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PCB SMT Assembly: Modern Era Technique in PCB Manufacturing

Electronics manufacturing is advancing every day, and thus, you need to look for better and more efficient processes for manufacturing components. One of the innovative processes in electronics manufacturing is the PCB SMT assembly.

 Let’s learn more about this technology and why it has become essential in modern PCB manufacturing. 

Table of Contents

What is PCB SMT assembly?

In SMT (surface mount technology), you need not insert the electronic components through PCB holes. Instead, you can simply solder them onto the PCB through reflow soldering.

 The process works as the SMT components are designed with small tabs so that you can easily apply solder to attach these SMT devices to the PCB surface. 

On the contrary, in the through-hole PCB assembly process, you used to mount components on the PCB through lead holes drilled on the surface.

 It was essential to size holes according to the component size so that a hole could hold the components tightly.

 Following this, you need to solder the grip. However, there is no hole-drilling process in the SMT assembly process. 

Sort the surface mount devices, attach them to the PCB surface, and you are done. 

SMT PCB assembly is time-consuming and boring if you do it manually, as it requires high precision. 

However, thanks to automated SMT assembly machines, it is possible to do SMT PCB manufacturing quickly and on a large scale. 

Automatic pick and place machine for SMT PCB manufacturing

Automatic pick and place machine for SMT PCB manufacturing

PCB SMT assembly manufacturing process:

Let’s discuss this in detail for a better understanding of the SMT process.

SMC and PCB preparation:

In this preparatory step, you collect all surface mount components and the design of the PCB. 

The circuit board you use is flat, mostly silver with gold plated or tin-lead pads (solder pads) without holes. 

These solder pads support the components pins like chips and transistors. Another tool that you need to keep handy is stencil. 

The stencil helps you to fix the components during the solder paste printing stage based on the position of the solder pads. 

Collect all these tools and materials and check them for any defects. 

Solder paste printing:

In this phase, you use a printer that applies solder paste on the pads at an angle of 45-60 degrees.

 You can do this with the help of a stencil and a cleaning tool. Solder paste looks like a putty mixture with a metal solder (powdered form) and a sticky flux. 

Here, the flux acts as a glue to fix the components in place and also clean the solder surface of any oxidation and impurities. 

The solder paste connects surface mount components and solder pads on the circuit board. 

You need to be careful while coating each pad with the appropriate amount of paste only. 

Excess or less solder will not make a secure connection when you melt the solder in a reflow oven. 

Component placement:

After applying solder paste, use pick-and-place automatic machines to mount surface mount components on the board. 

The pick machines will first remove each component from its packet through a gripper nozzle or a vacuum. Next, the placement machine put them in the designated locations. 

During this process, PCBs run on a conveyor belt, and these machines quickly and accurately place all the components, reaching up to 80,000 in an hour. 

Machines must work accurately in this step, as wrongly placed components lead to faulty PCB designs. 

Reflow soldering:

After putting all the components, PCBs run through the reflow soldering oven, an electronic device for heating the solder. 

Every PCB runs through the different zones of the oven mentioned under 

Preheat zone: this is the first zone where heating starts gradually. Here, all the components and the board’s temperature rise simultaneously and slowly. 

The temperature increases at a rate of 1-2 degrees per second until it reaches 140-160 degrees Celsius. 

Soak zone: the board remains in this zone for 1-1.5 minutes at 140-160 degrees Celsius. 

Reflow zone: in this zone, the temperature again starts rising 1-2 degrees per second until it reaches 210-230 degrees Celsius. 

At this temperature, the tin of solder paste starts melting and welds the components onto the PCB pads. 

While this connection occurs, the molten solder maintains the position of the components. 

Cooling zone: PCB comes out of the heating zone and is allowed to cool in this zone. The PCBs remain in this zone until the solder freezes completely to avoid any joint defects. 

Cleaning and inspection:

Finally, in this step, you must clean the board and check the PCBs for any issues. In case any problems arise, it need immediate repair. 

To examine the SMT process, you can use automated optical inspection, magnifying lenses, X-ray inspection, flying probe tester, etc. 

All these machines help you get accurate and quick results. 

Quality control inspector 

Quality control inspector 

In the case of double-sided printed circuit boards, you have to repeat the same processes on both sides. 

Through-hole assembly (THT) Vs. Surface mounting process (SMT): Which one to choose?

Initially, it seemed that the new SMT technology would make the thru-hole assembly process obsolete. However, the assembly still has many benefits. You have to choose the proper one as per your requirements. 

PCB assemblyTHTSMT
AdvantagesComponents mounted via through-hole technology offer high reliability. The components’ lead passes all the board layers and gets soldered in one place. Thus, the products are more robust and are resistant to environmental stresses. 
Most large units use this technology, so you need not worry about lowering costs using smaller components.
As the through-hole process does not involve soldering, you can use it for prototyping and testing because adjustments and changes are easier. 
SMT uses boards that are one-third in size and one-tenth in weight of the boards used in the through-hole assembly process. 
You can set more electrical connections with the SMT process as you can use both sides of the board to mount the components, utilizing more space.
Surface mounting of components is easier and faster than the through-hole assembly process. 
Further, the costs involved are lesser due to automated machines. 
DisadvantagesThrough-hole components are larger and use more space. If you use it for high-density boards, you must invest more for larger-size boards.Surface-mounted components are not as secure as those which pass through the board via holes. 
Applications Suitable for capacitors, semiconductors, and plug connectors.
Due to its high reliability and ability to withstand environmental stress, military and aerospace industries use a through-hole PCB assembly process. 
LED lights used on billboards and in stadiums use through-hole PCB manufacturing technology as they can withstand outdoor conditions. 
All modern devices like laptops, smartphones, the Internet of Things, and other electronic devices use the SMT process. 
It is also used in medical devices, industrial lighting, traffic and transportation components, commercial hardware, etc. 
PCB assembly manufacturing 

PCB assembly manufacturing 


Modern electronics are becoming smaller and more complex. Thus, manufacturers use SMT techniques to design fast, accurate, high-density circuit boards. 

Whether you are looking for high-density PCBs or more reliable ones, Cloom has hi-tech production machinery and advanced management systems for PCB assembling. 

We ensure high-quality products as we only source high-quality and durable components from certified distributors. Contact us with your detailed requirements. 

I am Lillian Yang, having been a sales manager for over 10 years.

I have received many positive reviews from customers. They have praised our excellent service, on-time delivery, and high-quality cable assemblies.

For your projects, please provide cable assembly files/images/smples, etc., so that I can send you a quotation within 24 hours.

Contact me now and let’s get started on building your wire harnesses!

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