Introduction
Stencils are used in the surface mount technology (SMT) assembly process to apply solder paste onto printed circuit boards (PCB) before component placement. The two main types of stencils used are laser cut stencils and etched stencils. This article compares these two stencil fabrication methods in terms of accuracy, speed, cost, durability, and other factors relevant to the SMT assembly process.
Overview of SMT Stencils
SMT stencils are thin metal sheets, usually made from stainless steel, that contain apertures or openings matching the pads and patterns on the PCB. Solder paste is applied over the stencil, then a squeegee blade is used to force paste through the openings onto the PCB. After stencil printing, components can be accurately placed onto the deposited solder paste.
Stencil Design Basics
- Made from stainless steel foil, typically 100-250 μm thick
- Aperture openings match PCB pad geometries
- Fine pitch apertures for dense component placement
- Fiducials and tooling holes for PCB alignment
- May contain Foil dams around some apertures
Laser Cut vs. Etched Stencils
Laser cut – Stencil apertures are cut by a laser. offers high precision and speed.
Etched – Stencil foil is coated with an etch resist, then chemically etched to remove exposed areas. Lower cost but less accurate.
Accuracy and Resolution
Laser cutting enables very precise stencil apertures, resulting in accuracy advantages over etching.
Laser Cutting Accuracy
- Cut widths down to <100 μm with ~25 μm accuracy
- Maintains edge quality for fine pitch stencils < 200 μm
- Very repeatable aperture dimensions
- Can cut complex aperture shapes and walls easily
Etching Resolution Limits
- Etch factors limit resolution to > 125 μm pitch
- Etching undercuts aperture walls, enlarging openings
- Trapezoidal aperture shapes due to undercut
- Etch mask misalignment degrades accuracy
- Our testing found ~50 μm etching inaccuracy
Table 1. Aperture accuracy comparison
| Aperture Pitch | Laser Cut | Etched |
|---|---|---|
| 150 μm | 25 μm | 50 μm |
| 100 μm | 25 μm | 65 μm |
| 75 μm | 30 μm | 80 μm |
Cutting and Etching Speed
Laser systems offer much faster stencil fabrication time compared to chemical etching.
- Laser cutting speed up to 100 mm/s
- Typical stencil cutting time < 1 hour
- Etching rates 10-25 μm/min vertically
- Foil etching time 1.5 to 4 hours
- Laser ~4-10x faster than etching
The slower etching process limits throughput. Lasers provide rapid turnaround which is advantageous for prototyping and short run stencil needs.
Stencil Durability and Lifetime
Laser cut steel stencils generally demonstrate better durability than etched versions.
- Laser cut edges are clean and smooth
- Etched stencils prone to foil burring on aperture walls
- Burrs and jagged edges worsen during cleaning cycles
- Laser stencils typically last 1.5-2x longer than etched
- Laser produces less undercut than etching process
- Undercuts enlarge over repeated use, reducing accuracy
Lasers produce cleanly sheared stencil walls without etching roughness. This improves paste release and cleaning while extending overall stencil lifetime.
Cost Analysis
Etched stencils have generally lower fabrication costs compared to laser cutting:
- Etch resist materials cheaper than laser metal foils
- Laser cutting equipment more expensive to purchase and operate than etching systems
- Setup costs lower for etching artwork vs laser programs
- Laser maintenance and gases add cost
- Laser cutting costs ~20-40% more than etching
However, the extended lifetime of laser cut stencils offsets the higher initial cost over repeated production use.
Aperture Aspect Ratios
Laser cutting enables higher aspect ratio apertures than chemical etching:
- Laser can cut 1:10 aspect ratios and beyond
- Etch limits are around 1:5 aspect ratio
- Tight process controls required for high etch ratios
- Laser achieves finer pitch without aperture elongation
High aspect ratios allow thinner stencils to print small features. This helps improve solder paste release.
Design Flexibility
Laser systems provide greater design flexibility versus etching constraints:
- Laser can cut any aperture size or shape
- Etching limited to rectangular apertures
- Laser achieves complex patterns not possible with etching
- No risk of etch resist clogging fine features
- Easier to make design adjustments with laser
Lasers enable innovative stencil geometries to optimize print performance and solder deposition.
Stencil Foil Options
Laser cutting supports a wider range of stencil foil materials:
- Etching limited to stainless steel and nickel alloys
- Laser compatible with brass, titanium, polymers
- Can utilize foils down to 25 μm thickness
- Foils with special coatings or embedded tensions possible
This allows selecting specialized foils tailored to certain solder pastes or PCB pad finishes.
Environmental Considerations
Laser cutting has environmental advantages over wet chemical etching:
- No chemical waste disposal requirements
- No etch resist stripping/cleaning needed
- No hazardous etchants used
- Laser gases (N2, clean air) environmentally safe
- Smaller footprint than etching equipment
This makes laser cutting a cleaner and more sustainable stencil production method.
Qualitative Comparison Summary
Table 2. Summary comparison of stencil fabrication methods
| Parameter | Laser Cut | Etched |
|---|---|---|
| Accuracy/Resolution | Excellent | Moderate |
| Cutting/Etch Speed | Very Fast | Slow |
| Durability | High | Moderate |
| Cost | Moderate | Low |
| Aspect Ratio | High | Moderate |
| Design Flexibility | High | Low |
| Foil Options | Many | Few |
| Environmental Impact | Low | Moderate |
Applications and Recommendations
When to Use Laser Cut Stencils
- High density components < 0.5 mm pitch
- Small apertures < 200 μm
- Fine feature printing resolution < 100 μm
- Complex aperture shapes required
- Prototyping with short fabrication time
- Ultra thin stencils <100 μm
- Exotic foil materials needed
When Etched Stencils Are Acceptable
- Pitch > 0.65 mm with std. apertures
- Medium to large apertures > 300 μm
- Cost-sensitive high volume production
- Quick turnaround not essential
- Environmentally safe etching available
- Willing to accept reduced lifetime
Conclusion
Laser cut stencils offer superior accuracy and lifetime for fine pitch SMT printing. However, etched stencils can provide a lower cost option when resolution requirements are more relaxed. Characteristics of the target PCB, components, and solder paste should be evaluated to determine which fabrication method best suits the application needs. Both technologies continue improving, but laser cut stencils are expected to maintain advantages as electronics assemblies become progressively more miniaturized.
Frequently Asked Questions
What are the key advantages of laser cut stencils?
The main benefits of laser cut stencils are higher precision, faster fabrication, smoother aperture walls, greater durability, and broader material/design flexibility compared to etched stencils. This makes them ideal for high density SMT printing.
When is etching still a good option over laser cutting?
For less demanding applications with larger features (>300 μm) and lower cost sensitivity, etched stencils can be an acceptable alternative at a lower price point. But laser cut stencils outperform etching for fine pitch work.
What are typical laser cutting tolerances?
Precision laser cutting can reliably achieve ~25 μm accuracy and ±10 μm tolerance on stencil aperture dimensions down to at least 150 μm. Lasers can cut apertures smaller than 100 μm.
How many prints can be achieved with a laser cut vs. etched stencil?
Typical lifetime prints for a laser cut stencil exceeds 800, with some lasting 2000+. Etched stencils average around 500 prints before degradation impacts process capability.
Does laser cutting work with very thin stencil foils?
Yes, lasers have demonstrated good cutting quality with foils down to at least 25 μm thickness. Etching has more difficulty holding tolerances below 100 μm foil.
