What is the difference between SMT and SMD?

The electronics manufacturing industry is a highly professional industry. Those who are familiar with it, prefer to use basic abbreviations to express their needs, such as PCB (printed circuit board), PCBA (PCB assembly), Through Hole Technology (THT), etc. This is not friendly to novices or electronics enthusiasts. In this article, FS Technology will explain two commonly used terms in the PCBA industry, SMT and SMD, from the concept, use, and combination of these three aspects.

What is SMT?

SMT assembly equipment

Basic Concept Explanation

The full name of SMT is surface mount technology, a new method of arranging components on PCB. This technology is widely used in the PCBA industry, and almost all companies that provide PCB assembly services can provide SMT assembly services. In the early 1970s, electronic manufacturers used through-hole assembly to realize component mounting and soldering work, that is, the leads of components were placed in drilled holes on bare PCBs, and permanent component through-hole arrangements were achieved through soldering guns. However, with the development of technology, people gradually found that through-hole PCB assembly is not the best solution, and SMT assembly may serve as a replacement.

Unlike plug-ins, SMT technology uses leadless assembly to solder components directly to the bare PCB, which is a faster and more cost-effective assembly method. For some consumer electronic devices (mobile phones, computers, etc.), their update frequency is less than a year, which means that electronics manufacturers urgently need automated PCB assembly to achieve large-scale production, and SMT technology simply meets this requirement.

SMT Assembly Process

  1. Printing: The SMT instrument aligns the prepared stencil with the PCB board requiring patching. As the squeegee spreads the stencil, the solder paste is deposited onto the corresponding solder joints through the holes in the stencil.
  2. Mounting: The SMD components are accurately positioned by a pick-and-place machine, utilizing the solder paste as a temporary adhesive.
  3. Reflow Soldering: At this stage, the solder paste remains in a semi-liquid state and needs to be melted and solidified to form a robust solder joint. Reflow soldering is commonly employed in the SMT process for handling delicate-pin components like BGA and QFN, providing precise temperature control and uniform heat distribution, resulting in highly reliable solder connections.
  4. Testing and Inspection: Once the entire SMT process flow is completed, the manufacturer conducts a range of inspection methods to ensure welding quality, including component alignment, solder bridges, circuit short circuits, etc. These inspections involve manual examination, AOI inspection, and other techniques.

Features of SMT

  • Reduce parasitic capacitance and parasitic inductance;
  • Reduce the demand for components in PCBA projects;
  • Automated new equipment enables fast and reliable large-scale production;
  • Facilitating the development of tiny electronics or implantable devices.
 

In summary, SMT refers to an assembly technique that involves applying the right amount of solder paste to mount each component. It also lends itself to automation as machines can be programmed to mount components on the PCB. This ensures accuracy, and it also speeds up the production process and delivery quicker. So far, our introduction to SMT is over. If you want to learn more about SMT, please read “What does SMT stand for in electronics”.

What is SMD?

Surface Mount Device

Basic Concept Explanation

Firstly, it is essential to comprehend the operational principle of a PCB, which comprises two main elements: the bare board and the components. The bare board serves as a platform for mounting the components and establishing electrical pathways. When current is applied, it traverses the circuitry and reaches the components, enabling their functionality. Therefore, it is the components themselves that actively contribute to the operational capabilities of the PCBA board. SMD represents a specific type of component.

SMD, short for Surface Mount Device, is an electronic component that is soldered to a printed circuit board and is designed to be mounted directly on the PCB. Currently, we have a variety of installation methods for PCB components. According to different installation methods, PCB components can be divided into various types, including DIP components and SMD components, that is, plug-in components and patch components. Affected by the trend of miniaturization, more SMDs are being used in the production of PCBA nowadays. 

Types of SMD Component

It took only 20 years for surface mount components to become popular, and they have dominated the field of components today. Tracing the developmental history of components, the components of electronic equipment used by consumers ranged from passive components to active components and integrated circuits, and finally, is now ruled by SMD. Surface mount equipment can be roughly divided into two categories: transistors and integrated circuits (SOP, SOJ, PLCC, LCCC, QFP, BGA, CSP, FC, and MCM). Several common SMD components are classified below.

  • Resistor

This type of SMD resistor is usually surface mounted in SMT technology and needs special attention when it is mounted. Taking SMDs on a ceramic PCB as an example, when the ceramic PCB is patched, it is necessary to apply a glass passivation film on the surface in order to prevent the metal material from overflowing by welding.

SMD resistor
  • Ceramic Capacitor

Applications such as engine engines, military communication equipment and aerospace equipment are very dependent on high frequency circuits, so we need to use SMD SMT chip technology to mount SMD ceramic capacitors.

Ceramic SMD Capacitor
  • Transistor (SOT)

The three- and four-lead SOT devices are considered one of the pioneers of surface mount active devices. Three-lead SOTs are identified as SOT 23 (EIA TO 236) and SOT 89 (EIA TO 243). The four-lead device is called SOT 143 (EIA TO 253).

nine different SMD transistors
  • Integrated Circuit

An integrated circuit, or IC chip, is a collection of components such as resistors, capacitors, and transistors. Engineers integrate a “small PCB” with specific functions through IC programming and semiconductor processing. Small-outline SMD active integrated circuits that are common in life generally use shrink packaging. Small outline ICs have lead centers of about 0.050 inches and are primarily used to accommodate larger ICs than SOT packages.

PWB Integrated Circuit

Features of Surface Mount Device

  • The SMD body is on the same side as the solder joints;
  • SMD has no leads or only short leads;
  • Smaller size but better performance;
  • Reduce the risk of PCBA manufacturing errors by reducing through holes.
 

In general, SMD refers to a specific type of component that is surface-mounted on the PCBA board, utilizing surface mount technology for its functionality. SMD components align with the industry’s trend of miniaturization and guarantee assembly quality, making them extensively employed in PCBA production. For a more in-depth understanding of SMD components, please refer to the article “What are SMD components?”

SMD vs SMT

Difference

  • Conceptual Difference: SMT refers to a specific type of assembly technology and is commonly associated with terms like assembly, soldering, and process. On the other hand, SMD represents a type of electronic component and is typically used in conjunction with words such as component, part, etc.
  • Usage Difference: The SMT process is specifically designed for SMD components and cannot accommodate through-hole components. In contrast, SMD components can be installed on circuit boards using various soldering processes, depending on their properties and characteristics. These processes include manual soldering, hot air melting, hot plate heating, and wave soldering, among others.
  • Purpose Difference: The primary objective of the SMT process is to achieve automation and precise installation, thereby enabling efficient production. Conversely, the utilization of SMD electronic components primarily aims at achieving miniaturization and high-density integration within electronic systems.

Combination

Although SMD and SMT represent distinct concepts, they complement each other in the realm of electronics manufacturing. Considering the developmental history of PCBA, the decline of DIP components can be attributed, in part, to the limitations of manual soldering, which prompted the emergence of pick-and-place machines. While manual soldering was previously used for traditional SMD component assembly, the advent of placement machines has rendered this method obsolete. In the present era, simplicity, efficiency, and cost-effectiveness are crucial factors for progress, and the amalgamation of SMT and SMD precisely fulfills these requirements.

  • The automated production mode aims to optimize the cost of PCB assembly, and SMD offers a cost-effective solution in this regard.
  • SMT equipment enables rapid installation of thousands of SMD components on the board within a short timeframe.
  • The compact size of SMD components allows for increased circuit board capacity, and SMT technology effectively harnesses this advantage.
  • SMD components use lead-free soldering, PCBA companies can use this to reduce the occurrence of failures during assembly and improve the overall reliability of the SMT process.

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