A Comprehensive Guide to Surface Mount Technology in PCB Assembly

Surface mount technology (SMT) has revolutionized how electronic components are assembled onto printed circuit boards (PCBs) by directly mounting components, known as surface mount devices (SMDs) onto the board’s surface. 

Unlike older techniques, like through-hole technology, which required components to be inserted through holes drilled into the PCB, SMT uses reflow soldering to attach SMDs without the need for drilling. 

This approach eliminates the requirement for component leads to pass through the board, reducing the complexity of the assembly process and allowing for more efficient, faster production. This is why SMT has become a key method in modern electronics manufacturing.

Originally developed in the 1960s by IBM as part of small-scale computer manufacturing, SMT did not become mainstream until the 1980s. By 1990, it had become the dominant assembly method for high-tech printed circuit assemblies (PCAs).

Today, SMT is integral to virtually every electronic device, from smartphones and laptops to household appliances and toys. While the manual assembly process can be labour-intensive, the rise of automated machinery has allowed manufacturers to meet the demand for mass production, further driving the widespread adoption of SMT in modern electronics.

What does the SMT manufacturing process involve?

The SMT manufacturing process uses a highly refined and systematic series of steps designed to efficiently assemble electronic components onto PCBs. It includes several crucial stages, each contributing to the creation of small, reliable, and high-performance electronic devices. The following are some of the key steps of the SMT manufacturing process:

1. The preparation of SMCs and PCBs

The SMT process begins with the preparation of both the surface mount components (SMCs) and the PCB itself. The PCB is designed with flat solder pads, which are typically made from materials like gold, tin-lead, or silver-plated copper. These pads serve as the attachment points for the SMDs, and are carefully aligned to match the component layout.

One of the key tools used in this preparation phase is a stencil, which must be aligned to the PCB to help guide the next step involving solder paste printing. 

Both the PCB and the SMDs must undergo strict inspection before the manufacturing process can begin to ensure that all the materials are defect-free and meet the required specifications.

2. Solder paste application

The next stage in the process is solder paste printing, which is vital for establishing electrical connections between the SMDs and the PCB. Using a squeegee, a thin layer of solder paste (a mixture of metal solder and flux) is applied to the PCB’s solder pads through the stencil.

The flux within the solder paste serves to clean the pads and ensure that they are free from contaminants, while the solder itself will later be melted to form solid connections.

Precision is critical during this stage, as improper paste application—whether too little or too much—can result in poor solder joints or component misplacement, which can lead to device failure. The stencil ensures the paste is applied accurately to the predetermined solder pad positions.

3. Component placement

Once the solder paste is applied, the pick and place machines come into action. These automated machines use vacuum nozzles or gripper tools to pick up the components from their packaging and place them precisely onto the solder pads on the PCB.

The components are carefully positioned by the machines, which are capable of placing thousands of components per hour with high precision. The components are typically arranged on a conveyor belt, where each one is picked and placed at its designated spot on the board. 

This stage is especially critical because even a slight error in the placement of an SMD can lead to costly defects.

4. Reflow soldering

The next step is reflow soldering, where the PCB, with its newly placed components, is moved into a reflow oven. The oven is divided into several temperature-controlled zones, which are as follows:

• Preheat zone: The temperature is gradually raised at a controlled rate, typically between 1.0°C to 2.0°C per second until the board reaches 140°C to 160°C, which helps the board and components to heat evenly. 
• Soak zone: The board is held at a stable temperature between 140°C and 160°C for a period of 60 to 90 seconds, allowing the flux to activate and prepare the solder paste for the next stage. 
• Reflow zone: The temperature increases rapidly to a peak range of 210°C to 230°C, where the solder paste melts, creating solid solder joints between the component leads and the PCB’s pads. The molten solder’s surface tension holds the components in place, ensuring reliable electrical connections. 
• Cooling zone: After the solder has reflowed, the PCB enters a cooling zone, where it is gradually cooled to solidify the solder joints and prevent defects like cold solder joints or bridging.

If a PCB is double-sided, these steps must be repeated to ensure both sides are properly soldered.

5. Cleaning and inspection

After the reflow soldering is complete, the PCB undergoes thorough cleaning to remove any residual flux or other contaminants. This ensures that the board is ready for inspection and does not suffer from issues like short circuits caused by flux residues.

Once cleaned, the PCB is subjected to rigorous inspection. This includes modern techniques, such as automated optical inspection (AOI), X-ray inspection, and flying probe testing, to quickly identify any defects in solder joints, component placement, or other potential defects in the PCB.

If defects are found, the affected areas are repaired before the product is either tested further or stored for shipment.

How Circuits Central Can Help You Apply Surface Mount Technology to Produce Top-Quality PCBs

If you would like to benefit from the advantages of surface mount technology within PCB manufacturing, you need a reliable manufacturer like Circuits Central to count on for high-quality PCB assembly.

At Circuits Central, we are incredibly well-versed in the SMT manufacturing process, as well as in all other areas of PCB design and development. We have over 25 years of experience in top-notch PCB assembly of all types for clients in several different industries, ranging from aerospace to medicine.

Thanks to our knowledgeable and talented PCB experts and state-of-the-art PCB production facility, we can guarantee that your PCBs will be manufactured economically, efficiently, and with a level of detail that is necessary for optimal results. 

Whether you need assistance with the PCB design process, PCB prototyping, PCB assembly, or post-manufacturing testing and development, we have you covered, as the full spectrum of services that we offer allows us to help in all of these areas.

No matter what your needs are when it comes to volume or PCB complexity, we will do whatever it takes to ensure your satisfaction with the end results. Not only will we exceed your expectations in terms of product satisfaction, but we will also go above and beyond to ensure a high level of customer service at every possible opportunity. 

For more information about our approach to SMT technology within PCB assembly, or to learn more about the different PCB-related services we offer, call Circuits Central at 1 (888) 602-7264 or contact us here.