machine after confirmation.
Import the compiled program into the SMT machine, then find the origin and make a mark.
Correct the coordinates, optimize the saving procedure, and recheck the component direction and data.

5. Solder paste printing

Solder paste printing is the first step in the PCB assembly process, involving the use of a stencil which precisely applies the required amount of solder paste onto the PCB’s surface so that the components can be soldered on the boards properly. As recommended by Viasion’s engineers, the stencil must be the exact dimensions of the circuit board, and the solder paste must be the correct type for the soldered components. The solder paste is then spread on the PCB at the designated pads using a squeegee or a knife edge to ensure an even coating. Many may ask, “Is the solder paste made by a small particle of solder paste mixed with scaling powder?” Yes, indeed. And most manufacturers will use stencils ( They come in a few types of sizes, shapes and specifications ). These stencils will cover the rest of the PCB just enough to open up the pads where the components need to be soldered so that it can print the correct amount of solder paste onto specific parts of the PCB in the right way.

Viasion is skillful in accomplishing the process quickly and accurately using the correct solder paste printing techniques with minimal mess or waste. Moreover, we are constantly improving our techniques to ensure successful PCB assembly.

6. Component placement

After the surface of the PCB is printed with solder paste, the next step is component placement. And different from the past, this step is now fully automated. In the past, the picking and placement of parts (such as surface mount components) were once done manually. Now, the machine will pick and accurately place components in the pre-planned area of the circuit board. Then, each component will be taken out by a vacuum nozzle or fixture nozzle from the component package, checked by the vision system and placed in the programmed position at high speed.

7. Pre-Reflow Automatic Optical Inspection ( AOI )

After all the components placement correctly and before the reflow soldering, the most important thing is to verify that there are no errors. The best method for this operation is to use an AOI machine to inspect, such as inspect whether component presence, type/value and polarity are correct or not.

8. Reflow soldering

After the above steps are completed, the reflow soldering will be performed. People with little knowledge of reflow soldering may wonder, “Is reflow soldering a process of bonding the components to the PCB using solder during the PCB assembly process?” The answer is yes. It is the most popular method of soldering used in the electronics industry today, and it is used to join the components and the board securely. In reflow soldering, the solder paste is first applied to the board. Next, the components are placed in the correct positions on the PCB, and the assembly is then heated with infrared radiation and convection. As the PCB is heated, the solder paste melts and reflows around the component leads and joints, creating a secure mechanical and electrical connection.

Let’s take a glimpse at the advantage of reflow soldering.

9. Automatic Optical Inspection ( AOI ) after reflow soldering

After the reflow process, we need to carry out Automatic Optical Inspection (AOI) to check the board’s function and inspect if there are any quality problems at the soldering point. This inspection can help to ensure if there is a poor quality connection, misplaced components and short cause by the PCB’s continuous movement during the reflow soldering. Inspecting those problems like dislocation may involve one of a few different inspection methods. Those methods are manual inspection, AOI, manual inspection x radiographic testing ( AXI ) etc. Use those methods to identify low-quality solder and any potential hazards. After inspection, the team will make critical decisions and usually scrap the PCB with function mistakes. On the other side, if there are just very small faults, the worker will mark and repair them.

10. X-ray Inspection for BGAs

X-ray inspection for BGAs is a powerful tool for detecting defects and flaws in electronic components. X-rays can penetrate the surface of a BGA package, providing an internal view of the solder balls, pad connections, and other features that can be difficult to inspect with traditional optical methods. As a result, x-ray inspections offer fast and accurate results while non-destructive to the component. X-ray inspection also makes it easier to detect hidden cracks or voids in solder joints which may not be visible by other means.

X-ray inspection allows production engineers to quickly and accurately identify flaws, allowing them to take corrective action before they become a significant problem. In addition, X-ray inspections provide detailed images, enabling production engineers to make informed decisions about the reliability and performance of their components.

11. Through-hole component insertion

Some types of PCB need to make the through-hole component insertion. This step is typically finished by manual operation or going through wave soldering.

Manual solder uses a soldering iron to melt the soldering tin and utilize gravity and liquid mobility to flow solder tin into the solder holes. After the tin’s natural cooling, the through-hole component is fixed. But the efficiency of this method is low, and liquid tin has stickiness, will stick in the iron head and not easy to fall. Iron head is easy to oxidize, then easily cause impurities affecting the welding quality. The high temperature of the electric soldering iron is easy to burn the components and PCB.

Wave soldering general use in batch through-hole component solder. Its working principle is to use a heater to melt tin into the liquid state, then sweep the bottom surface of the PCB with the components inserted by the head from the liquid solder, make the components and PCB solder point dip soldering tin, after tin cooling, the through-hole components are fixed. This technology is complex and expensive and also needs the bottom surface that dips soldering tin can not have SMT components or special treatment required if there have SMT components. And it’s easy to cause tin slag accumulation, affecting the soldering quality.

