What You Need to Know About SMT Surface Mount Technology

Almost every piece of commercially produced electronic equipment these days is filled with minute devices. These components are mounted onto the surface of the boards and many are very small in size, rather than being connected to wire leads as in home construction and kits.

This technology is known as surface mount technology, SMT and it has the associated SMT components or surface mount devices or SMDs.

Surface mount technology, SMT, is used virtually in all commercially manufactured equipment today, since it offers significant advantages during PCB manufacturing, and it allows for a much greater amount of electronics to be packed into a smaller space due to its large size.

Aside from the size, surface mount technology also allows automated assembly and soldering of PCBs, bringing significant reliability improvements as well as cost savings.

What is Surface Mount Technology (SMT)?

Surface mount technology (SMT) represents an innovative assembly and production method that revolutionizes the integration of electronic components directly onto a printed circuit board (PCB). In stark contrast to conventional techniques, SMT eliminates the need for components to be inserted through holes. Instead, electronic elements are seamlessly soldered onto the board through reflow soldering, streamlining the manufacturing process and enhancing automation capabilities.

Components affixed using SMT are termed surface-mount devices (SMDs), marking a departure from the traditional Through-Hole Technology. The inception of SMT can be traced back to the 1960s when IBM, in pursuit of constructing small-scale computers, introduced the concept initially known as Planar Mounting. However, it wasn’t until 1986 that surface-mounted components gained significant traction, reaching 10% market popularity. By 1990, SMDs became ubiquitous, featuring prominently in the majority of high-tech printed circuit assemblies (PCAs).

Unlike Through-Hole Technology, where components were anchored through drilled lead holes in the PCB, SMT components boast small tabs designed for solder application, facilitating swift attachment to the PCB surface. By circumventing the hole drilling step, SMT accelerates the device assembly process significantly.

The precision demanded by the SMT assembly process makes manual execution both tedious and time-consuming. To optimize efficiency, particularly in large-scale production, automated assembly machines are commonly employed in SMT manufacturing.

Notably, SMT components exhibit a substantially reduced size compared to their through-hole counterparts. This characteristic not only contributes to the sleek and appealing design of electronic devices but also aligns seamlessly with the demands of modern technology. Consequently, SMT has become a ubiquitous presence in nearly every electronic device, ranging from toys and kitchen appliances to laptops and smartphones, shaping the landscape of contemporary electronics.

Advantages of Surface Mount Technology

Surface Mount Technology (SMT) offers a multitude of advantages that have contributed to its widespread adoption in the electronics manufacturing industry. Here are some key advantages of SMT:

1.Space Efficiency:

One of the primary advantages of SMT is its ability to significantly reduce the space required on a printed circuit board (PCB). By mounting components directly onto the surface, SMT eliminates the need for holes and leads, allowing for a more compact and space-efficient design.

2.Higher Component Density:

SMT enables a higher component density on a PCB compared to traditional through-hole technology. This increased density allows for the integration of more complex and sophisticated electronic circuits, contributing to the miniaturization of electronic devices.

3.Improved Electrical Performance:

The shorter paths and reduced parasitic elements in SMT contribute to improved electrical performance. This results in lower inductance and capacitance, reduced signal distortion, and enhanced overall circuit efficiency.

4.Enhanced Speed and Precision in Manufacturing:

SMT processes are well-suited for automation, leading to faster and more precise manufacturing. Automated pick-and-place machines accurately position components on the PCB, reducing the likelihood of human error and increasing overall production efficiency.

5.Cost-Effectiveness:

SMT is often more cost-effective than through-hole technology, especially for high-volume production. The automated assembly processes and reduced material costs associated with smaller and lighter components contribute to cost savings.

6.Improved Thermal Performance:

The compact design of SMT components and their direct attachment to the PCB enhance thermal performance. This allows for better heat dissipation, reducing the risk of overheating and contributing to the overall reliability of electronic devices.

7.Versatility in Component Types:

SMT accommodates a wide range of component types, including resistors, capacitors, integrated circuits, and more. This versatility makes it suitable for various applications across industries, from consumer electronics to automotive and medical devices.

8.Reduced Signal Disturbance:

The absence of long leads and the closer proximity of components in SMT contribute to reduced signal disturbance and improved signal integrity. This is particularly advantageous in high-frequency applications.

9.Environmental Benefits:

SMT processes typically generate less waste compared to through-hole technology. The reduction in material usage and the elimination of leaded components contribute to environmentally friendly manufacturing practices.

10.Design Flexibility:

SMT’s flexibility in design allows for creative and innovative solutions. Designers have greater freedom in arranging components, optimizing layouts, and creating products with unique form factors.

Disadvantages of Surface Mount Technology

While Surface Mount Technology (SMT) has its advantages, it also has some drawbacks:

1.Reliability with Mechanical Stress:

When components face mechanical stress, relying solely on surface mounting for attachment to the PCB may not be reliable. This is especially true when using component connectors for devices that are frequently removed and re-attached.

2.Damage to Solder Connections:

The solder connections for Surface Mount Devices (SMDs) could be damaged through thermal cycles during operations.

3.Skilled Operators and Expensive Tools:

Skilled or expert-level operators and expensive tools are required for component-level repair and manual prototype assembly due to the smaller sizes and lead spaces.

4.Limitations on Component Installation:

Most SMT component packages cannot be installed in sockets that allow easy installation and replacement of failed components.

