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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 the exact volume of solder that gets deposited. The material, thickness, and surface condition of this piece of metal set the baseline for transfer efficiency and print repeatability. A good stencil is the difference between a stable, boring process and a daily war on bridging and inadequate paste volume.

1.3.1 Stencil Types: Performance vs. Cost

The choice of foil material and process is a direct trade-off between stencil cost and your required paste release quality at the smallest aperture.

Type

Foil Material

Performance Upside

Cost & Risk Trade-Off

Laser-Cut Stainless (LCS)

Stainless Steel, cut by laser.

Standard Workhorse. Fast turnaround, low cost, robust for general use (0.5 mm pitch and larger).

Aperture walls are rougher; limits transfer efficiency (TE) on fine pitch. Requires aggressive cleaning.

Electroformed (EF)

Nickel foil, grown by plating.

Ultimate Release. Super-smooth, rounded aperture walls (chimney effect) for superior release. Essential for Area Ratios < 0.66.

Higher cost, longer lead time. Requires gentler cleaning—coating damage is expensive.

Step Stencil

Standard LCS/EF foil with localized thinned (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.

Nano-Coated

Any foil type with a permanent, anti-stick fluoro-polymer layer.

ROI Play. Improves Transfer Efficiency (TE), reduces cleaning cycles (better UPH/throughput), and extends open time.

Added cost; must 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 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)

≥ 0.66

Area of Aperture / Area of Wall Surface. Below 0.66, wall friction starves the paste volume. Base thickness is driven by this.

Aspect Ratio (AsR)

≥ 1.5

Aperture Width / Stencil Thickness. Below 1.5, the aperture is too deep for the paste to easily exit.

The most common thickness used in modern EMS is 100 µm (0.004 inch), but the hardest part dictates the final choice:

Board Component Mix (Examples)

Recommended Base Thickness

Primary Focus/Trade-Off

General Mix / 0.65 mm QFP

120-130 µm (0.005 inch)

Forgiving, easy volume for 0402s/0603s. 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.

1.3.3 The Step-Stencil Playbook


When one uniform thickness doesn't work—say, you have sensitive 0.4 mm pitch BGA and a huge thermal slug—you 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 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 step transition to minimize the mechanical shock and pressure fluctuation as the squeegee moves over it.

1.3.4 Logistical Control: The Stencil is Tooling

A stencil is expensive, precision tooling. Treat it like a feeder or a test fixture, not a disposable item.

  • Standardize Frames: Use 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 ID, thickness, step-map revision, 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: Store stencils flat, tensioned, and protected in dedicated cabinets. Damage to the foil or frame warp due to leaning is a preventable source of yield loss.

Final Checklist: Ordering the Right Tool

  • Foil Type: Determined by the required Area Ratio of the hardest part (LCS or EF).
  • Base Thickness: Set by the AR of the smallest aperture (e.g., 100 µm).
  • Step Map: Designed (if necessary) with steps ≥ 3 mm from fine pitch, aligned with the squeegee stroke.
  • Nano-Coating: Decision made based on required throughput and expected cleaning frequency.
  • Documentation: Stencil ID, thickness, and cleaning specification recorded in the Golden Build Package.
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