1.3 Stencil Types, Thickness, and Lifecycle

The stencil is wherenot thea twostatic criticalplate; elementsit ofis SMTa —precision thetooling pastedie that degrades with every print stroke. Its foil thickness and thetechnology PCBdetermine — first meet. Its geometry defines the exactyour volume ofpotential, solderbut thatits gets deposited. The material, thickness,tension and surface condition ofdetermine thisyour pieceyield ofstability metalover settime.

Most printing defects (smearing, variation) blamed on "process parameters" are actually caused by a stencil that has lost tension or has micro-damage in the baselineapertures. forTreat transfer efficiency and print repeatability. A goodthe stencil is the difference betweenas a stable,controlled boringasset process andwith a dailydefined warlifecycle onfrom bridgingbirth andto inadequate paste volume.scrap.

StencilTechnology Types:Selection: PerformanceMatching vs.Tool Costto Pitch

TheSelect choice ofthe foil materialtechnology andbased processon isthe aTransfer directEfficiency trade-off(TE) betweenrequired stencil cost andby your required paste release quality at the smallest aperture.component.

TypeTechnology	Foil MaterialProcess	PerformanceUse UpsideCase	Cost & RiskEngineering Trade-Off
Laser-Cut Stainless (LCS)	Stainless Steel, cut by laser.Standard	StandardDefault Workhorse.Choice. Fast turnaround, low cost, robustRobust for general use (0.5 mm5mm pitch and larger).larger.	ApertureRougher aperture walls arereduce rougher; limits transfer efficiency (TE)release on fine pitch.features. RequiresStandard aggressivegrain cleaning.structure limits TE to ≈70-80%.
ElectroformedFine-Grain (EF)FG)	Refined Steel	High-Density. Required for 0.4mm pitch QFN/BGA and 0201s.	Smoother wall geometry improves release. Costs 20-30% more but prevents insufficient solder defects.
Electroform (Ni)	Nickel foil, grown by plating.Plating	UltimateUltra-Fine Release./ Wafer. Super-smooth, rounded aperture walls (chimney effect) for superior release. Essential for Area Ratios < 0.66.35mm pitch or µBGA.	HigherExpensive cost,and longerfragile. leadThe time.gasket Requires gentler cleaning—coating damageseal is expensive.
Stepperfect, Stencil	Standardbut LCS/EFthe nickel foil withdents localizedeasily thinnedif (step-down) or thickened (step-up) regions.	The Compromise. Allows one stencil to handle extreme variations (e.g., 0.4 mm) BGA and massive thermal pads).	Requires precise design and machine programming to ensure smooth squeegee crossing.mishandled.
Nano-CoatedCoating	AnyHydrophobic foil type with a permanent, anti-stick fluoro-polymer layer.Layer	ROIProductivity Play.Booster. ImprovesRecommended Transferfor Efficiencyall (TE),High-Volume reduces cleaning cycles (better UPH/throughput), and extends open time.lines.	AddedRepels cost;flux. mustReduces use gentlerunder-stencil cleaning fluidsfrequency from every 3 boards to preserveevery the20+ coatingboards. life.Pays for itself in throughput.

HowThickness Thick?Decision Quantifying the Volume ConstraintMatrix

Your choice of base thicknessThickness is constraineda bycompromise between volume (preventing opens) and release (preventing shorts). Do not guess; use the smallest, most difficult component on the 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) rule.	The Golden Rule: ≥ 0.66	Area Ratio = ( Area of ApertureOpening / Area of WallAperture Surface.Walls Below) ≥ 0.66 If AR < 0.66, wall friction starves the paste volume.will Basestick thickness is driven by this.
Aspect Ratio (AsR)	≥ 1.5	Aperture Width / Stencil Thickness. Below 1.5,to the aperturewalls isand too deep fornot the pastepad. Standard Thickness Map: 127 µm (5 mil): Target: Standard SMT (0805, 0603, SOIC, 0.65mm Pitch). Risk: Excessive volume on 0.5mm pitch parts leads to easily exit.bridging.

The most common thickness used in modern EMS is 100 µm (0.0044 inch)mil):,

• Target: but the hardest part dictates the final choice:

  Board Component MixMobile/Computing (Examples)	Recommended0402, Base Thickness	Primary Focus/Trade-Off
  General Mix /0201, 0.655mm mmBGA, QFP	120-130QFN). Risk: µm (0.005 inch)	Forgiving, easyInsufficient volume foron 0402s/0603s.large Needs aggressive aperture reductions for finer parts (next section).
  0.4 mm QFN / 0.5 mm BGA / 0201	90-100 µm (0.0035 – 0.0040 inch)	The sweet spot for dense, modern consumer boards. Use T4/T5 paste with this thickness.
  0.3 mm WLCSP / Ultra-Fine Pitch	60-80 µm (0.0024 – 0.0031 inch)	Requires Electroformed foil. If you can’t get the Area Ratio with 80 µm, you must thin it.

  The Step-Stencil Playbook

  
  

  When one uniform thickness doesn't work — say, you have sensitive 0.4 mm pitch BGAconnectors/shields. andMitigation: aUse hugeaperture thermalover-printing slug—you(1:1.1) must step the stencil.

