1.5 Printer Setup & Cleaning

PrinterThe screen printer is where perfect design meets physical reality. Its setup is a high-impact, low-visibility discipline. The goal is to make the quietmachine disciplineboring—to consistently produce stable paste deposits so that decideswhen whetherdefects pasteappear, printingyou staysknow invisibleit's a materials issue (Chapter 1.1) or becomesan aperture problem (Chapter 1.4), not a machine parameter that drifted. Repeatable recipes and SPI-driven control are the day’sfoundation biggestof headache.SMT Board support, stencil contact, and alignment form the foundation; squeegee parameters and separation speed refine the print; and understencil cleaning holds it steady across long runs. Each setting interacts with paste behavior and stencil design, but when tuned and locked into repeatable recipes, the printer fades into the background—consistently producing stable deposits that reflow without drama.success.

1.5.1 Board Support: Eliminating the "Squeegee Trampoline"

The setupsingle flowbiggest (fivesource calmof minutesrandom thatprint savefailure anis hour)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. BoardPin support first.Placement: Use support pins/vacuumpins or custom tooling soto hold the PCB isflat. flatPins must be placed under everylarge, pasteopen regionareas (and especially centerunder ofBGAs largeand panels).fine-pitch Nocomponents 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 nodistorting “squeegeethe trampoline.”panel.
2. Gasket check.Check: LightlyBefore printing, gently lower the stencil onto copper, jog the stage, and look: you wantverify tight, even contact at(the pads—no"gasket daylight,seal") across the entire panel. No rocking, no rocking.daylight.
3. Fiducials & offsets. Teach three global fiducials (or rail marks on panels) and verify X/Y/θ; save the offset trend so operators can spot drift.
4. Bead & wipe. Lay a small bead (narrow ribbon ahead of the stroke), do one conditioning swipe, and print the First Article panel for SPI.
5. Lock the recipe. Record angle/pressure/speed, separation speed, and cleaning interval in the job file before volume.
   

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1.5.2 AlignmentSqueegee thatParameters: survivesFill, real panels

• Teach globals, correct locally. Use three well-spaced fiducials for global alignment; if panels stretch, enable local correction (board or quadrant).
• Don’t chase bad rails. If rail fiducials are scuffed, switch vision to on-board fiducials you reserved in panelization.
• Watch the trend. If offsets “walk” during the shift, check board supportWipe, and clamp force before you blame vision.
  

Pressure

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1.5.3 Squeegee basics (angle, pressure, speed)

ThinkThe ofsqueegee thesesettings asare threethe slidersprimary thatcontrols controlfor paste fill (paste pushed into apertures) and top-surface wipe. (They must be tuned to achieve a clean topstencil surface):surface while completely filling the smallest apertures.

KnobParameter	StartRecommended hereStarting Point	IfRisk youof goGoing tooToo lowLow (Defect)	IfRisk youof goGoing tooToo highHigh (Defect)
Angle	~60° to the stencil	Shallow angle → smearing, paste rides under the blade	Steep angle → starved fill on fine apertures
Pressure	0.4 – 0.6 kg/cm“Just (or just enough to wipe”create (a cleanclean, bright stroke, no gray halo)wipe)	Incomplete FillLight →(starved residueapertures, poor volume) and Residual Paste on top,stencil incomplete filltop.	ScoopingHeavy → scoops(pulling paste out,out forcesof apertures) and Smearing (forcing paste under the stencilstencil).
Speed	20 – 50 mm/secModerate (youslow shouldenough seeto aallow controlledpaste roll of paste)roll)	Excessive ShearSlow → extra shear and heat in the bead (damages rheology).	Skips and Starved FillFast → skips 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, inconsistentblade fillwear.

TwoProcess quick tells:Tell: If you see a faint gray film behindor the blade = pressure too high; ragged aperture edgeshalo 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 too steep or speedslow toothe high.speed.

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1.5.43 Separation &and snap-offPrint (Release

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

• ContactSnap-Off mode:(Separation Gap): keepSet the gap to snap-offzero =(Contact 0Mode) unless yourexplicitly machinerequired by a thick PCB or boardnon-standard demandstooling. aLifting gap.the stencil straight up is ideal.
• Separation speed:Speed (The Peel): slow, steady peel so paste releases cleanly; larger QFN/BGA fields often likeUse 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 ms)milliseconds) after the print stroke and before peel.
• Tall/crowdedseparation areas:can if you see “stringers” between fine-pitch pads, slowhelp the peelpaste first;stabilize onlyand thenrelease shrink apertures (7.4).cleanly.
  

