PCB Assembly

SMT Assembly: Introduction, Applications, Steps & Common Errors

what is SMT assembly

What is SMT Assembly?

With so many terms and technical jargon, the world of PCBs (printed circuit boards) may get overwhelming, sometimes. In the process of PCB fabrication and assembly, SMT assembly i6s an important part of the job. Here is a comprehensive yet easy-to-understand guide to answer all your SMT-related questions once and for all. Keep scrolling to find what our experts have to say!

Introduction: What is SMT?

SMT assembly

After the completion of PCB lamination, the next step is to mount all the required components on the PCB bare board. For this mounting process, several technologies can be used. It mainly includes SMT (Surface Mount Technology) and THT (Through-Hole Technology).

In SMT, as the name suggests, the components are mounted directly on the surface of the PCB bare board. Pick-and-place machines are used for this process. Unlike through-hole PCB assembly, drilling and plated through holes are not required in SMT assembly. However, if both SMT and THT techniques are carried out for the assembly of a PCB, then it would require the above-mentioned processes.

SMT is often confused with SMD (Surface Mount Devices). While SMT is a technique of mounting components, SMD refers to the components that can be mounted via this technology. In other words, SMT is a process whereas SMDs are physical electronic components. SMDs are specially designed so that they are as small as possible in size. This allows the placement of a high number of components on the PCB bare board and eases the establishment of circuits via SMT among them. So, though SMT and SMD are interrelated, they are not the same.

Applications of SMT Assembly

This can be considered to be one of the advantages of SMT assembly technology as its versatility makes it suitable for various applications. They include:

  • Telecommunications
  • Consumer Electronics
  • Industrial Control Systems
  • Health and Medical Industry
  • Defence Systems
  • Aerospace Industry

Steps of SMT Assembly

You have already understood by now that the “pick-and-place” method of component placement is one of the most crucial aspects of the SMT assembly process. However, there are many steps involved in the SMT assembly. Below are the steps detailed briefly.

PCB Design, Preparation and Component Selection

Prior to SMT assembly, the initial step is to carry out PCB design, creating a schematic design using CAD (Computer-Aided Design) software. Therefore, designers and engineers start by clearly listing the functions that the PCB has to serve. Then, according to this requirement list, components are selected. Now, depending upon the components, a design is prepared that allows proper placement of all the SMDs along with ensuring enough space to establish the circuit. A PCB bare board is then manufactured as per this design. The board has solder pads where the pins of SMDs are mounted later on. For correct placement of solder paste, engineers use a custom-designed stencil and run an examination round to eliminate the chances of error.

SMT PCB design


At this stage, solder paste is printed on the PCB bare board. The “stencil” mentioned in the above step is used for this process. Solder paste helps mount the components precisely on the board and also ensures that the soldering surfaces are clean of oxidation and impurities. Note that the solder paste should be applied in an appropriate amount as per the requirement. Otherwise, the connections may not be established correctly and the SMT assembly may even fail. Therefore, get your SMT PCBs fabricated at Viasion only!

SMD Placement

Once the solder paste printing is done, the required components are placed in their positions using pick-and-place machines. These machines work with high accuracy and precision. Placing each component manually takes up a considerable amount of time and increases the chances of human error. Therefore, the process of SMT assembly is not only faster but also reduces the chances of error exponentially.

Reflow Soldering

After component placement, they are to be soldered on the board. For this process, a reflow soldering oven is used. In this oven, PCBs are conveyed through various stages of temperature exposure. Initially, the temperature is increased gradually in the preheat zone. Then, once sufficient temperature is achieved, it is maintained in the soak zone for around 60 to 90 minutes. The temperature is again raised gradually in the reflow zone. This helps melt the solder paste, ultimately soldering the SMD pins with the corresponding solder pads. Finally, the temperature is reduced gradually in the cooling zone to freeze and solidify the molten solder.

Final Cleaning and Inspection

You may think that all the crucial steps of the SMT assembly process are done. However, one should remember that every stage holds its own importance and may lead to flaws in case of negligence. Therefore, the technicians at Viasion Technology Co., Ltd. carry out all the steps with utmost care. This includes the next step of cleaning the board and running inspection tests for flaws. Several methods like X-ray inspection, flying probe testing, Automated Optical Inspection (AOI), etc. can be used for inspection purposes.

SMT assembly inspection

Note that each step requires utmost accuracy, precision and care. Any small error can lead to a faulty PCB. This further calls for rework which is a rather time-consuming and costly process. Therefore, hire expert PCB manufacturers, like Viasion, for all your PCB fabrication and assembly requirements.

Common Errors in SMT Assembly

The entire SMT assembly process is monitored closely and several inspection rounds are carried out to avoid errors. This is because no matter how experienced or advanced a manufacturer may be, errors are unavoidable. PCBs are tiny boards with several devices mounted on them. Therefore, flaws like short-circuiting and improper soldering may occur. Below are some common errors that may occur in this process, their potential causes and possible solutions.


When an unwanted connection is established between two conductors due to incorrect solder placement, an electrical short is caused. This error known as solder bridging leads to failure of the circuit. This is usually caused due to factors like faulty solder paste printing process, deposition of excess solder paste, incorrect placement of stencil, etc. To avoid this error, solder paste should be printed in sufficient amounts only, the reflow oven must be programmed correctly and stencil thickness and aperture dimensions should also be suitable to avoid excessive deposition of solder paste. Additionally, designers must keep in mind that the components are not placed too closely, that is, there is enough spacing between the components to maintain a decent component density in the PCB.

Electrical Opens

When the supposed connection between two points is not established due to any reason, it results in “electrical opens”. In other words, when there is a break in the circuit, it affects the functioning of the SMT assembly. This may occur due to several reasons including insufficient solder paste printing and a faulty PCB fabrication. Therefore, solder printing should be carried out carefully using a precisely crafted stencil to ensure sufficient printing. Also, the environment must be controlled to avoid solder paste contamination. To avoid faulty PCB fabrication, prefer getting your PCB fabricated and assembled in the same facility. Vision Technology Co., Ltd. provides a one-stop solution for all your PCB-related needs. Get in touch today to request a quote.

Many other errors like solder balling, tombstoning, de-wetting, solder beading, grainy joints, etc. may occur during the SMT assembly process as it is not a zero-defect soldering process. Experts at Viasion pay attention to every detail and take extra measures to prevent all such errors. So, if you are looking for error-free PCBs, look no further than Viasion!

errors in SMT assembly

SMT Development Trends

While the SMT assembly process is quite fast-paced, the future may see further improvement in the speed of SMT machinery. Exponentially increasing competition in various industries calls for faster operations that are flexible and have high responsiveness. Therefore, the trends are indicating drastic advancements in equipment systems which will in turn increase production and control efficiency.

With the world as a whole moving towards greener and more sustainable solutions, the PCB manufacturing industry is also expected to observe some changes in the same line. Currently, the raw materials used, the manufacturing and assembly processes, packaging materials and shipping practices are not very eco-friendly. So, the future calls for improvements in the PCB manufacturing industry while keeping environmental protection in mind. Manufacturing regulations emphasizing sustainability may also come into existence in the future.


SMT assembly is a technology that helps many devices run efficiently without coming into the limelight itself. It has multiple applications along with some chances of errors. Expert engineers at Viasion work in a way that they eliminate the possibility of these errors and craft fault-free PCBs suitable for your required application. SMT also has its own set of pros and cons. To know more about SMT and other assembly technologies like PTH, keep checking this space!

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