What is a SMT process?

Original Equipment Manufacturers (OEMs) are organisations that manufacture or sell products to another company. This company then uses these components in part of a wider process to create products for their customers under their own branding. A big part of creating products to sell on to other companies centres around the SMT process.

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What is SMT?

Surface mount technology (SMT) is the process wherein components are mounted onto the surface of a printed circuit board. The components are designed specifically to be directly mounted, rather than hardwired, onto the circuit board for the vast majority of electronics. 

Surface mount technology has been widely used since the s. This is because the SMT process is automated, meaning it saves time and costs for both the manufacturer and the customer. Previous methods used hand wiring through holes between the boards to bring components together, meaning this needed manual labour to do so and of course, risked human error. The SMT process is also more adaptable for technological advances in the future than the through hole wire method.  

Other advantages of surface mount technology include higher component density and smaller components for mounting alongside better performance under pressure. Using surface mount technology also means that either side of the board can have components mounted on it.

What is the typical SMT process?

There are three main steps to the process when it comes to surface mount technology which cover solder paste printing, component placing and reflow soldering.

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.

How is ABL different?

A tailored service:

ABL Circuits offers a tailored and personal service for all OEMs. We are a one-stop shop for all your PCB manufacturing needs, and can take the entire process from start to end off your hands, or alternatively, assist with an individual aspect along the way. 

Family values

We are a family run business which began modestly at our founder, Mark Leverett&#;s home. 36 years on, William Leverett is now director and is helping to run the company with his father. Family is at the heart of everything we do, which means you get a personal and passionate service. 

Decades of experience

With over 36 years of experience under our belt, ABL Circuits has a wealth of knowledge across our employees, alongside the newest state of the art technology. As one of the only manufacturers that offer the full start-to-end process, in-house, for PCB assembly, we can help to guide you through every step.

We respect your budget

We work with a range of SMEs and bigger companies and know that it is not &#;one size fits all&#; when it comes to budget and delivery. We work closely with each of our customers to understand what they are looking for and how we can provide this in the most cost efficient way, without compromising on quality.

If you&#;d like to find out more about how we work and how we can help you, get in touch for a free quote.

The need to mount components onto a printed circuit board (PCB) to turn them into a module and even a finished product gave rise to SMT manufacturing.

This blog presents the complete SMT manufacturing process.

But wait ... You may need to learn some critical terms related to SMT manufacturing before viewing the SMT process.

Terms Related to SMT

SMT: Surface mount technology. It places and solders components on the PCB pads that are on the surface of the circuit board. The PCB assembly process using SMT technology is called SMT assembly or SMT PCBA manufacturing.

PTH: Through-hole, is the other PCB assembly technology compared with SMT. The components with longer leads insert the PCB through holes and are soldered with the copper hole walls.

PCB: A printed circuit board is made of laminating and drilling multiple layers of fiberglass epoxy, prepreg, and copper layers that are etched to be the circuit graphics and plating copper in the hole walls. After PCB fabrication, it is a bare circuit board without components.

PCBA: When a PCB is surface-mounted or through-hole-mounted with components, we call this semi-finished product a PCBA. It can further assemble with other PCBAs and an enclosure to become a complete product or box-build we call it.

SMD: Surface mount device, is is an electronic component for SMT manufacturing. Compared with PTH components for PTH assembly, SMDs have a much smaller weight and size about 1/10 of a PTH component of the same function. This makes SMT manufacturing popular because of the much smaller size and finer circuits of the electronic product.

Understanding the terms, now walk through the SMT manufacturing process!

SMT Manufacturing Process

Hey, welcome to the PCB assembly factory. Do you want to view an SMT manufacturing process video?

Want plain text to explain the SMT manufacturing process? No worries, below is the step-by-step process.

Step 1. Transfer PCB into solder paste printing machine

So you are standing beside the start of an SMT assembly line. The first machine is a loader that holds all the PCBs in line for SMT manufacturing.

The loader automatically sends the bare PCB one by one through a transmission rod into a solder paste printing machine. When a PCB finishes, the next PCB is in turn.

Step 2. Solder paste printing

In a solder paste printing machine, there is an SMT stencil for silkscreen printing the solder paste on the PCB.

The PCB halts under the SMT stencil, and a scraper presses the solder paste through the stencil holes to "print" on the PCB.

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Then the PCB leaves the machine and transfers to the next.

Step 3. Solder paste inspection (SPI)

Does the SMT manufacturer place components on the PCB now? Not yet, slow down. Inspect the solder paste to avoid possible economic loss!

The PCB passes the infrared cameras of the SPI machine, which records hundreds of images in seconds and compares them with the reference.

