PCB Assembly

The Ultimate Guide to PCB SMT Assembly Process: Everything You Need to Know

PCBs (Printed Circuit Boards) are fabricated using various technologies viz. SMT, Through-hole, manual and mixed technologies. If you are passionate about learning about the SMT PCB assembly process, you have reached the right place. In this ultimate guide, you will find what SMT Assembly is, the pros and cons of the SMT assembly process, and the entire SMT assembly manufacturing process. So, why wait, continue reading to learn all about the PCB SMT assembly process by the best experts in the industry!

SMT Assembly: An Introduction

SMT or Surface Mount Technology is one of the most popular technologies used to assemble PCBs. As you already know, PCB assembly is the process of mounting and soldering all the required components to a PCB bare board. In the SMT PCB assembly process, the components are directly mounted on the surface of the PCB bare board. Unlike through-hole where holes are drilled to connect components, SMT processes do not require any such drilling of the PCB board. Here, the circuit is completed by simply mounting and soldering the components on the PCB board surface.

Steps involved in the SMT PCB assembly process

SMT PCB assembly process involves various steps most of which are automated i.e., conducted via machines. Below are the main steps.

Preparing and examining the PCB and the components

To assemble a PCB, a custom-designed PCB bare board and the components to be assembled are required. Therefore, the assembly process begins with the preparation and examination of these elements. Also, the circuit diagram and layout should be properly designed and examined before initiating the assembly process.

Cleaning the PCB

In case you are getting your PCB board manufactured at a different facility and assembled at a different one, confirm with the assembler that they thoroughly clean the PCB board before beginning the process. This is because PCB boards may get a layer of dust or impurities during packaging and shipment. This may further affect the efficiency and performance of the PCB assembly. Therefore, it is necessary to always clean the PCB boards.

Solder paste printing and inspection

Once the materials are prepared and the circuit design is examined, engineers move on to the next step which involves the application of solder paste to the stencil. This stencil is designed as per the PCB layout and has holes in the places where various components are to be placed. The solder paste passes through these holes and drops on the PCB bare board as per the design requirement. This step may seem easy to carry out but the solder application may go wrong when improper pressure is applied or when a wrong type of solder paste is used. Therefore, it is necessary to conduct this step with expertise and precision.

Once the solder paste is applied successfully, a round of inspection is carried out. This helps investigate misplacements of solder paste, excessive or insufficient amounts, and other such errors.

Placing the components

The next step in the SMT PCB assembly process after the solder paste application is to place the components correctly. For this step, manufacturers programme their automated machines as per the PCB layout. These robotic machines pick and drop the components in their specific positions not only quickly but also with high precision. However, no matter how precise or advanced the used equipment may be, it is necessary to conduct a round of inspection before moving further. This will help identify if any component is placed incorrectly before soldering them in place. The inspection round at this stage is very crucial as it helps detect errors early and helps avoid a high amount of rework. This is the most critical step in PCB SMT assembly process. Note that if a wrongly placed component goes unnoticed and proceeds further in the process of soldering, it will need a lot of rework to handle this error which will further require more time and money.

Soldering the components in place

Now that the components are placed at their designated positions, they need to be soldered. For this, the reflow soldering technique is utilised. In this process, the PCB board along with the components is conveyed into a reflow oven where they are exposed to heat uniformly. This helps solder the joints between the components and the printed circuit board.

The strength and quality of the solder joints ultimately define the functioning and the quality of the PCB assembly. Therefore, make sure you work with certified manufacturers, like Viasion Technology Co. Ltd. only. Such experts ensure that the temperatures in the reflow oven are increased and decreased gradually for proper melting and solidifying of the solder paste.

Final inspection

The SMT PCB assembly process is not complete without a final inspection. However, before that, the PCB may also require some cleaning. Once the PCB assembly is done with all such finishing touches, the final inspection phase begins. This involves testing via an AOI machine (automated optical inspection machine). Remember that all the inspection stages are quite necessary to carry out and should not be taken lightly. No matter how carefully the entire process is carried out, there are always chances of errors and defects. PCB assemblies form the foundation of electronic devices and therefore, one small error in their fabrication may lead to the failure of the entire device. So, in addition to the AOI inspection, functional testing and manual inspection may also be carried out depending on the PCB requirements.

