1.3 Stencil Types, Thickness, and Lifecycle

Technology Selection: Matching Tool to Pitch

Select the foil technology based on the Transfer Efficiency (TE) required by your smallest component.

Technology	Process	Use Case	Engineering Trade-Off
Laser-Cut Stainless	Standard	Default Choice. Robust for 0.5mm pitch and larger.	Rougher aperture walls reduce release on fine features. Standard grain structure limits TE to ≈70-80%.
Fine-Grain (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 Plating	Ultra-Fine / Wafer. Essential for < 0.35mm pitch or µBGA.	Expensive and fragile. The gasket seal is perfect, but the nickel foil dents easily if mishandled.
Nano-Coating	Hydrophobic Layer	Productivity Booster. Recommended for all High-Volume lines.	Repels flux. Reduces under-stencil cleaning frequency from every 3 boards to every 20+ boards. Pays for itself in throughput.

Thickness is a compromise between volume (preventing opens) and release (preventing shorts). Do not guess; use the Area Ratio (AR) rule.

Area Ratio = ( Area of Opening / Area of Aperture Walls ) ≥ 0.66

If AR < 0.66, the paste will stick to the walls and not the pad.

127 µm (5 mil):

100 µm (4 mil):

Target: Mobile/Computing (0402, 0201, 0.5mm BGA, QFN).
Risk: Insufficient volume on large connectors/shields. Mitigation: Use aperture over-printing (1:1.1) on large pads.

Step Stencils (The Hybrid Solution):

Scenario: A board has both a USB-C connector (needs volume) and a 0.4mm pitch BGA (needs control).
Action: Use a Step-Up (local thickening) for the connector or Step-Down (local thinning) for the BGA.
Constraint: Keep a "Keep-Out Zone" of 3mm - 5mm around the step to allow the squeegee to conform.

A stencil must be managed like a machine tool. Implementing a "Stencil History Record" prevents gradual process degradation.

Do not put a new stencil on the line until it passes these checks:

Fiducial Check: Verify fiducials match the PCB Gerber exactly.
Tension Check: Measure foil tension using a tension meter (e.g., Tetra).
- Pass Criteria: > 35 N/cm (New).
- Fail: < 30 N/cm (Reject immediately; loose foil causes "dog ears" and smearing).
Text Orientation: Verify readable text is on the "Squeegee Side" to prevent loading it upside down.

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.
Chemistry: Use pH-neutral saponifiers. High-pH cleaners dissolve the glue holding the foil to the frame.
Storage: Store vertically in a protective rack or hanging system. Never stack stencils horizontally; the mesh will warp.

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 "coining" (indentation from a component).
Blockage: Dried paste in apertures that cannot be removed by automated washing. Do not use metal tools to poke apertures clear.

Parameter	Specification / Limit	Action
Material	Laser (Std) vs FG vs E-Form	Select based on smallest pitch (e.g., < 0.4mm = FG/E-Form).
Nano-Coating	Yes / No	Yes if cleaning frequency is reducing line throughput.
Incoming Tension	> 35 N/cm	Measure upon receipt. Reject if loose.
Life Limit	< 20 N/cm or ~100k strokes	Measure monthly. Scrap if tension fails.
Step Keep-Out	≥ 3mm from step edge	Ensure no critical pads are in the transition slope.
Cleaning Mode	Verified Compatible	Ensure wash chemistry does not eat frame glue or Nano-coat.
Traceability	Unique ID Tag	Link Stencil ID to Batch Record in MES.