Overview

A custom invar lead frame is a precision metal skeleton that supports a semiconductor die, routes electrical connections and carries heat out of a package, etched from Invar 36 (an iron-nickel alloy with roughly 36% nickel). Photochemical machining cuts the die pad, lead fingers and tie bars without mechanical load, so the frame stays flat, burr-free and dimensionally stable. Invar's very low coefficient of thermal expansion closely matches silicon, which reduces stress on the die and on wire bonds during temperature cycling. Stock is typically 0.10–0.40 mm with lead width held to ±0.015 mm.

Package engineers specify a custom invar lead frame when thermal mismatch, not just conductivity, drives reliability. Because the geometry comes from a phototool rather than a progressive die, lead count, pad size and tie-bar layout can be revised without a tooling programme, and selective Ni, Au or Ag plating can be added where wire bonding or die attach needs it. A custom invar lead frame therefore suits both high-reliability prototypes and volume production of QFN, DFN and hermetic packages.

Parameter Window

• Available Materials: Invar 36 (Fe-36Ni), Kovar, 42 alloy, copper, 304 stainless
• Material Thickness Range: 0.10–0.40 mm (common 0.15, 0.20, 0.25, 0.30 mm)
• Tolerance: ±0.015 mm on lead width, ±0.02 mm on profile, ±10% on half-etched pad depth
• Minimum Feature: 0.10 mm lead width, 0.10 mm gap, 0.20 mm pitch
• Surface Options: degreased, passivated, selective Ni/Au/Ag plating, spot plating, roughened die pad
• Compliance: ISO 9001 process control, RoHS/REACH material traceability

Manufacturing Process / Workflow (8 Steps)

1. Artwork Review — Confirm lead count, pad size, tie-bar layout and plating zones against the package drawing.
2. Material Selection — Choose Invar 36 temper and thickness, verify flatness and check the CTE certificate of the coil.
3. Cleaning & Lamination — Degrease and laminate dry film on both faces, keeping the resist thin over fine lead fingers.
4. UV Exposure — Expose through registered high-resolution film with the die pad referenced to the panel datum.
5. Development — Wash out unexposed resist and inspect narrow leads and half-etched pad areas for complete opening.
6. Etching — Spray-etch with Invar-matched chemistry, controlling time and speed for straight lead sidewalls and recess depth.
7. Stripping & Finishing — Strip resist, neutralize, then passivate or selectively plate Ni/Au/Ag and roughen the die pad as specified.
8. Inspection & Packaging — Measure lead width, pad depth and flatness, verify plating coverage, then pack in trays with a report.

Core Advantages

1. CTE match to silicon: Invar's low expansion cuts thermal stress on the die and wire bonds across temperature cycles.
2. Burr-free fine leads: Chemical removal leaves smooth 0.10 mm fingers with no stamped burr that could short a neighbour.
3. Stable flat pad: A stress-free, flat die pad improves die-attach coverage and keeps bond heights consistent.
4. Selective plating ready: Ni, Au or Ag can be plated only where bonding needs it, controlling cost on precious-metal areas.
5. Tool-free package revisions: A new lead count or pad layout is a film change, so prototype iterations avoid die charges.
6. Material flexibility: A custom invar lead frame can also run in Kovar, 42 alloy or copper on the same etching line.

Typical Applications

• QFN and DFN packages: Lead frames with exposed pads for compact, low-profile integrated circuits.
• Power devices: Frames that carry high current and spread heat from MOSFETs and diode dies.
• MEMS and sensor packages: Low-stress frames that protect a fragile sensing structure during reflow.
• Optoelectronic packages: Frames for laser and photodiode dies where alignment must stay stable with temperature.
• Hermetic and RF packages: Metal frames that seal reliably and provide a controlled ground path.
• Prototype assembly: Small lots for package development before committing to stamped tooling.

Design Notes & Process Considerations

1. Lead width versus thickness: Keep the narrowest lead at 0.10 mm or 1.2 × thickness, whichever is larger, to avoid uneven etching.
2. Half-etch pad depth: State die-pad recess depth and tolerance, since it sets die height and wire-bond loop geometry.
3. Plating zones: Mark selective plating areas and thickness, and specify whether the lead tips or the whole frame is plated.
4. CTE traceability: Require a material certificate with measured expansion, because it is the reason for choosing Invar.
5. Flatness and tie bars: Call out flatness per 100 mm and define tie-bar break points for downstream singulation.
6. Inspection criteria: Agree lead width, pad depth and sampling plan; documenting these on the drawing cuts review cycles.