Overview

A custom kovar lead frame is a photochemically etched metal skeleton that carries the die and routes electrical connections inside hermetic semiconductor packages. Kovar (ASTM F15, Fe-Ni-Co) is specified because its thermal expansion closely matches borosilicate glass and alumina ceramic, so glass-sealed leads stay aligned and gas-tight through temperature cycling. Etching removes metal through a photoresist mask rather than punching it, so frames leave the line flat and burr-free, without the work-hardening or edge burrs that stamping would put into fine leads.

For a packaging engineer, a custom kovar lead frame solves three recurring problems: fine-pitch lead arrays a die cannot hold over long runs, die pads and leads that must be coplanar for reliable bonding and sealing, and a frame that must survive glass-sealing temperature without oxidizing or blistering. Geometry is defined by artwork, so a new pin-out is a new film rather than a new die. Supplied from 0.10 mm to 0.5 mm stock, frames are usually selectively nickel- or gold-plated to protect the Kovar surface and support wire bonding and glass sealing downstream.

Parameter Window

• Available Materials: Kovar (ASTM F15 / 4J29), Alloy 42, stainless 316L on request
• Material Thickness Range: 0.10–0.5 mm
• Tolerance: ±0.02 mm on lead width and pitch, ±0.03 mm on coplanarity
• Minimum Feature: 0.10 mm lead width and 0.20 mm pitch at 0.20 mm stock
• Surface Options: selective Ni/Au, electroless nickel, bright anneal, hydrogen anneal, passivation
• Compliance: ISO 9001 process control, RoHS/REACH material traceability

Manufacturing Process / Workflow (8 Steps)

1. Artwork & DFM Review — Convert the package drawing to vector artwork and confirm lead-to-thickness ratio, pitch and die-pad size.
2. Alloy & Temper Selection — Match Kovar lot, cold-rolled temper and flatness to the sealing method and bonding spec.
3. Cleaning & Lamination — Degrease and micro-etch, then laminate dry-film resist with no bubbles over fine leads.
4. UV Exposure — Expose both faces through registered film so front-side pads align to back-side leads.
5. Development & Inspection — Develop the resist and inspect leads for pinholes before etching.
6. Controlled Etching — Spray-etch in Fe-Ni-Co-matched chemistry; time, temperature and conveyor speed set sidewall and half-etch depth.
7. Stripping, Anneal & Plating — Strip resist, hydrogen anneal to relieve stress and activate the surface, then selectively plate Ni/Au.
8. Coplanarity & Pitch Verification — Measure lead width, pitch and coplanarity, then tray-pack or tape for automated packaging feed.

Core Advantages

1. Coplanar leads: Chemical removal leaves no roll-over, so whole rows stay coplanar and bonding and sealing yields rise.
2. No hard tooling: A new pin-out is a new phototool rather than a punch die, keeping low-volume programs affordable.
3. Stress-free edges: Etched leads carry no stamped micro-cracks, resisting distortion through sealing heat and thermal cycling.
4. Fine pitch: 0.10 mm leads at 0.20 mm pitch hold at 0.20 mm stock for high-density packages.
5. Seal-compatible: Kovar matches glass and ceramic expansion, and selective Ni/Au gives reliable bonds and stable hermeticity.
6. Batch repeatability: A custom kovar lead frame from one approved film etches identically from first article to volume lots, simplifying qualification.

Typical Applications

• Ceramic hermetic packages: Lead frames for CERDIP and CQFP-style packages sealed with glass frit.
• TO and metal-can headers: Lead frames in transistor and optoelectronic headers matched to Kovar lids.
• Hybrid integrated circuits: Interconnect frames in thick- and thin-film hybrids that survive sealing heat.
• MEMS and sensor packages: Lead frames for hermetic pressure and infrared sensor housings with low stress.
• RF and microwave devices: Feed-through frames in shielded cavities where stable geometry aids bond consistency.
• High-reliability multichip modules: Dense interconnect frames whose fine leads etch in one pass.

Design Notes & Process Considerations

1. Lead versus thickness: Keep the narrowest lead at 0.10 mm or 1.2 × material thickness, whichever is larger, to protect coplanarity.
2. Pitch and bridging: State pitch tolerance and minimum adjacent gap; tight gaps can merge during etch if allowance is missing.
3. Half-etch depth: When bond shelves or local thinning are needed, state depth and tolerance together with lead width.
4. Coplanarity callout: Specify row coplanarity and overall bow; thin frames distort during anneal if not fixtured.
5. Plating specification: State selective Ni/Au areas and thickness; bond zones need continuous gold and seal zones should stay wettable.
6. Panelization and marking: Group frames on a shared registration grid with orientation marks to speed automated feed and alignment.