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Choosing between SMT and through-hole assembly: what you need to know
For any printed circuit board (PCB) designer or manufacturer, understanding the advantages and disadvantages of surface mount technology (SMT) and through-hole technology (THT) is crucial. This blog post explores the key differences between these two PCB assembly methods to help you make a better choice for your next project.
For any printed circuit board (PCB) designer or manufacturer, understanding the advantages and disadvantages of surface mount technology (SMT) and through-hole technology (THT) is crucial. This blog post explores the key differences between these two PCB assembly methods to help you make a better choice for your next project.
SMT vs. Through-Hole Assembly: A Brief Overview
Surface Mount Technology (SMT) assembly is a technique that involves placing components directly onto a printed circuit board (PCB). This efficient method allows for higher density but lighter weight end products.
In contrast, through-hole assembly has been around for some time. In this assembly, components are inserted into pre-drilled holes on the PCB, and then wave soldering is performed from the bottom to fill the holes and establish the necessary interconnections. This seemingly small difference changes many things, such as the circuit board design, the materials and processes used, heat dissipation, and the associated labor and setup costs.
That's where this guide comes in. It helps you understand the differences between SMT and THT so you can choose the best method for your next project. Additionally, it provides some useful tips for designing SMT circuit board designs to ensure they are easy to assemble.
Key Points
• SMT and THT as Complementary Methods: Surface mount and through-hole components are often used together in a single product to leverage the strengths of each technology according to the specific application requirements.
• SMT Design for Manufacturability: Design for manufacturability involves considering aspects such as standard sizes and shapes, component selection, alignment and spacing, through-hole handling, and design for testability to simplify production, reduce costs, and improve product quality.
• THT Use Cases: THT is suitable for high-stress mechanical applications, high-power and high-temperature applications where reliability is a must.
Understanding the Basics of SMT PCB Assembly
SMT assembly refers to the process of assembling electronic devices by mounting components directly onto the surface of a printed circuit board through a process called bulk solder reflow.
The process begins with applying solder paste to the PCB. Components are then strategically placed on the board. The entire assembly is then heated in a controlled manner through a reflow oven. The heat melts, or "reflows," the solder paste, creating a connection between the component and the PCB. After cooling, the solder solidifies, forming a strong bond between the component and the PCB.
SMT assembly is highly automated. This can significantly reduce labor costs for large-scale production. However, this method may be less efficient for small-scale production. Furthermore, skilled workers are needed to manage the complex equipment and perform any necessary manual rework.
The components used in SMT are called surface mount devices (SMD) or surface mount components (SMC). The term SMD is also used in the electronics industry to refer to solder pads defined by solder mask. Unlike traditional components, SMDs have a unique package. They lack wired leads and instead have small metal or ceramic contacts on their bottom, allowing them to be directly soldered to pads on the PCB. This unique packaging gives SMDs certain characteristics:
They are smaller and lighter than conventional components.
They have shorter lead lengths, resulting in lower inductance and capacitance, thus improving electrical performance.
SMDs are more susceptible to cracking related to moisture.
There are three main types of SMT assembly:
Type I: This type uses only SMDs. Components can be mounted on one side (single-sided) or both sides (double-sided) of the PCB.
Type II: This type uses a mix of SMDs and through-hole components. This type is usually used when some components are not available in SMD form. This assembly type is more complex to manufacture as it involves many process steps.
Type III: This type replaces only discrete components (such as diodes or resistors) with surface mount components. Typically, these discrete surface mount components are fixed on the bottom side of the PCB, while THT components are placed on the top side.
Advantages of SMT Assembly
SMT assembly offers several design and manufacturing advantages, such as:
High Component Density: SMT assembly allows for more components per square inch of PCB space because SMD components are small and can be placed on both sides of the board. This is beneficial for more complex circuit designs without enlarging the PCBA.
Reduced Weight: SMT components can weigh up to 10 times less than traditional components. This weight reduction is especially important for the aerospace industry where weight reduction is key.
Improved Electrical Performance: Surface mount packages have lower parasitic effects (unwanted inductance and capacitance components). This can reduce propagation delays and noise, and this improved electrical performance becomes more important and critical as clock speeds increase.
Easier to Automate: SMT is highly compatible with automated assembly processes. Automated pick-and-place machines can place components faster and more accurately than manual labor, significantly speeding up the assembly process. This helps reduce prototyping time and improves scalability.
Cost Efficiency: SMT assembly reduces the need for drilling, thus lowering board costs. The reduced number of drill holes also benefits routing on the board.
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