1.5 Printer Setup, Cleaning, and Verification Checklist

The screen printer is not a "set and forget" machine; it is a dynamic system where variables drift with every stroke. 60% of SMT defects originate here, yet they are often blamed on the reflow oven or the pick-and-place machine later down the line.

To achieve deterministic printing, you must remove operator "feel" from the equation. Use this standardized setup and verification protocol to ensure the machine conditions—not luck—dictate the quality of the deposit.

Machine Setup Checklist (Pre-Run)

Before the first board enters the machine, the physical interface must be rigid and precise.

Component	Check	Pass Criteria / Standard
Squeegee Blades	Edge Inspection	Run a fingernail along the blade edge. If you feel a nick or dent, replace it. A damaged blade leaves streaks of flux that cause bridging.
Board Support	Pin Placement	Support pins must be placed directly under high-density connectors and BGA sites. Rule: If you can push the board down > 0.5mm with your finger, the support is insufficient.
Clamping	Z-height / Snugness	Top clamps (foils) must be flush with the PCB surface. Side clamps must hold the board firm without bowing it.
Stencil Alignment	Gasket Seal	Verify 0.00 mm gap between stencil and PCB. Any light visible between them indicates poor clamping or debris on the bottom of the stencil.

Process Parameters: The Physics of Transfer

Do not copy settings from a previous job blindly. Set parameters based on the physics of the current paste and stencil.

1. Squeegee Pressure

Goal: Clean wipe with minimum force.

• The Risk: Excessive pressure causes "scooping" (dishing) on large pads and accelerates stencil wear.
• Setting Rule: Start at 0.5 kg per 25mm of blade length.
  • Example: 250mm blade ≈ 5.0 kg starting pressure.
  • Tuning: Run a dry print. Increase pressure by 0.5 kg increments until the stencil surface is visibly clean (no paste smear) after the stroke. Stop adding pressure once the wipe is clean.

2. Squeegee Speed

Goal: Proper paste roll.

• The Physics: The paste must roll (diameter ≈ 15mm) to generate the hydraulic pressure that fills the apertures.
• Standard Window: 40 – 80 mm/sec.
  • Too Fast: Paste slides instead of rolling; incomplete filling (insufficient volume).
  • Too Slow: Cycle time loss; paste may seep under stencil (smearing).

3. Separation Speed (The Critical Release)

Goal: Vertical release without turbulence.

• The Physics: This is the speed at which the table lowers the PCB away from the stencil. It determines the definition of the brick walls.
• Standard: 0.5 – 3.0 mm/sec (slow separation) for the first 3mm of travel.
• Fine Pitch Rule: If printing 0.4mm pitch or µBGA, use < 1.0 mm/sec. Fast separation creates a vacuum suction that pulls spikes ("dog ears") on the solder deposit.

Cleaning Strategy: Logic, Not Superstition

Stop setting the Under Stencil Cleaner (USC) to "Wet-Dry-Vacuum every 1 board." It wastes cycle time, consumes expensive paper/solvent, and can actually cause defects by leaving solvent on the stencil.

Cleaning Frequency Triggers:

1. Baseline: Start with Wet-Vacuum-Dry every 3–5 prints.
2. SPI Feedback: Link cleaning to data.
   • Drift: If Volume % drops by > 10% over 3 boards, trigger a clean.
   • Bridge Warning: If Smear/Bridge defects appear on the 5th board, set frequency to every 4.
3. Process Warning: If you are using Nano-coated stencils, reduce cleaning frequency (e.g., every 20 boards). Aggressive cleaning abrades the coating.

First-Panel Verification Routine

The first board is the "Golden Gate." It does not proceed down the line until these checks are passed.

1. The Visual "Slump & Smear" Check (20x Magnification)

• Definition: Look at the 0.4mm/0.5mm pitch pads.
• Pass: Edges are sharp (brick-like). Paste is centered on pads.
• Fail:
  • Slump: Paste edges are rounded or touching (Check: Viscosity, Temperature, Humidity).
  • Smear: Paste halo around the pad (Check: Gasket seal, Support pins).
  • Scooping: Concave surface on large pads (Check: Pressure too high).

2. The Height/Volume Check (SPI)

• Mandate: If you have SPI, the first board must score CPK > 1.33 equivalent (no red flags).
• Manual: If no SPI, use a height gauge on the 5 critical fine-pitch sites. Target: Stencil Thickness ± 10%.

Print Stop Conditions

Empower the operator to stop the line immediately if these conditions occur.

Observation	Diagnosis	Action
Knead Roll < 10mm	Insufficient paste on stencil.	Add Paste. Do not scrape from sides (it's dried out).
Flux Separation	Clear liquid visible on stencil.	Scrap Paste. Remove all paste, clean stencil, apply fresh jar.
Squeegee Streaks	Lines of paste left after wipe.	Inspect Blade. Nicked blade or dried paste on edge.
"Double Print"	Ghost image of pads.	Board Support Failure. The board moved/slipped during separation.
Temp > 28°C	Viscosity breakdown.	Stop. HVAC failure. Paste is now too thin to print reliably.

Final Checklist

Parameter	Setting / Limit
Support Pins	Under BGA/Connectors
Squeegee Angle	60° (Standard) / 45° (High Volume)
Squeegee Pressure	Clean Wipe + 0.5 kg
Separation Speed	Slow (< 2.0 mm/s)
Cleaning Mode	W-V-D (Start with every 5)
First Article	No Smear, Sharp Edges