SMT assembly process

Close-up Shot of Printed Circuit Board on a Factory Assembly Line with Automated Robotic Arm for Surface Mounting Microprocessors to the Motherboard.
Printed Circuit Board on a Factory Assembly Line with Automated Robotic Arm for Surface Mounting Microprocessors to the Motherboard.

When the components for your printed circuit boards (PCBs) have been sourced and gathered, they need to be physically mounted to the board itself. This is known as the assembly process. 

Generally speaking, there are two main approaches to this process: surface mount technology (also known as SMT assembly), or through-hole technology (also known as THT assembly). 

Both of these assembly methods have their own positives and drawbacks. As a result, the right assembly service for your needs will depend on the scope of the project and the needs of the PCB.

In this blog, we’ll outline the SMT process in detail. In doing so, we’ll help you understand what it is, when it’s needed and what the process looks like. We’ll also discuss how it’s different from the THT assembly process.

What is SMT and when do you need it?

Surface mount technology (SMT) involves soldering PCB components into their required locations on the PCB’s substrate. With this method, surface mount components (SMCs) are soldered onto the board directly through reflow soldering.

Due to this, the SMT process is much quicker than the THT assembly process, which requires passing the conductive ‘legs’ of your components through small apertures and then soldering them to pads on the reverse face.

Plus, the SMT process can be made even more efficient through the use of a pick and place PCB assembly machine. This machine can be used to arrange all of the components accurately and at speed before the soldering process begins.

On top of this, it’s also worth keeping in mind that components that are designed for SMT layouts often tend to be smaller and cheaper than those that are designed for THT builds. Due to this, our SMT assembly service is usually the preferred option when people want to keep costs low and save space.

What does the process look like?

The SMT process starts during the design phase, when the components are selected and the PCB itself is designed. By starting the process at such an early stage, all aspects of assembly can be considered. Starting the SMT process at this point also makes production more straightforward.

Once the PCB has been designed and you’ve decided the SMT process is the right one for your needs, the assembly can begin. This involves the following three stages (and multiple inspection points):

Solder paste printing

The solder paste printing process is carried out by a machine to ensure accuracy and speed.

During this part of the assembly, a printer applies solder paste using a pre-made stencil of the PCB and squeegees. This solder paste is usually a mixture of flux and tin and it’s used to connect the SMC and solder pads on the PCB.

During this part of the process, it’s vital that each pad is covered in the correct amount of paste. If not, a connection will not be established when the solder is melted in the reflow oven (more on that later).

Controlling the quality of the solder paste printing process is vital. This is because, if any printing defects are left undetected at this stage, they will lead to other defects further down the line. For this reason, the design of the stencil is key and care must be taken by the assembly team to ensure that the process is repeatable and stable. Thankfully, to smooth the process, most solder paste printers have the option of including an automatic inspection.

However, sometimes external machines are used to assess the quality of the printing. These solder printer inspection machines use 3D technology and allow for a more thorough inspection. This is because they check for things like solder paste volume per pad, rather than just the print area.

Components placement

Once the PCB has passed inspection, it moves to the component placement phase of the SMT assembly process.

During this phase, each component that will be mounted on the PCB is removed from its packaging using a vacuum or a gripper nozzle. Following this, a machine places it in its programmed location. Not only are the machines that carry out this process highly accurate, but they’re also incredibly quick. Some of the most advanced machines can place 80,000 individual components every hour.

When all the individual components have been placed on the PCB, they must be inspected to ensure that they’ve been placed correctly. This is an incredibly important step in the process, because if any placement errors go undetected and the parts are soldered into that position, then this can lead to high volumes of rework, which can be both costly and time-consuming.

Reflow soldering

Once the placed components have passed their inspection, the process moves to the reflow soldering phase. During this part of the SMT process, the PCB is placed into a reflow soldering machine (some people refer to them as reflow ovens).

Here, all the electrical solder connections are formed between the components and the PCB. Using heat, the solder paste applied earlier is converted into a solder. Again, accuracy is vital during this stage of the process because if the PCB is heated to a temperature that’s too high, the parts or assembly could become damaged and the PCB won’t function as intended. If the temperature is too low, a connection may not be established.  

To ensure the best results, all PCBs within the soldering machine are placed on a conveyor belt. They’re then heated gradually in a series of zones before being passed through a cooling zone.

To avoid joint defects, the PCBs must stay in each zone for the correct period of time. The PCBs must then also be entirely cooled before they’re handled or moved. If not, they may warp.

After the PCBs have been through the reflow solder machine, they are inspected one final time. This inspection is usually carried out by a 3D automated optical inspection machine (AOI). This is to ensure that the solder joint quality is as expected and that no mistakes have been made during the SMT process. Machines are used for this process because they’re much quicker than humans, and more accurate in their analysis.

What is THT and how is it different?

The SMT process involves soldering components into their required locations directly on the board’s substrate. However, through-hole technology (THT) involves passing the conductive ‘legs’ – fine wire leads – of your components through small apertures that have been machined into the substrate at specific points. Following this, the ‘legs’ are then soldered onto pads on the reverse face.

Although SMT assembly has its merits, THT assembly is an ideal option when a sturdier physical bond is important at component solder points. Certain types of connectors or hardware devices can benefit greatly from this, depending on their intended application.

THT assembly takes longer than SMT assembly because additional steps are involved in both the design and manufacturing stages of the project. That said, the soldering on a THT-assembled board can be done entirely manually or it can be automated to an extent using a dedicated insertion mount machine.

You should also be aware that components that are designed for THT mounting are often larger and command slightly higher prices than their SMT equivalents. This is in part because they need to be produced with the necessary conductive ‘legs’ so that they can be inserted and secured through the PCB substrate.

Whichever of the two PCB assembly methods you use, quality soldering is essential. For your PCB to perform optimally and efficiently in its designated role, you’ll need a blend of craftsmanship, experience, tools and attention to detail. For this reason, you should always consult an expert like us.

How can ABL help?

Here at ABL Circuits, we’re one of the UK’s leading PCB assembly companies. As a result, we’re proud to offer a full range of PCB assembly options, including an SMT assembly service.  

Our skilled, qualified and highly-trained team can take care of all aspects of the PCB assembly process, from component procurement and mounting, to physical soldering and testing. Our staff have decades of hands-on experience in this area and are experts at working with precise briefs and specifications.

We have more than 30 years of experience in the industry and we pride ourselves on getting the job done right the first time, every time. On top of this, our turnaround times and your satisfaction are guaranteed.

Book a free quote today!

Ready to get started? Request a free quote from the team today. Alternatively, if you’d like to speak to a member of the team about our SMT assembly service, please don’t hesitate to get in touch with us. We’d love to help you.