Overview

An etched wafer bumping stencil is a fine-pitch stainless steel stencil whose apertures deposit solder paste or flux onto wafer pads to form solder bumps. In wafer-level packaging the bumps themselves become the interconnect, so every aperture must release the same volume of material, at the same position, across the entire wafer. Photochemical etching builds these apertures from both sides of full-hard 304 stainless foil, producing openings that are burr-free, dimensionally stable and repeated with photographic accuracy.

What sets an etched wafer bumping stencil apart is the combination of fine pitch and release consistency. Apertures reach 0.08 mm with pitch down to 0.15 mm, and the double-sided etch with electropolished walls gives each opening a clean taper that lets paste and flux release uniformly — a requirement for uniform bump height after reflow. Foil thickness is held to ±0.005 mm so deposited volume stays constant across the wafer. These stencils serve solder bump, copper pillar and wafer-level flux deposition, where a single bridged or starved aperture can fail an entire die.

Parameter Window

• Available Materials: full-hard 304 stainless steel, 301 stainless steel
• Material Thickness Range: 0.05–0.2 mm foil
• Tolerance: ±0.01 mm on aperture size, ±0.005 mm on foil thickness
• Minimum Feature: 0.08 mm aperture and 0.15 mm pitch
• Surface Options: electropolished, nano-coated, nickel plated
• Compliance: ISO 9001 process control, RoHS/REACH material traceability

Manufacturing Process / Workflow (8 Steps)

1. Bump Map Review — Import the wafer bump map and convert it to aperture artwork, confirming pitch, aperture ratio and die pattern.
2. Material Selection — Choose full-hard 304 stainless foil at the specified thickness for strength and dimensional stability.
3. Cleaning & Lamination — Degrease both sides and laminate dry film under controlled pressure for clean aperture areas.
4. UV Exposure — Expose both sides through registered films so each aperture opens symmetrically from front and back.
5. Development — Wash out unexposed resist to open the bump apertures, then inspect for residual resist and under-development.
6. Etching — Etch from both sides in matched chemistry, controlling time and temperature so apertures meet with clean walls.
7. Stripping & Electropolishing — Remove the resist and electropolish to smooth aperture walls and radius edges for uniform release.
8. Inspection & Framing — Verify aperture size, wall quality and thickness under vision, then frame and measure tension.

Core Advantages

1. Uniform bump deposition: Consistent aperture geometry and thickness deliver the same paste volume at every pad, supporting uniform bump height.
2. Burr-free apertures: Chemical etching leaves openings free of burr, so paste and flux release without smearing or bridging.
3. Fine pitch for bump arrays: Apertures of 0.08 mm and pitches of 0.15 mm hold repeatably across the full wafer map.
4. Smooth release walls: Electropolished, taper-controlled walls let paste and flux release cleanly stroke after stroke.
5. No hard tooling: Each wafer map is produced from film, so prototype and revised bump patterns carry no die cost.
6. Stable thickness: Foil at ±0.005 mm keeps deposited volume constant, reducing bump height variation after reflow.

Typical Applications

• Solder bump deposition: Aperture arrays for printing solder paste to form solder bumps on wafer pads.
• Copper pillar bumping: Stencils for depositing paste used in copper pillar bump formation.
• Wafer-level flux printing: Fine apertures for printing flux before ball placement or reflow.
• Micro-bump arrays: Ultra-fine apertures for micro-bump interconnects in advanced packaging.
• Redistribution bumping: Openings aligned to redistribution layer pads for bump deposition.
• Pillar and stud stencils: Apertures for solder studs and pillars in power and RF devices.

Design Notes & Process Considerations

1. Aperture area ratio: Keep opening area above 0.66 of wall area, or paste and flux will not release cleanly on fine bump apertures.
2. Bump height control: Specify target deposited volume or bump height so foil thickness and aperture size can be balanced.
3. Wall finish: Electropolished or nano-coated walls are essential for uniform release on fine bump arrays.
4. Die pattern alignment: Confirm the aperture array matches the wafer die layout and fiducials for stencil-to-wafer registration.
5. Frame tension: State mesh tension and frame size so the stencil stays flat over the full wafer area during printing.
6. Thickness tolerance: Call out foil thickness tolerance, since it directly drives bump height uniformity across the wafer.