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

  RulesStep forStencils the(The Printer:Hybrid Solution):
  Keepouts:

  • Scenario: PlaceA theboard stephas edgeboth ≥a 3USB-C mmconnector away(needs fromvolume) theand nearesta fine-0.4mm pitch aperture.BGA The(needs pressure change near the edge can cause scooping or smearing.control).
    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:
  • Action: Use a gentle taperStep-Up (local thickening) for the connector or ramp)Step-Down on(local thinning) for the BGA.
  • Constraint: Keep a "Keep-Out Zone" of 3mm - 5mm around the step transition to minimize the mechanical shock and pressure fluctuation asallow the squeegee movesto over it.conform.

  Logistical Control: The Stencil isLifecycle ToolingManagement

  A stencil ismust expensive,be precision tooling. Treat itmanaged like a feedermachine ortool. Implementing a test"Stencil fixture,History Record" prevents gradual process degradation.

  1. Incoming Inspection (The Gate)

  Do not put a disposablenew item.stencil on the line until it passes these checks:

  • StandardizeFiducial Frames:Check: UseVerify fiducials match the samePCB frameGerber sizeexactly.
  • Tension andCheck: Measure foil tension systemusing acrossa alltension your linesmeter (e.g., standardTetra).
    • Pass Criteria: > 35 N/cm29" x(New).
    • Fail: 29"< 30 N/cm (Reject immediately; loose foil causes "dog ears" and smearing).).
  • Text FramelessOrientation: foilsVerify arereadable cheaptext is on the "Squeegee Side" to storeprevent butloading introduceit setupupside variabilitydown.
  (tensioning)

  2. Cleaning & Storage Control

  • Ultrasonic Risk: Do not use aggressive ultrasonic cleaning on Electroformed or Nano-coated stencils; it can delaminate the plating or coating. Use Spray-Under-Immersion.
  • Label and Track:Chemistry: StencilsUse mustpH-neutral besaponifiers. labeledHigh-pH withcleaners their ID, thickness, step-map revision, anddissolve the cleaningglue cycleholding spec.the Logfoil itsto usagethe (and cleaning cycles) in a central database or MES, just like a maintenance log for a machine part.frame.
  • Storage: Store stencils flat, tensioned, and protectedvertically in dedicateda cabinets.protective Damagerack or hanging system. Never stack stencils horizontally; the mesh will warp.

  3. Retirement Criteria (When to Scrap)

  Stencils have a finite fatigue life. Monitor these triggers to retire a stencil before it causes yield loss.

  • Tension Loss: If tension drops below 20–25 N/cm, the foil cannot snap off the PCB cleanly. Result: Smearing/Bridging.
  • Damage: Any visible dent, crease, or frame"coining" warp(indentation duefrom a component).
  • Blockage: Dried paste in apertures that cannot be removed by automated washing. Do not use metal tools to leaningpoke isapertures a preventable source of yield loss.clear.

  Final Checklist: OrderingStencil the Right ToolControl

  This table summarizes the non-negotiable specifications that must be confirmed and documented before the stencil is ordered. These checks prevent yield loss caused by incorrect paste volume (bridging or starvation).

  Parameter	Mandate/SelectionSpecification Criteria/ Limit	Verification PointAction
  Foil TypeMaterial	Must be selected based on the required Area RatioLaser (AR)Std) ofvs theFG hardestvs part: Electroformed (EF) for AR ≤ 0.66 (premium release) or Laser-Cut Stainless (LCS) (standard).E-Form	ConfirmedSelect bybased DFM review againston smallest aperture size.
  Base Thickness	Set by the largest thickness that still satisfies the Area Ratio (≥ 0.66) and Aspect Ratio (≥ 1.5) of the smallest aperture.	Corresponds to the largest viable paste volume to minimize tombstoning.
  Step Map	Inclusion Criteria: Required only when the board contains extreme variationspitch (e.g., < 0.44mm mm= BGA and a large PIH connector)FG/E-Form).	Rules: Step edges placed ≥ 3 mm from fine-pitch features and aligned parallel to the squeegee stroke.
  Nano-Coating	DecisionYes based/ on No	OpEx/ThroughputYes goals. Justified if cleaning cyclesfrequency mustis bereducing reducedline for high volume/high-mix.	Cost of coating must be balanced against labor savings from reduced under-stencil cleaning.throughput.
  LogisticsIncoming &Tension	> ID35 N/cm	StencilMeasure mustupon bereceipt. labeledReject withif itsloose.
  Life uniqueLimit	< 20 N/cm or ID,~100k thickness,strokes	Measure monthly. Scrap if tension fails.
  Step Keep-Out	≥ 3mm andfrom cleaningstep specification.edge	AllEnsure datano mustcritical bepads recordedare in the transition slope.
  GoldenCleaning BuildMode	Verified PackageCompatible	Ensure wash chemistry does not eat frame glue or Nano-coat.
  Traceability	Unique ID Tag	Link Stencil ID to ensureBatch traceabilityRecord andin correct setup.MES.