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What is the difference between SMT and SMD?

Both SMT and SMD belong to professional terms in the PCB assembly industry. SMT, that is, surface mount technology, is one of the PCB assembly technologies; SMD, that is, surface mount devices, is one of the components used in the SMT process.

SMT stands for Surface Mount Technology, the entire technology of mounting and soldering electronic components, such as resistors, capacitors, transistors, integrated circuits, onto a printed circuit board or PCB. The components used are also referred to as surface-mount devices (SMD, surface-mount devices). It should be noted that SMT does not need to reserve corresponding through holes for component pins, and SMD is much smaller than through-hole insertion technology.

PCB Fabrication
PCB Fabrication

Features of SMT:

  • Components have no leads or only short leads;
  • The main body of the component and the solder joint is on the same side of the PCB;
SMT PCB Assembly
SMT PCB Assembly Machine

Features of SMD:

  • miniaturization;
  • No lead (flat or short lead);
  • Suitable for surface assembly on PCB;
SMD Components
SMD Components

In the early stages, SMD was soldered manually by hand. Then the first batch of pick and place machines could only handle a few simple components. The more complex and smaller components still needed to be placed manually. Not long before the introduction of surface mount components over 20 years ago, a new era was born. From simple resistors to complex ICs, almost every through-hole component now has an SMT equivalent.

The actual process of SMT

The actual process of SMT
1080The actual process of SMT

Firstly, a PCB stencil is aligned on the surface of the boards, and solder paste is applied using a squeegee to ensure the pads are coated with a uniform and controlled amount of solder paste.

Secondly, via a pick and place machine or hand placement, the components are mounted onto the boards in their respective locations. The wet solder paste will act as a temporary adhesive but it is still important to ensure that the boards are moved gently to prevent misalignment.

Thirdly, the boards are passed through a reflow oven which subjects the boards to infrared radiation, melting the solder paste and forming solder joints. Then the boards are passed through an AOI machine, or Automatic Optic Inspection machine which runs a number of quality checks on the boards visually, such as component alignment and checking for solder bridges. The boards then proceed to further test.

In the 1980s, SMT production technology became increasingly more refined and as a result, is widely used in mass production. As costs were reduced and technical performance enhanced, more advanced but economical equipment became available.

Types of SMD

  • Passive components
  • resistance
  • capacitance
  • inductance
  • Composite device

Discrete components

  • integrated circuit
  • Active device

Electromechanical devices

  • Switch/relay
  • Connector
  • Micro-motor

Advantages of SMT

  • Reduce PCB area;
  • Reduced weight
  • Installation is easy to automate;
  • Reduce parasitic capacitance and parasitic inductance;
  • Improve product quality and reliability;

SMT brought with it a new generation of electronic assembly technology, which is widely applied in applications from aviation, communications, automotive, and medical electronics, down to household appliances and other fields.

JHYPCB Can offer PCB manufacturing and PCB assembly services, including PCB prototype fabrication, LED PCB assembly, through-hole PCB assembly, feel free to contact us.

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