Why the Question Matters for Semiconductor Packaging
Chip packaging fixtures locate, clamp and support lead frames, substrates and encapsulated devices through moulding, plating, singulation and testing. Because these operations run at tight tolerances, high temperatures and strictly controlled cleanliness levels, the fixture material must be stable, clean and predictable. Designers sometimes ask whether an ordinary cold-rolled steel coil, being cheap and easy to machine, can serve as fixture stock. The short answer is that commodity cold-rolled carbon steel fails several core requirements, and its use is limited to a narrow set of non-critical carrier parts.
Four Technical Reasons Ordinary Cold-Rolled Coils Are Not Recommended
1. Surface Cleanliness
Cold-rolled coil surfaces carry rolling oil residues, iron fines and a mill finish that can shed particles under abrasion. In a packaging environment, any particle larger than a few microns can contaminate the die attach or moulding interface and lower yield. Achieving semiconductor-grade cleanliness from commodity coil stock requires extra grinding, plating and passivation steps that erase the cost advantage.
2. Outgassing and Volatile Release
During moulding and reflow, fixtures are heated well above 150 degrees C. Unsealed carbon steel surfaces, especially with residual oil in pores and scratches, release volatiles that can outgas inside the mould cavity and cause voids or discolouration on the package. Materials chosen for fixtures are selected partly for their low and stable outgassing behaviour at operating temperature.
3. Magnetic Response
Ferritic carbon steel is strongly magnetic. In packaging lines that use magnetic handling, magnetic chucking or electrical inspection, a magnetic fixture disturbs part positioning and can retain residual magnetism that attracts ferrous debris. Austenitic stainless steel and aluminium are non-magnetic and avoid this interference.
4. Dimensional Stability
Cold-rolled coil has residual stress from rolling and levelling. When material is machined into a fixture, stress relief shifts dimensions, and the coefficient of thermal expansion of carbon steel is high enough to cause positioning errors when the fixture cycles between room temperature and moulding temperature. Fixture materials are chosen so that the fixture and the part expand at a controlled, matching rate.
Limited Cases Where Cold-Rolled Coils Can Be Considered
Cold-rolled carbon steel can still appear in non-critical jigs, magazine guides, stack trays and transport carriers that do not touch the die surface, do not enter the mould, and run at ambient temperature. For these parts, the coil should be specified with clean pickled and oiled or lightly oiled surfaces, and the parts should be degreased, phosphated or painted after machining. Even then, dimensional stability and particle shedding should be verified with a pilot run before committing to a production batch.
Mainstream Materials for Chip Packaging Fixtures
| Material | Typical Role | Key Property |
|---|---|---|
| Austenitic stainless steel (304/316 type) | Mould bases, clamps, precision carriers | Non-magnetic, corrosion resistant, clean |
| Nickel-iron low-expansion alloy (FeNi36) | Mould frames, reference tooling for encapsulation | Very low thermal expansion matching package dimensions |
| Aluminium alloys (6061, 7075) | Lightweight carriers, handling fixtures | Low mass, good machinability, anodisable |
| Engineering plastics | Contact pads, nest inserts, handling rails | Non-marring, low outgassing grades available |
| Titanium alloys | High-temperature contact tooling | Heat resistance, low thermal expansion, non-magnetic |
Nickel-iron low-expansion alloys deserve special attention: they combine a coefficient of thermal expansion of roughly 1-2 x 10-6 per degree C with good machinability, which is why they are used where the fixture must hold a near-constant dimension while the mould heats and cools. Where magnetic interference is the concern, austenitic stainless steel is the first choice; where weight and machinability dominate, aluminium 6061 or 7075 with a hard anodised surface is typical.
How to Make the Final Decision
Start from the process: what temperature will the fixture see, what touches the package, and what cleanliness level does the cleanroom require? Draw up a material shortlist from the table above, compare outgassing data, thermal expansion, magnetic permeability and cost, then run a qualification test with the actual package and moulding parameters. In most modern packaging lines, the answer is a combination of stainless steel for precision parts and engineering plastics or aluminium for handling elements, with ordinary cold-rolled coil reserved only for truly non-critical ambient-temperature carriers.
Frequently Asked Questions
Can galvanized or coated cold-rolled sheet improve suitability?
Zinc and organic coatings reduce rusting but do not fix cleanliness, outgassing or magnetic issues, and coating particles can shed during wear. Coatings do not convert commodity coil into semiconductor-grade fixture stock.
Why is low thermal expansion so important in moulding fixtures?
Moulding cycles heat the fixture by more than 100 degrees C. If the fixture expands more than the package materials, wire bonds, die positions and encapsulation thickness shift, causing rejects. A low-expansion alloy keeps the cavity geometry stable through the whole cycle.
Is stainless steel always better than aluminium for fixtures?
Not always. Aluminium is lighter, machines faster and costs less, and hard anodising gives a wear-resistant surface. Stainless steel wins where stiffness, non-magnetism and thermal stability under repeated heating are critical.
What is the practical surface finish requirement?
Contact surfaces are typically ground or polished to a roughness of Ra 0.4 or better and cleaned to particle specifications set by the cleanroom class. Any finish specification should be agreed with the fixture maker in writing.
How should fixture materials be verified on delivery?
Request material certificates showing grade, heat treatment and chemical analysis, and verify hardness and dimensional stability on a sample. A fixture that moves more than the specified tolerance after the first heat cycle should be rejected.