5.Concerns with Solder Joint Reliability:

Using less solder for solder joints in SMT raises concerns about the reliability of solder joints. Void formation may lead to solder joint failures.

6.Challenges in Component Identification:

SMDs are generally smaller than through-hole components, providing less surface area for marking part IDs and component values. This makes identifying components challenging during prototyping and PCB repair.

7.Solder Melting with Heat Exposure:

Solder in SMT can melt when exposed to intense heat, making it unsuitable for electrical load circuits with high heat dissipation.

8.Higher Installation Costs:

PCBs using SMT technology incur higher installation costs. This is due to the expensive nature of SMT equipment, such as the hot air rework station, pick and place machine, solder paste screen printer, and reflow oven.

SMT Manufacturing Process

The SMT manufacturing process is typically divided into three main stages: solder paste printing, components placement, and reflow soldering. However, due to the intricacies of SMT production, each stage is further detailed in the following breakdown:

1.SMC and PCB Preparation:

This initial stage involves the selection of Surface Mount Components (SMCs) and the design of the Printed Circuit Board (PCB). The PCB features flat copper pads, typically silver, tin-lead, or gold-plated, known as solder pads, which support component pins. A stencil is crucial in this phase, aligning with predetermined positions of solder pads for the upcoming solder paste printing. All materials used must undergo meticulous examination for potential flaws.

2.Solder Paste Printing:

A critical phase in SMT, solder paste printing utilizes a printer, stencil, and squeegee. The printer applies solder paste at an angle between 45° and 60° using the stencil and squeegee. Solder paste, a mixture of powdered metal solder and flux, acts as a temporary glue, securing surface mount components in place and cleansing soldering surfaces. The solder paste connects SMCs and solder pads on the PCB, requiring precise application for effective connections during reflow soldering in a reflow oven.

3.Components Placement:

Pick-and-place machines are employed in this stage to position components on the PCB. Components are extracted from their packaging using a vacuum or gripper nozzle, and the placement machine situates them in their designated spots. Conveyor belts transport the PCB, and these machines, some capable of placing 80,000 components per hour, ensure swift and accurate placement. Precision is vital to avoid costly and time-consuming rework.

4.Reflow Soldering:

After component placement, the PCB enters a reflow soldering oven, progressing through several temperature zones:

• Preheat zone: Gradual temperature increase to 140℃-160℃.
• Soak zone: Maintaining a temperature of 140℃-160℃ for 60–90 seconds.
• Reflow zone: Temperature rise to 210℃-230℃ to melt the tin in the solder paste, welding component leads to the PCB pads. Components remain in place due to the surface tension of molten solder.
• Cooling zone: Ensures solder solidifies, preventing joint defects.

If the PCB is double-sided, these processes may be repeated using either solder paste or glue to secure SMCs.

The reflow soldering machine accounts for about 25% of the total cost of a Surface Mount Technology (SMT) device.

This SMT machine is primarily used for enabling solder reflow from the Surface Mount Device (SMD) to the circuit board. This is done after the reflow soldering is done with solder paste.

5.Cleaning and Inspection:

Following soldering, the board undergoes cleaning and inspection. Flaws are identified, repaired, and the product is then stored. Inspection methods include magnifying lenses, Automated Optical Inspection (AOI), flying probe tester, X-ray inspection, etc., employing machines for quick and accurate results instead of relying on the naked eye.

What Is the Difference Between SMT and SMD?

SMT (Surface Mount Technology) and SMD (Surface Mount Device) are related terms, but they refer to different aspects of electronic components and manufacturing processes. Here’s a breakdown of the differences:

1.Surface Mount Technology (SMT):

• Definition: SMT is a manufacturing process that involves mounting electronic components directly onto the surface of a printed circuit board (PCB). It eliminates the need for leads or wires to pass through the board.
• Process: In SMT, components are typically attached by soldering them directly onto the surface of the PCB. This process is facilitated by solder paste, which is applied to the board before components are placed. The assembly is then heated in a reflow oven to melt the solder and secure the components.

2.Surface Mount Device (SMD):

• Definition: SMD refers to the individual electronic components that are designed to be mounted directly onto the surface of a PCB. These components are characterized by their small size, flat surfaces, and lack of leads or wires.
• Examples: SMDs include various types of components such as resistors, capacitors, diodes, transistors, integrated circuits (ICs), and other semiconductor devices. These components are designed for surface mounting and are an integral part of the SMT process.

Key Differences:

• Scope: SMT is a broader term that encompasses the entire manufacturing process, while SMD specifically refers to the individual components used in that process.
• Process vs. Component: SMT describes the assembly process, focusing on how components are mounted on the PCB. SMD refers to the type of components designed for surface mounting.
• Application: SMT is a manufacturing methodology applicable to the entire electronic assembly process. SMD refers to the specific components that are an essential part of the SMT process.
• Examples: SMT involves various stages like solder paste printing, components placement, and reflow soldering. SMD includes components like ICs, resistors, and capacitors designed for surface mounting.

Closing Thoughts

With its smaller size, faster production, and reduced weight, SMT leads to an easier design and production of electronic circuits, particularly in complex circuits. As a result of this higher level of automation, electronics manufacturers have been able to save time and resources. Although new technologies are always possible, SMT has undoubtedly secured its relevance in the electronics industry.