1.3 Stencil Types & Thickness

Laser-cut, step, nano-coatings—and the thickness rules that fit your component mix.

Stencil choice is one of the earliest—and most permanent—decisions you make in SMT printing, and it locks in your paste transfer efficiency before the first board hits the line. The right match of foil type, thickness, and optional step or nano treatments has to serve the smallest apertures on the board without starving larger pads, while surviving production cleaning cycles and printer setup quirks. Laser-cut stainless works for most builds, electroformed nickel shines on fine-pitch or low-area ratios, step stencils balance mixed needs, and nano-coatings help paste release stay clean over long runs. Thickness is set by your tightest pitch, then adapted with local step-ups/downs for thermal pads or paste-in-hole. Get the foil right now, and aperture design, printer setup, and SPI control become far easier to tune in the following steps.

1.3.1 What kind of stencil do you actually need?

Type	What it is	Why (and when) it wins	Watch-outs
Laser-cut stainless	The everyday workhorse foil, cut by fiber/laser and tensioned in a frame.	Robust, fast to source, great for most 0402–0.5 mm pitch, QFPs, “normal” QFNs.	Aperture walls are rougher than electroformed; fine-pitch transfer relies more on thickness & apertures (see 7.4).
Electroformed (nickel)	Plated, super-smooth apertures with slightly “radiused” edges.	Better release at low area ratios (tight BGAs/WLCSP, tiny QFNs); lets you keep thickness a hair thicker without starving.	Cost/lead time; baby the foil (no harsh brushing).
Step stencil	Same foil, but locally thinned (step-down) or built-up (step-up).	One board, mixed needs: thin for 0.4–0.5 mm BGA/QFN; thick islands for big pads or paste-in-hole.	Plan ramps/keepouts; squeegee must cross steps cleanly (see notes below).
Nano-coated (add-on)	Fluoro-polymer surface treatment on any foil.	Less paste sticking = cleaner releases, longer cleaning intervals, nicer edges on fine apertures.	Don’t scrub off the coating; use gentler solvents & wipes; re-coat on a schedule.

Pair type + aperture design (next section) + printer setup/cleaning (7.5) to make printing boring—in the best way.

1.3.2 How thick? (choose by the hardest part on the board)

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.

Board mix (examples)	Base thickness	Notes that save you a respin
WLCSP 0.3–0.35 mm pitch; 01005 passives	60–80 µm (0.0024–0.0031”)	Often electroformed + nano; Type 5 powder; be ruthless on area/aspect ratios (7.4).
BGA/QFN at 0.4–0.5 mm; 0201–0402 passives	90–100 µm (0.0035–0.0040”)	Sweet spot for dense consumer boards; step-up islands for big thermal pads or shield lands.
General mixed (QFP 0.5–0.8 mm; 0402–0603; modest QFN)	120 µm (0.0047”)	The default many 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 slugs	Keep base for SMT; add step-up islands to 180–200 µm where needed	Avoid full-thickness foils at 180+ unless it’s a PIH-only panel; local islands print better + keep fine-pitch happy.

If one foil can’t satisfy everything, step stencil beats compromising on the base. We’ll prove it at SPI and iterate apertures in 7.4, then lock cleaning/pressure in 7.5, and control it with SPC in 7.6.

1.3.3 Step-stencil rules (so the squeegee doesn’t 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.
Mark it. Put the step map on the drawing and in the stencil spec inside your Golden Pack so setup & cleaning teams don’t guess.

1.3.4 Nano-coating: when it earns its keep

Choose it when you’ve got tight 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.

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.
Label it like a part. Put foil ID, thickness, step map rev on the frame and in the stencil cabinet log. It’s part of your controlled tooling set (shows up in 7.5 cleaning WIs and 7.6 SPC).

1.3.6 Ties to the next pages (what you’ll tune there)

Apertures: area/aspect ratios, window-pane, home-plate, chimney—your transfer efficiency levers live in 7.4.
Printer setup: squeegee angle/pressure/speed and understencil cleaning cycles in 7.5 keep the same foil stable across shifts.
SPI closed loop: 7.6 sets volume/height/area limits and auto-adjusts print to hold your window.

1.3.7 Release checklist (stick this in the stencil drawer)

Foil type chosen (laser SS / electroformed Ni) and nano decision made.
Base thickness matches the tightest pitch on the board.
Step map (if used) shows depths, ramps, and keepouts—aligned with squeegee stroke.
Aperture rules documented (pointer to 7.4); printer setup/cleaning notes linked (7.5).
SPI targets and closed-loop enabled (7.6).

Bottom line: pick the foil for your hardest features, set a thickness that feeds them, and use steps/nano only where they earn their keep. Then back it up with good apertures (7.4), disciplined printing (7.5), and SPI feedback (7.6). That’s how you get clean releases, calm bridges, and a printer crew that smiles.