In the field of electronic manufacturing, many people often confuse the concepts of SMD and SMT. This blog will help you understand what SMD and SMT are and how to choose in electronic manufacturing.
In this article:
Part 1. What are the Surface Mounted Devices (SMDs)? Part 2. What is Surface Mount Technology (SMT)? 3. What are the common SMDs? Part 4. SMT Process StepsWhat are the Surface Mounted Devices (SMDs)?
SMDs(Surface Mounted Devices) are electronic components designed for surface mounting. Like chip resistors, chip capacitors, QFP chips, BGA packaging components, etc., they are a special type of component that needs to be soldered on the circuit board to function, which is different from another kind of DIP component (i.e., a plug-in component).
Unlike traditional components, which require wires to be inserted into holes drilled on a printed circuit board, SMDs do not require wires to be inserted into holes, thus simplifying the assembly process and improving efficiency.
What is Surface Mount Technology (SMT)?
SMT(Surface Mount Technology) refers to a production process technology that is used to quickly and accurately mount SMD components on the PCB surface and complete soldering through processes such as reflow soldering.
Process: solder paste printing → component mounting → reflow soldering → inspection.
Core Differences：
Comparison Items |
SMD |
SMT |
Definition |
Specific electronic components |
Production technology |
Use |
PCB functional unit |
Efficient, compact assembly process |
Connection |
Installation by SMT technology |
SMD as processing object |
Example |
SMD LED, SOP package chip |
SMT machine, reflow oven, solder paste |
What are the common SMDs?
There are many types of SMD (Surface Mount Device) components, which can be classified from different angles. For example, they can be divided into passive and active components according to their functions; they can be divided into chip components, transistors (SOT), and integrated circuits (IC) according to the packaging form; they can be divided into L-shaped pins such as SOIC, QFP, J-shaped pins, and leadless packages (BGA) according to the pin shape.
To have a more intuitive understanding of the types of SMD components, we use pictures to show some common types:
Chip components (Chip):
1. Resistors:
Function: Limit the current in the circuit. The larger the resistance value, the smaller the current.
Features: Used for voltage division, current limiting, circuit protection, etc. Accuracy and stability vary depending on materials and processes. The smaller the size, the lower the power handling capacity.
2. Capacitor:
Function: Stores charge and block direct current from passing through, allowing alternating current to pass through.
Features: Used for filtering, coupling, decoupling, energy storage, etc. Capacitance, withstand voltage and temperature characteristics are critical parameters. Capacitors with different dielectric materials have different characteristics.
3. Inductor:
Function:
Stores magnetic field and can block AC and allow DC to pass.
Features: Used for filtering, oscillation, choke, etc. Inductance-rated current and quality factors are important parameters. When used in high-frequency circuits, the self-resonant frequency needs to be considered.
Transistor (SOT):
Small Outline Transistor (SOT):
Function: Amplify or switch electrical signals.
Features: Divided into bipolar transistor (BJT) and field effect transistor (FET). BJT controls collector current through base current. FET controls drain current through gate voltage. Widely used in amplifiers, switching circuits, voltage regulator circuits, etc
Integrated Circuit (IC):
Small Outline Integrated Circuit (SOIC):
Features: Miniaturization: SOIC package is smaller than traditional DIP package and is suitable for high-density circuit boards. Surface Mount: Facilitates automated production and improves production efficiency. Multi-function: Many transistors, resistors, capacitors and other components are integrated internally to achieve complex circuit functions.
Quad Flat Package (QFP):
Features: High Pin Count: The QFP package has many pins and is suitable for complex integrated circuits.
Fine Pitch: The pin pitch is small, which further increases the package density. Surface Mount: Also suitable for automated production
Ball Grid Array Package (BGA):
Features:
High Density: The pins of the BGA package are located at the bottom of the package, which significantly increases the pin density. Good heat dissipation: Spherical solder joints help dissipate heat. High Reliability: The solder joints are strong and have strong vibration resistance. Suitable for high-frequency circuits
SMT Process Steps
SMT is a complete technical system, including equipment (chip mounter, reflow oven), process (solder paste printing, mounting, welding) and standards (such as IPC specifications), which is used to process SMD components efficiently and accurately. SMD is the processing object of SMT, and SMT is the assembly method of SMD; both are indispensable.
The following are examples of specific SMT processes:
Screen Printer: The process begins with the "Screen Printer," where solder paste is applied to the printed circuit board (PCB) using a stencil. This ensures precise placement of solder paste on the designated pads.
Solder Paste Inspection (SPI): Following the screen printing, the "Solder Paste Inspection (SPI)" stage verifies the quality and accuracy of the solder paste application. This step ensures that the correct amount of solder paste has been applied to each pad. Chip Placement:
Next, the "Chip Placement" machine precisely places small surface mount devices (SMDs), such as resistors and capacitors, onto the PCB.
#1 IC Placement: This step is the first stage of the IC (Integrated Circuit) placement. Where the first set of ICs are placed onto the PCB.
#2 IC Placement: This step is the second stage of the IC placement. Where the second set of ICs are placed onto the PCB.
Automatic Optical Inspection (AOI): After component placement, the "Automatic Optical Inspection (AOI)" system examines the PCB for any placement errors, such as misaligned or missing components.
Reflow Process Inspection (RPI): This is an inspection of the reflow process. It checks the quality of the soldering after the reflow oven.
Reflow Oven: Finally, the PCB is passed through a "Reflow Oven," where controlled heating melts the solder paste, creating permanent connections between the components and the PCB.
Post-reflow Automatic Optical Inspection (AOl) It is performed after the reflow soldering stage (where components are soldered onto a printed circuit board, or PCB, using heat) to automatically detect defects in the assembled PCBs
The above is a general and detailed explanation of the difference between SMD and SMT. In short, in the rapidly developing electronic world, SMT has become a mature and common process.
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