Overview

A custom kovar probe spring is a micro-fabricated cantilever contact, produced by photochemical etching, that provides repeatable elastic force in wafer probe cards and test sockets. Kovar (ASTM F15, Fe-Ni-Co) is specified because its coefficient of thermal expansion tracks silicon and alumina substrates, so probe tips stay registered across the −40 °C to 150 °C window used in wafer-level and burn-in testing. Photochemical machining removes metal through a photoresist mask rather than punching it, so beams leave the line flat and burr-free, without the work-hardening or root micro-cracks that stamping would put into a 0.05 mm spring.

For a test engineer, a custom kovar probe spring addresses three recurring limits: cantilevers that must hold uniform force and recovery on stock as thin as 0.05 mm, dense arrays whose pitch is too fine for a punch die, and contacts that must not drift out of alignment during temperature cycling. Because the geometry is defined by artwork, a new pin-out is a new film rather than a new die, so iteration from prototype to volume stays economical. Supplied from 0.05 mm to 0.5 mm stock, springs are usually nickel- or gold-plated after etch to lower contact resistance and support brazing or assembly downstream.

Parameter Window

• Available Materials: Kovar (ASTM F15 / 4J29), Alloy 42, stainless 301 spring temper on request
• Material Thickness Range: 0.05–0.5 mm
• Tolerance: ±0.01 mm on beam width, ±0.02 mm on outline profile
• Minimum Feature: 0.08 mm beam width and 0.06 mm gap at 0.10 mm stock
• Surface Options: electroless nickel, electrolytic Ni/Au, bright anneal, stress-relief anneal, passivation
• Compliance: ISO 9001 process control, RoHS/REACH material traceability

Manufacturing Process / Workflow (8 Steps)

1. Artwork & DFM Review — Convert the pin map to vector artwork, check beam-to-thickness ratios and confirm root radii and array pitch.
2. Alloy & Temper Selection — Match Kovar lot, cold-rolled temper and flatness to the force target and temperature-cycling spec.
3. Cleaning & Lamination — Degrease and micro-etch the sheet, then laminate dry-film resist under controlled heat and pressure.
4. UV Exposure — Expose both faces through registered high-resolution film so beam outlines align to back-side thinning zones.
5. Development & Inspection — Develop the resist and inspect for pinholes or scumming on fine beams before etching.
6. Controlled Etching — Spray-etch in ferric-chloride chemistry tuned for Fe-Ni-Co; time, temperature and conveyor speed set sidewall and half-etch depth.
7. Stripping, Anneal & Plating — Strip resist, hydrogen anneal to stabilize spring force, then apply Ni/Au to the contact tips.
8. Force & Dimensional Verification — Measure beam width, force-deflection curve and flatness, then pack in trays or on tape.

Core Advantages

1. Uniform spring force: Chemical removal leaves no roll-over, so beam width and recovery match across an entire array.
2. No hard tooling: A new pin-out is a new phototool rather than a punch die, keeping low-volume programs affordable.
3. Crack-free roots: Etched beams carry no stamped-in micro-cracks, so they survive tens of thousands of compression cycles.
4. Dense fine arrays: 0.08 mm beams with 0.06 mm gaps hold at 0.10 mm stock for high-density probe layouts.
5. Low thermal drift: Kovar tracks silicon and ceramic expansion, keeping tip registration stable across temperature cycling.
6. Batch repeatability: A custom kovar probe spring produced from one approved film etches identically from first article to volume lots, simplifying incoming qualification.

Typical Applications

• Wafer probe cards: Micro-spring arrays in vertical and cantilever probe cards for wafer-level testing.
• Test and burn-in sockets: Elastic contacts in aging and final-test sockets needing low resistance and long insertion life.
• MEMS and sensor probing: Fine elastic tips that contact delicate MEMS structures without scratching them.
• RF and high-frequency fixtures: Ground and signal springs whose stable geometry limits parasitic variation.
• Thermal-cycling fixtures: Elastic links inside temperature chambers that hold alignment despite drift.
• Ceramic substrate interconnects: Compliant transitions between ceramic packages and test boards that absorb tolerance stack-up.

Design Notes & Process Considerations

1. Beam width versus thickness: Keep the narrowest beam at 0.08 mm or 1.5 × material thickness, whichever is larger, to avoid root over-etch.
2. Root radii: Use internal radii ≥ 0.10 mm to reduce stress concentration and resist trapping at beam roots.
3. Half-etch depth: When local thinning is needed, state depth and tolerance — typically 30–60% of stock — together with beam width.
4. Force and deflection: Specify target force, working travel and fatigue cycles so alloy temper and anneal are chosen correctly.
5. Plating specification: State nickel underplate and gold thickness and whether tips only or the whole part is plated.
6. Panelization and marking: Group springs on a shared registration grid with orientation marks to speed automated placement and pin alignment.