SMT PCB assembly

The Advantages and Disadvantages of SMT PCB assembly

Like most things under the sky, the SMT PCB assembly process also comes with both advantages and disadvantages. PCB fabricators keep these pros and cons in mind while choosing an assembly process. They consider all the factors and specific PCB requirements for various devices and thus, employ the best technology. This helps generate the maximum benefit and reduce the chances of facing disadvantages.

Find below the pros and cons of the PCB SMT assembly process.

Advantages of SMT PCB assembly

The SMT technology comes with several benefits which make it a highly chosen one in most PCB assembly projects.

  • Is faster and simpler: Since the components are mounted directly on the surface, the process is short, simple, direct and fast. As compared to the through-hole assembly process, where there are additional steps of drilling holes and connecting component leads in them, the SMT PCB assembly process is quite simple and less complicated. This also reduces their production and manufacturing time, thus, speeding up the entire process.
  • Helps mount small and delicate components: In through-hole technology, small parts are difficult to connect via their leads into the drilled holes. Therefore, when very small and delicate components are needed to be mounted on a PCB bare board, SMT technology is employed.
  • Is economical and affordable: The PCB SMT assembly process is usually lower in price as compared to other technologies, especially the manual process. The cost depends upon various factors like the number of components to be assembled, their types, the size of the order, etc. Therefore, it may vary widely from one order to another. However, in most cases, SMT technology proves to be quite affordable.
  • Helps assemble small-size, compact PCBs: Both through-hole and manual technologies are preferred in the cases of larger PCBs. In cases where PCBs with a compact design are to be assembled, SMT is used. Additionally, even a large number of components can be mounted on a small PCB bare board, thanks to the SMT PCB assembly process.
  • Is suitable for medium to large-scale projects: While the manual assembly process has its own benefits, it is not feasible to employ the process for large-scale orders. SMT technology is widely used to handle various order sizes ranging from medium to large.
  • Is more efficient and less error-prone: Most of the SMT assembly steps are automated, i.e., they are carried out via machines and not humans. This increases the efficiency of the process and reduces the chances of errors.

Now, it may seem that the SMT PCB Assembly process is perfect and has no drawbacks. But as every coin has two sides, SMT technology also has some disadvantages due to which other technologies are used in combination. Ahead are the cons.

Disadvantages of SMT PCB assembly

The SMT technology is not fool-proof, which is why a combination of different assembly technologies is used often. One of the major drawbacks of surface mount technology is that a PCB board assembled via this process may not perform well in extreme heat and/or pressure. Therefore, if a device is expected to heat too much, SMT PCB Assembly processes are rarely employed.

In addition to temperature and environmental stress, PCBs assembled via SMT may not tolerate mechanical stress. Also, manual handling may become difficult due to small-sized PCBs and delicate joints. Therefore, to ensure the proper functioning of a PCB in such conditions, SMT is combined with through-hole technology in the same PCB board. This helps attain the benefits of both techniques and minimises the chances of error.

Note that no matter what technology you opt for, it is crucial that you work with expert designers, technicians, and electronics contract manufacturers only. Remember, the proficiency of staff is directly proportional to the quality of the PCB assembly.

PCB SMT Assembly

Situations when SMT assembly should and should not be considered

Now that you have learned in detail about the SMT PCB assembly process, you may wonder whether you should go for this technology or not. As you have already learnt even though extensively used, SMT is not the answer to all PCB assembly requirements. Therefore, below is a simplified list to help you make a wise decision.

Consider opting for SMT assembly technology when:

  • You need to assemble a lot of SMD components on a small PCB board
  • The SMT components are too small or delicate to handle
  • The PCB design is too compact
  • The final product is not expected to heat up to extreme levels
  • You need a high-speed and high-efficiency production
  • You need to get the PCB assemblies in high volume

Consider avoiding the SMT PCB assembly process or prefer a combination technology when:

  • The device is designed with PTH components
  • You are working on a prototype with small quantity
  • Very simple PCB assembly prototype with quick turn delivery
  • The components are too large in size
  • The PCB dimension is too large for SMT equipment
  • The components cannot be soldered by machines and need manual handling

The Bottom Line

While this guide covers almost all the necessary information related to PCB SMT assembly processes, you may still be in need of personalised guidance and more information regarding the subject. For expert assistance and related services, visit the website of Viasion Technology Co. Ltd. Viasion is one of the most trusted PCB fabricators in China that offers custom PCB-related services globally. With 16+ years of experience, the company holds expertise in handling small-to-medium volume projects. Get in touch today for more information!


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