1.5 Printer Setup, Cleaning, and Verification Checklist

The screen printer is wherenot perfecta design"set meetsand physicalforget" reality.machine; Its setupit is a high-impact,dynamic low-visibilitysystem discipline.where Thevariables goaldrift iswith 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 makeensure 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.

boringComponent	Check	Pass Criteria / Standard
Squeegee Blades	Edge Inspection	—Run toa consistentlyfingernail producealong stablethe pasteblade depositsedge. so that when defects appear,If you know it'sfeel a materials issuenick or an aperture problem, not a machine parameter that drifted.dent, Repeatablereplace recipes and SPI-driven control are the foundation of SMT success. Board Support: Eliminating the "Squeegee Trampoline" The single biggest source of random print failure is poor board supportit. IfA thedamaged PCBblade isleaves notstreaks perfectlyof flatflux underthat thecause stencil,bridging.
Board it will deflect under squeegee pressure, leading to starved volumeSupport or smearing.	Pin Placement:Placement	Use supportSupport pins or custom tooling to hold the PCB flat. Pins must be placed directly under high-density connectors and BGA sites. large, open areasRule: andIf especiallyyou undercan 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 securepush 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 causingbowing it.
railStencil bowAlignment	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: orClean distortingwipe thewith panel.minimum force.

• The Risk: Excessive pressure causes "scooping" (dishing) on large pads and accelerates stencil wear.
• GasketSetting Check:Rule: BeforeStart printing,at gently0.5 lowerkg 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 andsurface verifyis tight,visibly even contactclean (theno "gasketpaste seal")smear) acrossafter the entirestroke. panel.Stop Noadding rocking,pressure noonce daylight.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.

SqueegeeFirst-Panel Parameters:Verification Fill, Wipe, and PressureRoutine

The squeegeefirst settingsboard 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 primary0.4mm/0.5mm controlspitch forpads.
• 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 pasteCPK fill> 1.33 andequivalent (no red flags).
• top-surfaceManual: wipeIf no SPI, use a height gauge on the 5 critical fine-pitch sites. Target: Stencil Thickness ± 10%.

They

Print mustStop beConditions

tuned

Empower the operator to achieve a clean stencil surface while completely fillingstop the smallestline apertures.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	RecommendedSetting Starting/ Point	Risk of Going Too Low (Defect)	Risk of Going Too High (Defect)Limit
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.

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 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.

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 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.

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.

Yes, the Final Checklist is necessary here.

For a chapter on Printer Setup & Cleaning, the content covers a high number of interconnected, time-sensitive parameters (Pressure, Speed, Cleaning Cadence, Support Pins). An EMS operator or supervisor needs a fast, simple reference guide (a checklist) to ensure all critical variables are confirmed during setup or when troubleshooting a print failure.

Since the original content did not include a final checklist section, I will rewrite the essential final table for this chapter, integrating the key parameters from all previous sections (Board Support, Squeegee, Separation, and USC).

Final Checklist: Printer Recipe Lock

This table summarizes the mandatory verification points for achieving the "Golden Recipe" print stability. These steps must be signed off during the First Article (FA) and verified upon every machine changeover.

Category	Parameter	Mandate/Target	Verification Method
Board Support	PCBUnder Flatness	Perfect Gasket Seal confirmed; no rocking or daylight at the stencil.	Physical check; support pins placed directly under all BGAs/fine-pitch areas.BGA/Connectors
Squeegee Angle	Pressure	Adjusted to achieve a clean, bright wipe without visible gray film60° (smearing).	SqueegeeStandard) pressure/ set45° to(High 0.4 – 0.6 kg/cm starting point.
	Speed & Angle	Tuned to ensure complete aperture fill without excessive shear/heat in the bead.	Machine log confirms speed is consistent with the Golden Recipe.Volume)
ReleaseSqueegee Pressure	Clean Wipe + 0.5 kg
Separation Speed	Set to a slow, controlled peelSlow (1< – 52.0 mm/sec).	Prevents stringing or spiking that leads to fine-pitch bridging.s)
Cleaning (USC)Mode	CadenceW-V-D &(Start Mode	SPI-driven:with Cleaningevery interval is set to maintain Cpk ≥ 1.33 on critical features.	Recipe specifies Wet + Vac frequency; Dry only used for maintenance.5)
RecipeFirst IntegrityArticle	ProgramNo Lock	AllSmear, parametersSharp (Speed, Pressure, Clean Cadence) are saved and locked with the Golden Recipe ID in the job file.	Final audit ensures the settings used for the FA match the recipe loaded on the line.Edges

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