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

Understencil cleaning (drymaintains / wet / vacuum)

Clogged wallsconsistent print badly.quality Setby removing paste residue that accumulates on the stencil's underside, which causes smearing and bridging.

1. Base Cadence: Start with a defaultscheduled interval,cleaning theninterval, lettypically every SPI5–15 prints tell(depending youon whenthe 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 tighten/relax.
   clean immediately, regardless of the print count.
3. Cleaning Modes:
   • Dry wipe:Wipe: firstStandard line of defensemaintenance for nano-coated or clean-running pastes.or Fast,nano-coated gentlestencils. onFast coatings.and gentle.
   • Wet + vac:Vacuum: useUsed when residue is detected or $C_{pk}$ drops. A small amount of solvent wipeis dispensed, followed by a strong vacuum whento youlift seeall bridging/volumeresidue. drift;Do don’tnot over-wet (—solvent poolsresidue =can smear).smear.
   • DecisionVac rule (simple):Only: startPrimarily dryused everyto pull paste debris and paper dust through the apertures.

Special Case: Stencils used with XWater-Soluble printsflux (e.g.,Chapter 5–10),1.1) escalatemust tohave wet+a more aggressive and frequent wet/vac ifcycle, SPIas transferthe efficiencyactivators dropsare orhighly bridgingcorrosive appears;and resetmust not be allowed to dry when stable. Log changes inon the job file so the next crew inherits the lesson.stencil.

Special cases

• Water-soluble flux: needs more frequent wet/vac; always follow with a dry pass.
• Step stencils: clean after runs that cross steps; ramps collect paste.
• Nano-coating: longer intervals, gentler solvents; avoid scrubbing.
  

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1.5.65 BeadRecipe managementManagement & First Article (little and often)FA)

• Keep a narrow bead ahead of the blade (≈10–15 mm wide). Big pancakes over-shear and overheat paste.
• For pauses >15–20 min: scoop the bead, cap it, run an understencil clean, and restart with a fresh bead.
• Never return stencil-exposed paste to the original jar (7.2 has the hygiene rules).
  

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Recipe

repeatability

1.5.7 SPI-driven tuning (close the loop)

Use SPI asis your speedometer:defense against human error during changeovers.

• Volume low on fine features → ease pressure, slow speed, check bead and cleaning.
• Bridging hotspots → slow separation, add a cleaning cycle; if persistent, apply the anti-bridging aperture tweaks (7.4.6).
• Lot-to-lot drift → confirm stencil wear, gasket, and support pins; then revisit paste age/handling (7.2).

Save the winning settings into the job recipe and enable closed-loop corrections if your SPI/printer combo supports them (7.6).

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1.5.8 First Article & changeovers (make success repeatable)

• FA printProcedure:: Print one pass,board, SPIsend review,it tweakto SPI (Chapter 1.6), and review the chart. Tweak only one knobparameter at a time, reprint, then lock.
• Record everything: angle/pressure/speed, separation, cleaning cadence, bead policy, support map, and any local quirks (e.g., “addreduce dwellpressure atslightly), U17print QFN”)again, and re-review. Lock the settings immediately when the Volume $C_{pk}$ is maximized.
• Golden photo setRecipe:: top-sideThe printfinal, close-upsoptimized ofmachine the tightest features so night shift knows what “good” looks like.
  

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1.5.9 Pocket checklists

Start of run

• Support pins/tooling placed under large paste fields
• Stencil gaskets clean; fiducials taught; offsets saved
• Bead laid; First Article printed; SPI green
  

During run

• Angle/pressure/speed unchangedsettings (unlesssqueegee noted)
• Cleaningspeed/pressure, oncleaning schedulecadence, (dry→wet+vacseparation ifprofile) SPImust drifts)
• Beadbe small;saved, pauses handled (scoop + clean)
  

Changeover / end

• Final clean; stencil stored per coating type
• Recipe updated with final winning settings
• SPI charts archivednamed to the lotwork recordorder/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.

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

Conclusion: Standardize printer setup with proper support, balanced blade settings, and SPI-driven cleaning routines. This consistency removes variation at the source, ensuring repeatable prints and a production line that runs smoothly from start to finish.