Common Defects in SMT Solder Paste Printing and Improvement Measures

In SMT solder paste printing, ensuring print quality is a complex task. Defects may originate from several interconnected factors, including equipment calibration, stencil design, solder paste characteristics, workshop environment, and operator handling.

1. Solder Paste Slumping

Definition:
Solder paste fails to maintain a stable shape, causing the edges to collapse and flow outside the pad area. It may even bridge adjacent pads, leading to solder shorts.

Causes and Countermeasures:

•  1: Excessive squeegee pressure compresses the solder paste, causing it to flow through stencil apertures into adjacent pad areas.
  Improvement: Reduce squeegee pressure.
•  2: Solder paste viscosity is too low to retain the intended printed shape.
  Improvement: Use solder paste with higher viscosity.
•  3: Solder powder is too fine. While smaller powder offers better release, it compromises shape retention.
  Improvement: Use solder paste with larger solder powder particles.

2. Solder Paste Misalignment

Definition:
Printed solder paste is not properly aligned with the intended pad area. This may cause solder bridging or printing on the solder mask, potentially resulting in solder balls.

Causes and Countermeasures:

•  1: PCB is not adequately supported or clamped, leading to misalignment between stencil apertures and pads during printing.
  Improvement: Use multi-point clamping to fix the PCB securely.
• 2: Mismatch between the PCB and stencil, possibly due to poor stencil aperture quality or misalignment with pad locations.
  Improvement: Recreate the stencil with precise aperture alignment.

3. Solder Paste Insufficient Coverage (Insufficient Printing)

Definition:
Solder paste coverage on the pad is less than 80% of the aperture area, resulting in insufficient solder or no solder on the pad.

Causes and Countermeasures:

•  1: Squeegee speed is too fast, causing incomplete filling of stencil apertures, especially on PCBs and stencils with small or fine features.
  Improvement: Reduce squeegee speed.
•  2: Separation speed is too fast after printing, causing solder paste to be pulled away from the pad, leading to insufficient printing or solder tails.
  Improvement: Adjust separation speed to an optimal range.
•  3: Solder paste has excessive viscosity, preventing it from flowing into fine stencil apertures.
  Improvement: Use solder paste with suitable viscosity.
•  4: Stencil aperture is too small, and combined with fast squeegee speed, results in insufficient solder paste transfer.
  Improvement: Optimize stencil aperture size.

4. Solder Paste Depression (Uneven Surface)

Definition:
The solder paste print has an unclear or uneven outline on the pad, creating an uneven or sunken surface, which may lead to cold solder joints.

Causes and Countermeasures:

•  1: Excessive squeegee pressure or fast separation speed causes depressions in the solder paste.
  Improvement: Adjust squeegee pressure and separation speed appropriately.
•  2: Dust or debris in stencil apertures causes uneven solder paste release.
  Improvement: Clean the stencil thoroughly.

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