12. Inspection before wave soldering

When the PTH components are inserted on the PCBs, we need to inspected if these components are placed correctly before staring wave soldering. Normally we can do visual inspection and AOI inspection.

By following these pre-soldering inspections, manufacturers can guarantee quality results with every wave-soldered joint. That ultimately leads to fewer problems, lower costs, and higher customer satisfaction. With careful inspection before wave soldering, manufacturers can ensure that each product is made to the highest standards of excellence.

Manual assembly

When the PTH components are inserted on the PCBs and inspected we will solder the components by wave soldering.

Wave soldering is an efficient and cost-effective method of attaching PTH components to PCBs. This process uses a wave of molten solder that is applied to the PCB by passing it through a bath of hot liquid solder. Then hot will will run up into the PTH holes, and components pins will be soldered on the PCB after cooling down.

As a result, wave soldering can provide reliable connections between components pins and PCBs. It also offers excellent wetting characteristics for high-quality joints with minimal defects. Consequently, wave soldering is used in many industries, including automotive electronics, medical devices, consumer products, telecommunications equipment, networking hardware and military applications.

Viasion sums up the evident advantages of wave soldering as follows. Wave soldering provides fast production speeds while ensuring accuracy and reliability in each joint. Wave soldering is also more cost-effective than other methods of joining components to PCBs. Therefore, wave soldering is the preferred method for large production runs PTH assembly, given that it offers higher throughput speeds and lower manufacturing costs than traditional hand-soldering techniques.

14. Inspections after wave soldering

Inspections after wave soldering are an essential part of the manufacturing process. They help ensure that products meet quality standards, and they can identify problems before they become costly issues. Inspections after wave soldering involve visual inspections, AOI inspection and tests to measure electrical characteristics such as continuity, voltage drop, and resistance etc. By doing these checks regularly, manufacturers can be sure their products will function correctly once in use.

Inspections after wave soldering can identify issues such as misaligned components, shorts, and opens. These issues can be caused by poor board layout, inadequate soldering technique, or faulty components. If identified early in the manufacturing process, these defects can be corrected before becoming costly problems.

15. Conformal coating

Some customers will need a conformal coating for the finished PCBA. Conformal coating is a protective and non-conductive dielectric layer on the printed circuit board assembly. The function is to protect the boards and prevent components from being eroded by pollution, salt spray, humidity, fungi, dust and harsh or extreme environment.

The conformal coating process is relatively straightforward and can be carried out using a brush, aerosol spray, dip tank or automated machine. When using the brush or aerosol method, the board should be pre-cleaned, and the appropriate coating material should be selected based on the application’s needs. The liquid coating should then be applied in the required thickness, and the board should be left to dry before the next step of the assembly process can begin. Likewise, the boards must be pre-cleaned for the dip tank or automated machine approaches, and the correct type of material must be selected based on application requirements. The boards are then placed in the appropriate solution, and the coating material is applied.

Once the boards have been coated, the excess liquid should be removed to ensure the correct level of thickness is achieved. Finally, the board should be left to dry before the remainder of the assembly process can commence. Conformal coating is a crucial step in the PCB assembly process as it helps protect the board from environmental elements and static electricity. In addition, utilizing the correct coating material during the assembly process helps ensure the final product’s quality and reliability, saving time and money in the long run.

16. Final inspection, ICT testing and function test

After the PCB finish assembly and conformal coating, our quality control team will do the final inspection according to the notes required by the customer or a comprehensive inspection to avoid any faults not being found in the above steps.

ICT test may needed to make sure the components are soldered correctly by electronic connection testing. And if the customer asks for a function test, test method, test software and tools are usually provided by customers, we also can make test fixtures according to the customer’s requirement.

17. Clean and drying

The production process is filthy. Solder paste will remain with some scaling powder. So before sending the boards to the customer, you must completely clean the oil and dirt on the surface. First, clean them with polychlorinated biphenyls in deionized water or ultrasonic washing. After cleaning, the use of compressed air will completely dry the PCB.

18. Packing and transportation

The PCBA must be packed in an anti-static bag, and Viasion will put it in the custom foam bag after packing it in an anti-static bag to avoid any damage during transportation. After packing, we can arrange the transportation according to customer requirements.

Above are almost all the processes of PCB assembly, in actual assembly, it will be more complicated, and you will encounter a lot of problems, but as an experienced PCB assembly supplier, all problems are growth for us.

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Viasion Technology is your trusted partner & one-stop shop for PCB fabrication, components sourcing, PCB assembly and electronic manufacturing. With more than 16 years of experience, we have been supplying high-quality PCBs with competitive pricing for 1000+ customers worldwide. Our company is ISO9001:2015 Certified & UL Listed, and all our products are 100% E-tested and inspected by AOI & X-RAY to meet the highest industry standards. So please get an instant quote from our sales team now, and we will take care of the rest.