SMT (Surface Mount Technology) is an electronic manufacturing process that directly mounts electronic components onto the surface of a printed circuit board (PCB). It has largely replaced traditional THT (Through-Hole Technology) and has become the core technology in modern electronics manufacturing.
SMT production is a systematic process that mainly includes the following key steps:
1. PCB Pre-treatment
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Cleaning: Removes oil, oxide layers, or impurities from the PCB surface to ensure solderability of pads (commonly using plasma cleaning or chemical cleaning).
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Pad Inspection: AOI (Automatic Optical Inspection) is used to check whether the pads have defects such as deformation, oxidation, or scratches.
2. Solder Paste Printing (Stencil Printing)
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Principle: A laser-cut stainless steel stencil, with openings corresponding to the PCB pads, is used. A squeegee presses solder paste through the stencil openings and deposits it precisely onto the PCB pads.
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Key Point: The printing quality directly determines the quality of the final solder joints. This is one of the most critical steps in the SMT process.
3. Component Placement
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Principle: The placement machine uses a vision system to recognize fiducial marks on the PCB and the component package features, calculates the exact placement positions, then picks up SMD components with a nozzle and accurately places them onto the solder-pasted pads.
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Key Point: High speed, high precision, and flexibility. Modern placement machines can handle components from tiny 0402 (0.4mm x 0.2mm) chips to large BGAs, QFPs, connectors, and more.
4. Reflow Soldering
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Principle: The PCB with placed components passes through a reflow oven. Inside, it follows a carefully designed temperature profile (preheating, soaking, reflow, cooling). The solder paste melts during the reflow stage and then solidifies during cooling, forming permanent solder joints.
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Typical Temperature Profile:
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Preheat Zone: Slowly rises to 100–150°C to remove solvents in solder paste and prevent thermal shock to components.
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Soak Zone: Maintains 150–180°C to activate flux and remove oxides from pads and component leads.
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Reflow Zone: Quickly rises to solder’s melting point (e.g., Sn63/Pb37 at 183°C; lead-free solder ~217–227°C), melting solder and wetting pads and leads.
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Cooling Zone: Rapidly cools to room temperature, solidifying solder joints and avoiding coarse grain structures.
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Common Defects: Cold joints, voids, solder balls, component warpage (caused by uneven thermal stress).
5. Cleaning and Inspection (Optional but Important)
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Cleaning: Removes corrosive flux residues after soldering.
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Inspection: Includes AOI (Automatic Optical Inspection) and AXI (Automated X-ray Inspection). AOI checks placement errors, missing parts, wrong components, and solder joint appearance defects; AXI is mainly used for hidden solder joints such as BGA solder ball quality.
Applications of SMT
SMT has become the mainstream technology in electronics manufacturing and is widely applied in:
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Consumer Electronics: Smartphones, computers, tablets, smart wearables, etc.
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Communication Equipment: Base stations, routers, switches, etc.
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Automotive Electronics: Navigation systems, sensors, autonomous driving modules, etc.
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Industrial Electronics: PLCs (Programmable Logic Controllers), instruments, meters, etc.
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Medical Electronics: ECG machines, portable medical devices, etc.
SMT technology marks the transition of the electronics industry from “extensive” to “precision” manufacturing. Its core lies in achieving efficient and reliable component assembly through automation, high precision equipment, and strict process control.