What is Surface Mount PCB Assembly and How Does It Work?

This article is majorly created to offer a great resource to surface mount PCB engineers, as well as those that are interested in surface mount technology.

The main scope of this article will be the processes involved in surface mount PCB assembly. These include the solder paste printing and inspection, the placement of the component, reflow soldering, and then automatic optical inspection.

Finally, we will answer some frequently asked questions relating to the topic. Before that, let us know what smt PCB assembly is and how it works.

What is Surface Mount PCB Assembly?

Surface mount technology is a powerful and flexible assembly process, which has given manufacturers the opportunity to manufacture modern devices having ever-smaller footprints.

For SMT, the electronic components required for the devices of today are attached to the PCB’s surface. SMT could be automated; therefore this is useful for clients that require high volume circuit boards. If you want circuit boards that are assembled in line with some special requirements, then the most appropriate choice should be SMT.

How Does SMT Work?

Explaining briefly, the surface mount technology is an electronics assembly process whereby the supply of the electronic components are mounted or placed onto the printed circuit board’s surface. This process is a highly flexible one which lets the producers construct the circuit boards making use of different components.

In contrast to the similar techniques of PCB manufacturing, surface mounting features some benefits. Most importantly, this prototype PCB assembly process allows for products that are much smaller as well as increased connection densities.

Furthermore, with SMT, manufacturers can place circuits that are increasingly complex into the small assemblies. Therefore, it has always been a significant factor in rapid production of small devices used for daily living.

Furthermore, it is also easier and faster to assemble surface mount boards. Also, they are more cost-effective in contrast to boards that were constructed using through-hole technology. PCB assemblies using through hole technology is a much older technology that deals with attaching the leads to the holes that are drilled inside the circuit board.  

The process yields products that are very durable, because these are sturdy connections. Compared to the SMT, they could be costlier, although the density of the components is higher.

Four major steps are necessary during the smt PCB assembly. These include preparing the PCB, placing the components, the reflow soldering, and lastly, inspection.

Importance of Quality Control

Professional contract manufacturers know that quality is very important as clients look up to them to deliver customized units, which meets the best standards for reliability and quality.

These manufacturers have a large production space, and their facility is well designed. This is to make sure that there is a high quality control for any given job.

Furthermore, they work with the best industry-recognized practices, and make use of different monitored and scalable processes. This is to encourage transparency and accountability in their job.

Advantages of the Surface Mount PCB Assembly

Making use of SMT comes with many advantages. These include the following:

Small components

One great benefit of SMT is that small components can be attached to it and it can be done at high densities. This is why SMT results in smaller units, which may be constructed making use of other processes of PCB manufacturing. Utilizing contact terminals rather than leads, you can miniaturize the SMT components to reach a sand grain’s size.

Two-sided circuit boards

For through-hole, you can only place the components on one of the board’s sides. However, with SMT, you can attach to a side, and then to the other. This allows for extra complex circuitry and not increase the overall unit’s size significantly.

Less drilling

With through-hole technology, there is a need for holes to be drilled into the printed circuit board. Though this doesn’t create any secure connection, the drilling process adds more money to the manufacturing process.

Easily automated

SMT has lots of manufacturing benefits, which makes the entire process a very automated one. Machines will be able to perform component placement, printing of the solder paste, reflow soldering, and inspection scanning.

Highly customizable

For smt PCB assembly, you can attach different components to the circuit boards. The companies present in different industries have utilized this process of smt assembly. This is because it helps them with customization of boards for virtually all applications.

What is the Process of Surface Mount PCB Assembly?

Below are the processes involved in surface mount PCB assembly.

Machine Programming

After receiving the PCB components and panels, the next move is setting up the different machines that are used during the process of manufacturing. Machines like AOI and placement machines will require the creation of a program. This is best created from CAD data; however, this isn’t usually available

Almost all the time, the Gerber data is always available. This is because it is very necessary for the manufacturing of the bare PCB. Also, if the only data present is Gerber data, then the CPL file/centroid/XY file creation could take a lot of time. This is why surface mount offers this service to help in generating this file

Solder Paste Printing

During the manufacturing process, the first machine you should setup is the solder paste printer. This helps in applying solder paste making use of squeegees and a stencil to the right pads on the printed circuit board. This method is the most widely used during the application of solder paste. However, jet printing is gaining more popularity. This is prevalent in the sub-contract field, because stencils are not needed and it is easier to make modifications.

Ensuring that the smt process is controlled is important as printing defects, and if not detected, can cause other defects along the line. Also, as assemblies are now more complex, the stencil’s design is important and there must be proper care to make sure that the process is stable and repeatable.

Solder Paste Inspection (SPI)

Majority of machines used for solder paste include the option of adding automatic inspection. However, depending on the PCB’s size, this can be a time consuming process and therefore it is usually preferable to use a different machine.

Furthermore, the systems for inspection which are present in the solder paste printer make use of 2D technology. The SPI machines on the other hand make use of 3D technology. This helps better and thorough inspection.

Component Placement

Immediately you have confirmed the printed PCB to have the right amount of solder paste that has been applied, then the next section of the manufacturing comes into play which is the component placement.  For this, you pick each component from the packaging making use of either a gripper or vacuum nozzle; this is checked using the vision system, then placed inside the location at a very high speed.

Furthermore, there are different machines present for this. This depends largely on business as well as the machine type you have chosen. To make sure that there is repeatable and accurate component placement, it is very important to support the populated PCB fully.

Pre-Reflow Automated Optical Inspection

After you have completed the process for component placement, make sure that you verify that there are no mistakes. Also, make sure that you have placed all the parts correctly before the process of reflow soldering. To achieve this, the best way is to use an AOI machine. This helps in making checks like component presence, polarity and type/value.

First Article Inspection (FAI)

One challenge out of many that subcontract manufacturers usually face is the first smt assembly verification to the information of customers or first article inspection (FAI). This is because it can take a lot of time. This step is a significant one during the process, because if there are any errors that are not detected, this can result in large rework volumes.

Reflow Soldering

Once you have checked the component placements, then the next stage of the smt assembly will be the use of the reflow soldering machine. Here the forming of the electrical solder connections takes place. This is done between the printed circuit board and the components by applying heat to the pcb assembly at the right temperature.

This seems to be one of parts that is less complicated in the smt assembly process. However, the right reflow profile is important in ensuring that the solder joints are acceptable without bringing damage to the smt assembly or parts as a result of excessive heat.  

When making use of lead-free solder, it is very important to be careful and profiled. This is because the necessary reflow temperature could be very close to the majority of components’ highest rated temperature.

Post-Reflow AOI (Automatic Reflow Inspection)

Here comes the final part involved in the surface mount PCB assembly process. This involves going back to check again to make sure that you have not made any mistake while making use of the AOI machine in checking the quality of the solder joint.

After introducing the 3D technology, the process is now more reliable compared to the 2D inspection there. With 3D inspection, there is now better and accurate measurement. It also provides a more stable process for inspection.

Furthermore, one new feature of these inspection machines is their ability to be networked together. This permits instant feedback to allow you to make automatic adjustments.


This is the end of our article on surface mount PCB assembly. is a powerful and flexible assembly process, which has given manufacturers the opportunity to manufacture modern devices having ever-smaller footprints. If there is any aspect of this article you need some clarification on, please contact us immediately. We always have all the answers!

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