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1.5 Printer Setup & Cleaning

The screen printer is where perfect design meets physical reality. Its setup is a high-impact, low-visibility discipline. The goal is to make the machine boring—to consistently produce stable paste deposits so that when defects appear, you know it's a materials issue (Chapter 1.1) or an aperture problem (Chapter 1.4), not a machine parameter that drifted. Repeatable recipes and SPI-driven control are the foundation of SMT success.

1.5.1 Board Support: Eliminating the "Squeegee Trampoline"


The single biggest source of random print failure is poor board support. If the PCB is not perfectly flat under the stencil, it will deflect under squeegee pressure, leading to starved volume or smearing.

  1. Pin Placement: Use support pins or custom tooling to hold the PCB flat. Pins must be placed under large, open areas and especially under BGAs and fine-pitch components to prevent localized bowing. 2. Vacuum/Clamping: Use vacuum support or adjustable magnetic pins for maximum rigidity. The rail clamping mechanism should secure the board without causing rail bow or distorting the panel.
  2. Gasket Check: Before printing, gently lower the stencil and verify tight, even contact (the "gasket seal") across the entire panel. No rocking, no daylight.


1.5.2 Squeegee Parameters: Fill, Wipe, and Pressure


The squeegee settings are the primary controls for paste fill and top-surface wipe. They must be tuned to achieve a clean stencil surface while completely filling the smallest apertures.

Parameter

Recommended Starting Point

Risk of Going Too Low (Defect)

Risk of Going Too High (Defect)

Pressure

0.4 – 0.6 kg/cm (or just enough to create a clean, bright wipe)

Incomplete Fill (starved apertures, poor volume) and Residual Paste on stencil top.

Scooping (pulling paste out of apertures) and Smearing (forcing paste under the stencil).

Speed

20 – 50 mm/sec (slow enough to allow paste roll)

Excessive Shear and heat in the bead (damages rheology).

Skips and Starved Fill on tight apertures (paste can't settle fast enough).

Angle

55° – 65° (relative to stencil surface)

Smearing/Hydroplaning (paste rides under the blade).

Tired Fill on small apertures, blade wear.

Process Tell: If you see a gray film or halo on the stencil top behind the blade, your pressure is too high—it's forcing paste under the stencil. If the print edges are ragged/fuzzy, adjust angle or slow the speed.


1.5.3 Separation and Print Release


The moment the stencil lifts from the PCB is where bridging is either created or eliminated.

  • Snap-Off (Separation Gap): Set the gap to zero (Contact Mode) unless explicitly required by a thick PCB or non-standard tooling. Lifting the stencil straight up is ideal.
  • Separation Speed (The Peel): Use a slow, controlled peel speed (e.g., $1-5 \text{ mm/sec}$). Fast separation can cause the paste to string or spike, leading to bridges between fine-pitch pads or starved centers on large pads.
  • Dwell Time: For dense BGAs or QFN fields, a brief dwell (hundreds of milliseconds) after the print stroke and before separation can help the paste stabilize and release cleanly.


1.5.4 Understencil Cleaning (USC): The SPI-Driven Cadence


Understencil cleaning maintains consistent print quality by removing paste residue that accumulates on the stencil's underside, which causes smearing and bridging.

  1. Base Cadence: Start with a scheduled cleaning interval, typically every 5–15 prints (depending on the complexity and paste type).
  2. The Decision Rule: Let SPI decide. If your Transfer Efficiency (TE) on critical features drops below the specified minimum, or if your volume $C_{pk}$ falls below 1.33, that is the trigger to clean immediately, regardless of the print count.
  3. Cleaning Modes:
    • Dry Wipe: Standard maintenance for clean-running or nano-coated stencils. Fast and gentle.
    • Wet + Vacuum: Used when residue is detected or $C_{pk}$ drops. A small amount of solvent is dispensed, followed by a strong vacuum to lift all residue. Do not over-wet—solvent residue can smear.
    • Vac Only: Primarily used to pull paste debris and paper dust through the apertures.

Special Case: Stencils used with Water-Soluble flux (Chapter 1.1) must have a more aggressive and frequent wet/vac cycle, as the activators are highly corrosive and must not be allowed to dry on the stencil.


1.5.5 Recipe Management & First Article (FA)


Recipe repeatability is your defense against human error during changeovers.

  • FA Procedure: Print one board, send it to SPI (Chapter 1.6), and review the chart. Tweak only one parameter at a time (e.g., reduce pressure slightly), print again, and re-review. Lock the settings immediately when the Volume $C_{pk}$ is maximized.
  • Golden Recipe: The final, optimized machine settings (squeegee speed/pressure, cleaning cadence, separation profile) must be saved, named to the work order/job file, and documented for all subsequent runs.
  • Audit: Conduct periodic audits to ensure that the operator's actual settings match the Golden Recipe stored in the job file.

Would you like to move on to the final chapter in this section: SPI Metrics & Closed Loop?


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