1.3 Stencil Types & Thickness

The stencil is where the two critical elements of SMT — the paste and the PCB — first meet. Its geometry defines howthe exact volume of solder pastethat entersgets thedeposited. assembly process, making it one of the most critical enablers of print quality. ItsThe material, thickness, and surface treatmentscondition of this piece of metal set the baseline for paste transfer efficiency acrossand everyprint component footprint, from ultrafine BGAs to bulky thermal pads.repeatability. A well-chosengood stencil balancesis thesethe competingdifference needsbetween througha foilstable, type,boring step-downs or step-ups,process and optionala nano-coatings,daily ensuringwar cleanon releasesbridging and consistentinadequate depositspaste over long runs. Locking this decision early stabilizes downstream printing, inspection, and yield performance.volume.

1.3.1 WhatStencil kindTypes: Performance vs. Cost

The choice of foil material and process is a direct trade-off between stencil docost youand actuallyyour need?required paste release quality at the smallest aperture.

Type	WhatFoil it isMaterial	WhyPerformance (and when) it winsUpside	Watch-outsCost & Risk Trade-Off
Laser-cutCut stainlessStainless (LCS)	TheStainless everyday workhorse foil,Steel, cut by fiber/laser and tensioned in a frame.laser.	Standard Workhorse.Robust, fastFast toturnaround, source,low greatcost, robust for mostgeneral 0402–use (0.5 mm pitch,pitch QFPs,and “normal” QFNs.larger).	Aperture walls are rougherrougher; than electroformed; fine-pitchlimits transfer reliesefficiency more(TE) on thicknessfine &pitch. aperturesRequires (seeaggressive 7.4).cleaning.
Electroformed (nickel)EF)	Plated,Nickel super-smoothfoil, aperturesgrown withby slightlyplating.	Ultimate “radiused”Release. edges.Super-smooth, rounded aperture walls (chimney effect) for superior release. Essential for Area Ratios < 0.66.	BetterHigher releasecost, atlonger low area ratios (tight BGAs/WLCSP, tiny QFNs); lets you keep thickness a hair thicker without starving.	Cost/lead time;time. babyRequires thegentler foilcleaning—coating (nodamage harshis brushing).expensive.
Step stencilStencil	SameStandard foil,LCS/EF butfoil locallywith localized thinned (step-down)down) or built-upthickened (step-up) regions.step-up	The Compromise.) Allows one stencil to handle extreme variations (e.g., 0.4 mm) BGA and massive thermal pads).	OneRequires board,precise mixeddesign needs:and thinmachine forprogramming 0.4–0.5to mmensure BGA/QFN; thick islands for big pads or paste-in-hole.	Plan ramps/keepouts;smooth squeegee must cross steps cleanly (see notes below).crossing.
Nano-coatedCoated	Any foil type with a permanent, anti-stick fluoro-polymer layer.	ROI Play. Improves Transfer Efficiency (TE), reduces cleaning cycles (add-on)better UPH/throughput), and extends open time.	Fluoro-polymerAdded surfacecost; treatmentmust use gentler cleaning fluids to preserve the coating life.

1.3.2 How Thick? Quantifying the Volume Constraint

Your choice of base thickness is constrained by the smallest, most difficult component on anythe foil.board. You are playing a game of Volume Balance.

• Too Thick – Too much paste – Bridging (Shorts)
• Too Thin – Too little paste – Tombstoning (Opens)

You must ensure every aperture meets two minimum geometric thresholds:

Constraint	Minimum Target	What it Prevents
Area Ratio (AR)	≥ 0.66	LessArea of Aperture / Area of Wall Surface. Below 0.66, wall friction starves the paste sticking =volume. cleanerBase releasesthickness is driven by this.,
Aspect longerRatio (AsR)	cleaning≥ intervals1.5, nicer edges on fine apertures.	Don’tAperture scrubWidth off/ Stencil Thickness. Below 1.5, the coating;aperture useis gentlertoo solventsdeep &for wipes;the re-coatpaste onto aeasily schedule.exit.

PairThe typemost +common thickness used in modern EMS is aperture100 designµm (next0.004 section)inch), + printer setup/cleaning (7.5) to make printing boring—in the best way.

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1.3.2 How thick? (choose bybut the hardest part ondictates the board)

final

Start with the tightest pitch / smallest aperture you must print well; everything else adapts (with steps or shapes). Use these as starting points, then prove with SPI.choice:

Board mixComponent Mix (examples)Examples)	Recommended Base thicknessThickness	NotesPrimary that save you a respinFocus/Trade-Off
WLCSPGeneral Mix / 0.3–0.3565 mm pitch; 01005 passivesQFP	60–80120-130 µm (0.0024–0.0031”)005 inch)	OftenForgiving, electroformedeasy +volume nano;for Type0402s/0603s. 5Needs powder;aggressive beaperture ruthlessreductions onfor area/aspectfiner ratiosparts (7.4)next section).
BGA/0.4 mm QFN at/ 0.4–0.5 mm;mm 0201–0402BGA passives/ 0201	90–90-100 µm (0.0035–0035 – 0.0040”)0040 inch)	SweetThe sweet spot for densedense, modern consumer boards;boards. step-Use upT4/T5 paste islandswith forthis big thermal pads or shield lands.thickness.
General0.3 mixedmm (QFPWLCSP 0.5–0.8/ mm;Ultra-Fine 0402–0603; modest QFN)Pitch	12060-80 µm (0.0047”)	The0024 default– many0.0031 factories standardize on; tune apertures for tombstone/bridging (7.4).
0603–1206, connectors, power SO-8/LFPAK with big pads	130–150 µm (0.005–0.006”)	Use windowed thermal pads; consider step-down pockets around nearby fine-pitch.
Paste-in-hole (PIH), huge thermal slugsinch)	Keep base for SMT; addRequires step-up islandsElectroformed tofoil. 180–200If µmyou wherecan’t needed	Avoidget full-thicknessthe foilsArea atRatio 180+with unless80 it’sµm, ayou PIH-onlymust panel;thin local islands print better + keep fine-pitch happy.it.

