PCB Design For Assembly

Design for Assembly (DFA): ensures components on PCB can be assembled correctly during pick and place and reflow stages, preventing failures such as BGA crack, insufficient solder, component to component touching, component moving out of pads or tombstoning ,etc.

Common Check list

• Stencil Design

  • it's a metal piece with component footprint stamped out that can be used with a squeegee to transfer solder paste to a bare PCB.

• Solder Paste Selection

  • BGA: collapsing or non-collapsing process will affect pad geometry

• PAD: Soldermask defined, the paste is equal to the mask. NSMD uses the metal pad size as the paste stencil aperture

  • Solder Mask Defined PAD vs. Non Solder Mask Defined PAD

    • The choice is often a function of pitch where 0.4 mm and below are often mask defined. This ensures that BGA or an IC  component terminals can be safely soldered on the PCB land pattern.

• Component outline 

• Component Clearance: The assembly house equipment will be the main determinant to ideal spacing.

  • Passive to Passive

    • E.g 0201 RLC to 0201 RLC is 100 uM

    • E.g 0201 RLC to 0402 RLC is 150 uM

  • Passive to IC: This will depend on potential rework equipment and underfill requirements.

  • Passive to Mechanical Component: Getting the tools on the hardware and the iron on the component are the keys here.

    • Mechanical component can be a shield Can.

  • IC to IC. 250 microns is a safe bet.

  • IC to PCB Edge: 2 mm is desirable from pad to board edge. The edges of a PCB get hotter during reflow and solidify at a different rate than the inner areas.

  • Connectors include space for the mating connectors.

  • Tall components can cause a shadow in the IR oven. Smaller components in close proximity may not reach reflow temperatures

  • Small passive components are prone to tomb-stoning during reflow when the pad geometry is not identical including the width and launch direction of the traces.