2.4 The Population – Component Taxonomy

The transition from a bare board to a functional device involves the precise placement of the component population. Components are categorized by their attachment method, which dictates the labor and machinery required for assembly.

Surface Mount Technology (SMT)

SMT components are designed for robotic automation. They sit directly on the surface of the board.

• Characteristics: Small, leadless or short-leaded, high density.
• Assembly: Placed by high-speed machines (Pick & Place) and soldered simultaneously in a reflow oven.
• Cost Profile: High setup cost (stencils, programming) but extremely low unit cost at volume. This is the dominant technology for modern electronics.

Through-Hole Technology (THT)

THT components have metal leads that pass through drilled holes in the board.

• Characteristics: Large, mechanically robust, able to handle high power or physical stress (e.g., connectors, heavy transformers).
• Assembly: Often requires manual insertion by human operators or slower specialized machines. Soldering is done via Wave Solder or Selective Solder.
• Cost Profile: High labor cost per unit. Designers should minimize THT usage unless mechanical strength is critical.

Final Checklist