SMD: Surface Mount Devices

NextPCB Vocabulary

Definition: An SMD is an electronic component designed to be mounted directly on the surface of a printed circuit board (PCB). The assembly process used for SMDs is called surface-mount technology (SMT). Unlike through-hole components, SMDs are typically smaller, lighter, and have no leads or only very short leads.

Simply put, an SMD is soldered directly onto PCB pads without requiring drilled holes.

Learn PCB SMD Component Types

Key Advantages

• Space and weight savings, enabling high-density layouts and double-sided assembly.
• Faster automated production using pick-and-place machines and reflow soldering.
• Better electrical performance at high frequencies due to shorter interconnections.
• Cost efficiency through high-volume automated assembly and reduced material use.

SMD Package Types (by lead or solder-end structure)

1. Chip-type (leadless): Flat end-terminations; common for resistors and capacitors; ideal for high-density layouts.
2. J-lead: Leads curl under the body in a “J” shape (e.g., PLCC, SOJ); sturdy joints with moderate footprint. 
3. L-lead / Gull-wing: Leads extend outward and down (e.g., QFP/SOP families); easy visual inspection and rework. 
4. BGA (Ball Grid Array): Grid of solder balls on the underside; supports very high pin counts with good electrical/thermal paths. 
5. BTC (Bottom-Termination Component): Leadless packages with bottom pads (e.g., QFN/DFN), often with a thermal pad for heat transfer; requires specific design/assembly controls. 
6. Castellated (edge-plated module) packages: Boards or modules with plated half-holes along the edge for solder-down, board-to-board connection (common in RF/wireless modules). 
    

Basic SMD Assembly Flow

• Solder paste printing: A stainless/nickel stencil deposits controlled paste volumes onto pads.
• Pick-and-place: Machines position SMDs at high speed on pasted pads.
• Reflow soldering: A controlled thermal profile melts, then solidifies the paste to form joints.
• Inspection: AOI/X-ray verify alignment, bridges, voids, and hidden joints (e.g., BGAs). 

Typical Uses & Tips

Where SMDs shine: Phones, wearables, networking gear, automotive ECUs, aerospace and medical electronics—applications that demand compact size, low weight, and solid performance. 

Design hints: Choose packages to fit density and rework needs (e.g., gull-wing for easy inspection; QFN/BGA for compact, high-pin designs) and confirm thermal requirements early, especially for BTCs. 

Conclusion

SMDs are the component format that enables today's compact, lightweight, and high-performance electronics. Selecting the right package and pairing it with a robust SMT process delivers throughput, reliability, and consistent quality.

Recommend Reading: What is the difference between SMT and SMD?

Looking to bring your SMD-based designs to life with precision and reliability?

Get an instant PCBA quote from NextPCB and start your project today!

Learn our Turnkey PCBA Services.