Overview

A custom invar wafer stencil is a high-precision mask and printing template for wafer-level packaging, bumping and thin-film deposition. Photochemical etching forms fine-pitch aperture arrays in thin Invar foil without mechanical force or a stamping die. Invar is an iron-nickel 36 low-expansion alloy (4J36 / Invar 36, Fe-Ni36) with a room-temperature coefficient near 1.2 ppm/K, far below conventional stainless, so aperture positions barely drift during exposure, reflow or deposition heating. A photoresist mask is exposed with the array artwork, developed, and the unprotected metal is dissolved in chemistry matched to the alloy, leaving edges that are burr-free, stress-free and free of heat-affected zones.

For packaging and process engineers, a custom invar wafer stencil addresses three recurring problems. Bump and redistribution-layer pitch has reached tens of microns, so any aperture drift or size spread makes paste volume and bump height uneven across the wafer; deposition and reflow both raise temperature, and a conventional metal stencil expands out of registration with the wafer pattern; and a thin-foil mask must be flat and stress-free, while stamping and laser cutting introduce curl, burrs and distortion that prevent it seating on the fixture. Chemical machining forms every aperture on a sheet at the same rate and pairs with stress-relief anneal, holding position consistent from the first article through volume production.

Parameter Window

• Available Materials: Invar 36 (4J36 / Fe-Ni36 / UNS K93601) as standard, also Kovar 4J29, Alloy 42, 316L stainless and electroformed nickel
• Material Thickness Range: 0.03–0.25 mm
• Tolerance: ±0.005 mm on aperture position, ±0.01 mm on aperture size
• Minimum Feature: 0.04 mm aperture, 0.05 mm web, pitch down to 0.08 mm
• Surface Options: stress-relief anneal, electropolishing, flatness lapping, ultrasonic cleaning, anti-stiction coating
• Compliance: ISO 9001 process control, RoHS/REACH material traceability

Manufacturing Process / Workflow (8 Steps)

1. Artwork & Nesting — Convert the aperture array to vector art, check position against the wafer map, and plan layout, registration targets and process borders.
2. Material Certification & Prep — Verify Invar grade, mill certificate and foil thickness, check scratches, flatness and expansion, then shear to exposure size.
3. Cleaning & Lamination — Degrease both sides and laminate dry film resist under matched heat and pressure so no bubbles sit under fine webs.
4. UV Exposure — Expose both sides through high-resolution film, registering the array to the foil edge and alignment targets.
5. Development — Wash out unexposed resist to open the apertures, and inspect for residue that would cause blind holes.
6. Controlled Etching — Spray Invar-matched chemistry on both sides, tuning time, temperature and line speed to hold walls and avoid aperture merging.
7. Stripping & Anneal — Remove resist and neutralize, stress-relief anneal and lap for flatness, then electropolish to clear hole-mouth burrs.
8. Inspection & Packaging — Measure position, size and pitch on a vision system, verify flatness and open area, then vacuum-pack in clean trays.

Core Advantages

1. Ultra-low expansion: Invar barely grows with heat, so apertures stay registered to the wafer through exposure, reflow and deposition.
2. High position accuracy: ±0.005 mm position tolerance and fine-pitch arrays formed together keep paste volume and bump height uniform.
3. Flat and stress-free: Chemical removal plus stress-relief anneal leaves thin foil flat, so it seats on wafer and fixture without warping.
4. No die cost: Each new array is a new film, keeping prototypes and frequent revisions economical without a stamping tool.
5. Clean apertures: An etch-clean surface with no dross or heat-affected zone releases paste cleanly and prints crisp deposition edges.
6. Volume repeatability: Every aperture etches together, so position spread stays tight from first article to production, backed by vision reports.

Typical Applications

• Wafer bumping: Solder-paste and flux printing stencils whose fine apertures keep bump height consistent.
• Wafer-level packaging: Alignment masks for redistribution and ball-attach steps, where low expansion tracks silicon through heating.
• Thin-film deposition: Shadow masks for MEMS and sensors, with clean walls giving sharp deposited edges.
• Semiconductor alignment plates: Datum-hole plates for lithography and inspection tools, where stable position protects tool accuracy.
• Fine-line printing: High-resolution paste-printing stencils whose uniform apertures control line width and film thickness.
• Optical detectors: Focal-plane and detector window masks that stay flat in clean vacuum environments.

Design Notes & Process Considerations

1. CTE matching: Confirm process temperature rise and wafer material before choosing 4J36 or Kovar, and note grade and certificate requirements.
2. Aperture-to-thickness ratio: Keep aperture at or above material thickness; on thin foil approach 1:1, or undercut distorts shape and steepens wall taper.
3. Position and registration: Give position datums, alignment targets and cumulative tolerance, and state face-to-face registration for two-sided exposure.
4. Flatness and stress: Call out finished flatness, anneal condition and lapping needs so clamping does not cause poor contact or uneven print thickness.
5. Nesting and tie bars: Leave enough tie bars and process borders, and define the final cut method so fine webs survive stripping and cleaning.
6. Documentation: With a custom invar wafer stencil, attach aperture tolerances, position datums and release direction to the drawing for repeatable first-article approval.