If the solder paste position, surface, shape, and thickness on the PCB pads are qualified, the result is "good", and the PCB goes to the next step.

If it is "not good", the engineers pause the whole line and troubleshoot until everything's fine.

Step 4. Pick and place components

Now you come to the component mounting machines.

On the machines, some feeders send SMDs, and the robot arms in the machines place them at the expected positions, where the suction nozzle will come over to pick them up and place them onto the PCB.

On an automatic SMT line, there are at least two component mounting machines.

One is the high-speed mounting machine. It places SMDs with smaller footprints, such as resistors, ceramic capacitors, and inductors.

The other is the functional mounting machine. It places SMDs with larger footprints, such as electrolytic capacitors, any kind of ICs, and connectors.

(Only for BGA PCB) Step 5. X-ray inspection

If there is any BGA placed on the PCB, it is necessary to scan the hidden solder balls beneath the BGA with an X-ray. This ensures the solder balls have no breaks, pinholes, or other defects and are qualified for soldering.

Step 6. Reflow soldering

The PCB has solder paste and components, so now solder them? Yes! On the automatic SMT line, the PCB is sent into the reflow oven.

Depending on the different temperature and gas requirements, the reflow ovens have 8 to 12 temperature zones and can be normal or nitrogen reflow ones.

At the EMS PCBA manufacturer PCBONLINE, there are both normal and nitrogen reflow ovens.

For nitrogen reflow soldering, the standard of oxygen content is &#; 4%.

In the reflow soldering process, the temperatures are programmed according to the reflow profile. You can see an example below.

The process includes preheating, baking until the solder paste and PCB pads form the intermetallic compound, and cooling down to be the PCBA.

Then the PCBA is sent out of the oven and transferred toward the AOI step.

Step 7. AOI (automatic optical inspection)

Here's a note. From your SMT manufacturing provider, the AOI process had better be in-line not independent to ensure PCBA quality.

At PCBONLINE, all the SMT lines are equipped with permanently in-line AOI machines that are three-dimensional. (PCBONLINE fabricates and assembles all types of PCBs. Contact PCBONLINE the PCBA manufacturer at .)

The PCBAs pass the AOI machines' infrared cameras one by one, and all the surface details are scanned and analyzed by the computer.

Besides, each AOI process has a specialized technician to visually check the PCBAs and mark the issues if there are any.

At PCBONLINE, the not-good PCBAs have been sent for repair and separated from the good ones ever since then.

For SMT manufacturing of a double-sided PCB, the above steps finish its SMD mounting on one side. It will go back to the start of the SMT line for surface mounting on the other side.

Above is the complete SMT manufacturing process.

For a PCBA prototyping project, after the PCB assembly, the SMT manufacturer randomly selects a PCBA as a sample for the first article inspection (FAI).

FAI

The FAI equipment includes a computer and an FAI machine connected with two probe heads.

Each time the FAI technician uses the probe heads to touch the solder joints of the PCBA, the value of the component is read aloud and displayed on the screen.

If the component value is within the allowed tolerance, the result is good.

Otherwise, the result is "non-good", and the PCBA manufacturer needs to troubleshoot the entire prototype project until everything's fine.

Now you've learned the complete SMT manufacturing process and the additional FAI inspection.

Is SMT manufacturing the end of PCB assembly? No, if it also needs PTH assembly. We've briefly explained it in the above-mentioned terms, but you can learn more details from "Through Hole PCB Assembly Comprehensive Introduction".

The last question - Are SMT assembly and PTH assembly all of a PCBA manufacturing project? No.

At PCBONLINE at the post-assembly, there are value-added services, including IC programming, functional test, conformal coating, thermal aging, and box-build assembly until you receive the modules and ready-to-use electronic products.

Besides, you can save fabrication costs without function or quality sacrifice, as PCBONLINE can choose alternative components with the same function and certifications, if you need, to replace the expensive ones.

What's more, professional engineers with more than 20 years of experience provide one-on-one engineering support and optimize your design to ensure product success.

Conclusion

This blog explains critical terms related to SMT assembly and reveals every detail of the SMT manufacturing process. SMT manufacturing is a standardized process in electronics contract manufacturing, but a lot of details in the process can lead to the success or failure of the project. For successful SMT manufacturing and EMS PCBA, you can work with PCBONLINE the 24-year-old PCBA manufacturer with expertise and control over every detail.

© This article is an original work of the PCBONLINE team. Please indicate the author PCBONLINE if you reprint. If the article is reproduced without permission or indicating the author's source, PCBONLINE reserves the right to investigate the infringement.

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