1.3.3 The Step-Stencil Playbook

IfWhen one foiluniform can’thickness doesn't satisfywork—say, everything,you have sensitive 0.4 mm pitch BGA and a huge thermal slug—you must step the stencil.

• Step-Down: step stencilThins beatsthe compromisingfoil locally for fine-pitch features to prevent bridging. This is the most common use.
• Step-Up: Thickens the foil locally to dump extra volume into large thermal pads, heavy connectors, or Paste-in-Hole (PIH) features to ensure a robust joint.

Rules for the Printer:
Keepouts: Place the step edge ≥ 3 mm away from the nearest fine-pitch aperture. The pressure change near the edge can cause scooping or smearing.
Alignment: Always align the step ramps parallel to the squeegee stroke. Climbing or descending the ramp should happen along the direction of travel, not across it.
Taper: Use a gentle taper (or ramp) on the base.step We’lltransition proveto itminimize atthe SPImechanical shock and iterate apertures in 7.4, then lock cleaning/pressure influctuation 7.5, and control it with SPC in 7.6.

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1.3.3 Step-stencil rules (soas the squeegee doesn’tmoves launch the paste)

• Keep steps away from tiny apertures. Aim for ≥ 3–5 mm of flat land between a step edge and fine-pitch features.
• Ramp with the stroke. Align long step edges so the squeegee climbs/descends along, not across, the ramp.
• Mind the delta. Step depths of 25–75 µm are common; bigger jumps need longer ramps to avoid scooping.
• Markover it. Put the step map on the drawing and in the stencil spec inside your Golden Pack so setup & cleaning teams don’t guess.
  

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1.3.4 Nano-coating:Logistical whenControl: itThe earnsStencil itsis keepTooling

ChooseA itstencil whenis you’veexpensive, gotprecision tighttooling. area ratios or long runs with minimal cleaning. Expect: cleaner release (better SPI transfer efficiency), fewer bridging/tail issues at fine pitch, and longer stable open time. Pair with gentler wet/vac cycles in 7.5 so you don’t scrub the coating off early.

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1.3.5 Frame choices & logistics (boring, but they bite)

• Framed (mesh-mounted): rigid, consistent tension; great for mainline.
• Frameless foils (tension systems): cheaper to ship/store; nice for quick turns and variants—just mind tension settings and gasket health.
• LabelTreat it like a part.feeder or a test fixture, not a disposable item.
  • Standardize Frames: PutUse the same frame size and tension system across all your lines (e.g., standard 29" x 29"). Frameless foils are cheap to store but introduce setup variability (tensioning).
  • Label and Track: Stencils must be labeled with their foil ID, thickness, step step-map revrevision, and the cleaning cycle spec. Log its usage (and cleaning cycles) in a central database or MES, just like a maintenance log for a machine part.
  • Storage: onStore thestencils frameflat, tensioned, and in the stencil cabinet log. It’s part of your controlled tooling set (shows upprotected in 7.5dedicated cleaningcabinets. WIs and 7.6 SPC).
    

  
  

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  1.3.6 TiesDamage to the nextfoil pagesor (whatframe you’llwarp tunedue there)

  to
  • Apertures:leaning area/aspectis ratios,a window-pane,preventable home-plate,source chimney—yourof transferyield efficiency levers live in 7.4.loss.
  • Printer
  setup:

  Final squeegeeChecklist: angle/pressure/speed and understencil cleaning cycles in 7.5 keepOrdering the sameRight foil stable across shifts.

• SPI closed loop: 7.6 sets volume/height/area limits and auto-adjusts print to hold your window.
  

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1.3.7 Release checklist (stick this in the stencil drawer)Tool

• Foil typeType: chosenDetermined (laserby SSthe / electroformed Ni) andrequired nanoArea Ratio decisionof made.the hardest part (LCS or EF).
• Base thicknessThickness: matchesSet by the AR of the tightestsmallest pitchaperture on(e.g., the100 board.µm).
• Step mapMap: Designed (if used)necessary) showswith depths,steps ramps,≥ and3 keepouts—mm from fine pitch, aligned with the squeegee stroke.
• Aperture rulesNano-Coating: documentedDecision (pointer to 7.4); printer setup/cleaning notes linked (7.5).
• SPI targets and closed-loop enabled (7.6).
  

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Conclusion: Select stencil type and thicknessmade based on therequired mostthroughput demandingand featuresexpected ofcleaning frequency.

Documentation: Stencil ID, thickness, and cleaning specification recorded in the board,Golden thenBuild refine with step regions and coatings only where necessary. This approach maximizes transfer efficiency, simplifies printer setup, and delivers predictable results across diverse assemblies.

Package.