Surface Mount Technology (SMT) is an assembly method that mounts electronic components directly onto the surface of a printed circuit board (PCB). These components are also known as surface mount devices (SMDs). SMT is popular in the market due to its low cost and high quality efficiency. Furthermore, in recent years, SMT PCBs have become one of the common and cost-effective methods of circuit creation.

　　 What is an SMT PCB?

　　Understanding SMT PCBs isn't as simple as one might think. However, as the name suggests, the method of PCB assembly involves placing components onto a blank PCB. When building complex PCBs, SMT printers can usually automate this while adopting a specific design orientation. The accuracy this technology offers is what makes it so important. Before SMT, regular PCBs could only be so small until they became difficult to handle. Currently, the SMT method allows PCBs to become smaller.

　　Unlike previously created PCBs, this is usually mechanized, saving everyone time and money.

　　In SMT assembly, components are not inserted into holes; instead, they are soldered directly onto the circuit board using reflow soldering.

　 　How does Surface Mount Technology work?

　　Surface-mounted transistors, diodes, capacitors, and other components are placed on a printed circuit board (PCB assembly) using pick-and-place machines. These machines pick the components from their storage containers and place them onto the PCB.

　　Sometimes surface mount components come pre-applied with solder paste. You can place these components directly onto the PCB without using any extra solder paste. The components are then soldered together to establish connections. Usually, soldering is done by placing a hot soldering iron on top of the component, melting the solder, and connecting it to the PCB.

　　Typically, a small piece of tape is applied to the bottom of each SMT component to secure it to

　　its place. This also prevents the components from sliding off the PCB during assembly (especially when using pick-and-place machines). The tape should be removed before soldering.

　 　SMT Assembly Process

　　The three stages of the SMT assembly process include solder paste printing, component placement, and reflow soldering. However, due to the needs of the SMT manufacturing process, the following outline details these stages more thoroughly:

　 　1. Preparation of SMCs and PCBs

　　The initial steps involve selecting the SMCs and PCBs. The pads are usually flat without holes, and the copper pads are usually silver-plated, tin-lead-plated, or gold-plated. The pads protect the pins of components such as transistors and semiconductors.

　　The stencil is another important tool, used to create fixed positions for the next step of the process (i.e., solder paste printing), depending on the location of the pads on the PCB. These materials, and any other materials that will be used in the manufacturing process, must be carefully inspected for defects.

　　 2. Solder Paste Printing

　　Solder paste application is a crucial stage in the SMT process. In this stage, solder paste is applied using a squeegee (a tool used to clean printing plates) at an angle of 45° to 60°. To make solder paste, metal solder powder and flux (similar to glue) are mixed together. The flux acts as a temporary adhesive, bonding the surface mount components in place while also cleaning contaminants and oxides from the soldering surface.

　　Solder paste is also needed to connect the SMCs to the solder points on the PCB. It is important to have the right amount of solder paste on each pad. Otherwise, when the solder melts in the reflow oven, it won't connect to anything. A reflow oven is an electrically heated device used in surface mount technology (SMT) to connect electronic components to a printed circuit board (PCB).

　 　3. SMC Placement

　　Now it's time to mount the electronic components into the PCB. Manufacturers use a machine called a "pick and place" machine to do this. The machine carefully places the SMCs onto the circuit board. Once the mounting process is complete, the PCB is ready for reflow soldering.

　　 4. Reflow Soldering Process

　　Once the SMCs are in place, the PCB is fed into a reflow soldering oven and goes through the following stages:

　　Preheating Zone: In this part of the oven, the temperature of the circuit board and all its components is slowly increased. The temperature rises at a rate of 1.0°C to 2.0°C per second until it reaches between 140°C and 160°C.

　　Soak Zone: The circuit board will remain at a temperature between 140°C and 160°C for 60 to 90 seconds.

　　The circuit board then enters the reflow zone, where the temperature rises at a rate of 1.0°C-2.0°C per second to a high of 210°C-230°C to melt the tin in the solder paste, fusing the component leads to the PCB pads. During this process, the surface tension of the molten solder keeps the components in place.

　　Cooling Zone: This is the final section, ensuring that the solder freezes as it leaves the heating zone to prevent connection problems.

　　If the printed circuit board has two sides, repeat these steps. Solder paste or glue can be used to secure the SMCs in place.

　 　5. Inspection and Cleaning

　　The board is then inspected for flaws and cleaned after soldering. After soldering, the PCB cleaning step is crucial. Once the cleaning procedure is complete, the operator inspects the PCB for defects. They must make any necessary repairs or rebuilds before storage.

　　 What is the difference between SMT and SMD?

　　People often confuse the two terms SMT and SMD, and the two terms are often used interchangeably. In fact, any technology and its physical components can be so intertwined that uncertainty arises. SMT and SMD fall into this category. That is why it is important to understand the difference between SMT components and individual SMD components.

　　In short, SMT PCB technology is the method, while SMD is the equipment of that technology. SMT refers to the process of placing and soldering electrical components directly onto a PCB. Surface Mount Devices (SMDs) are another name for these components. The purpose is to place them on a printed circuit board (PCB).