Overview

A custom invar wafer bumping stencil is a low-expansion precision mask used to print solder paste or flux, or to define plating openings, across a semiconductor wafer before bump formation. Photochemical machining cuts the aperture array into thin Invar 36 foil: the artwork is exposed onto photoresist, developed, and the unprotected alloy is dissolved in a matched etchant, so opening size, pitch and wall taper are set by the film and the bath. Foils run from 0.05 mm to 0.3 mm. Because Invar 36 has a very low coefficient of thermal expansion, the stencil keeps aperture-to-pad alignment stable as the wafer and tooling warm up during printing — the dominant error source at fine pitch.

For buyers, custom invar wafer bumping stencil production addresses the limits of stainless and laser-cut masks at fine pitch. Ordinary stainless drifts with temperature, so a 0.10 mm pitch can misregister as the line heats; laser cutting leaves dross, taper and a recast edge that trap paste; and mechanical punching cannot hold the wall quality that clean paste release needs. Photochemical machining holds ±0.005 mm on hole position, produces smooth, slightly tapered walls, and can be paired with a nano release coating for consistent deposit volume. The flat, stress-free foil mounts evenly on a frame, so a stencil repeats the same print result from the first prototype wafer through high-volume production.

Parameter Window

• Available Materials: Invar 36 (Ni36Fe), Ni-plated Invar; also stainless 304/316L for cost-sensitive grades
• Material Thickness Range: 0.05–0.3 mm
• Tolerance: ±0.005 mm on hole position, straight to slightly tapered wall
• Minimum Feature: 0.05 mm aperture, 0.10 mm pitch, tapered wall for paste release
• Surface Options: electropolishing, nano release coating, flatness control, tension mounting on frame
• Compliance: ISO 9001 process control, RoHS/REACH material traceability

Manufacturing Process / Workflow (8 Steps)

1. Bump Map & Aperture Design — Convert the wafer bump map into aperture size, pitch and area ratio, and set the taper direction.
2. Stress-Relieved Foil Selection — Choose low-expansion Invar 36, verify thickness, flatness and stress state, and review the mill certificate.
3. Cleaning & Lamination — Degrease and laminate fine-grain dry film resist in a clean area so fine-pitch openings resolve sharply.
4. UV Exposure — Expose through high-resolution film with collimated light, registering every aperture to the wafer datum.
5. Development — Wash out unexposed resist and inspect openings under magnification for residue that would starve a deposit.
6. Precision Etching — Spray matched chemistry with tight time and temperature control to hold wall taper and aperture size.
7. Stripping, Polishing & Mounting — Remove resist, electropolish, apply nano release coating, then tension-mount the foil on its frame.
8. Inspection & Packaging — Verify hole position, aperture, wall quality and tension on a vision system, then pack in clean, rigid trays.

Core Advantages

1. Thermal alignment stability: Invar's low expansion keeps apertures registered to pads as the wafer and stencil warm during printing.
2. Fine-pitch accuracy: ±0.005 mm hole position and 0.10 mm pitch support dense bump maps without bridging.
3. Clean paste release: Smooth, slightly tapered walls plus a nano coating give consistent deposit volume and easy release.
4. Flat, even tension: Stress-free foil mounts evenly on a frame, so print height and deposit are uniform across the wafer.
5. Fast pattern revisions: A new bump map is a new film, keeping prototype and design-change cycles short without tooling.
6. Material options: custom invar wafer bumping stencil parts can use Invar 36, Ni-plated Invar or stainless to balance drift, cost and life.

Typical Applications

• Wafer-level CSP solder bumping: Printing paste or flux across a full wafer before reflow into solder bumps.
• Flip-chip bump printing: Fine-pitch deposits for flip-chip interconnects where alignment drives yield.
• LED wafer processing: Deposit masks for LED pad metallization and phosphor or solder printing.
• MEMS packaging: Fine apertures for MEMS wafer bonding, sealing and interconnect pads.
• Micro-bump plating masks: Masks that define electroplated micro-bumps and under-bump metallization openings.
• TSV and fan-out interposers: Large, low-distortion masks for through-silicon via and fan-out redistribution printing.

Design Notes & Process Considerations

1. Area and aspect ratio: Define aperture area ratio and width-to-thickness ratio, since they govern whether paste releases cleanly.
2. Position and pitch: Give hole position and pitch tolerance relative to the wafer datum, and account for expansion over the print temperature.
3. Taper and release: State taper direction and the release coating, as both set deposit volume and de-molding behavior.
4. Flatness and tension: Specify foil flatness and frame tension, because sag or slack changes print gap across a large wafer.
5. Cleanliness and ions: Require low particle and ionic residue, since contamination at a 0.05 mm opening prints a defect.
6. Inspection datum: With custom invar wafer bumping stencil work, fix the measurement datum and vision acceptance for position, aperture and wall quality.