Bonding Ribbon for Semiconductor Packaging: Choosing the Material

What is ribbon bonding?

Ribbon bonding is a form of wedge bonding in which a rectangular strip, rather than a round wire, is pressed onto a bond pad by a grooved tool and welded in the solid state by ultrasonic energy, sometimes with heat. The bonder forms the first bond, loops the ribbon to the second pad, bonds it and cuts it. No solder or adhesive is involved.

A flat conductor has two useful properties. For the same cross-section it has much more surface area than a round wire, which lowers inductance and resistance at high frequencies, where current flows near the surface. And it lies low over the chip, so loops are short and stable. Wedge bonds can also be compact: published comparisons suggest the bond footprint can be only slightly larger than the ribbon width, which helps pad layout.

The material choice is driven by the application. Power modules switch tens to hundreds of amps and need low DC resistance and long life under thermal cycling; RF and microwave circuits need low inductance and stable, corrosion-free contacts at gigahertz frequencies; battery packs need fast, reliable connections between cells and busbars. No single metal is best for all three.

Aluminium, gold, copper and silver ribbon compared

Bulk property values for the pure metals from standard reference data (for example the CRC Handbook of Chemistry and Physics and ASM). Ribbon properties such as breaking load and elongation depend on purity, size and temper and are reported on each lot certificate.

The thermal expansion row explains much of the reliability story. Silicon expands at about 2.6 ppm/K and silicon carbide at roughly 4 ppm/K, so every temperature swing shears the bond between the chip and an aluminium ribbon that expands six to nine times as much. Copper's lower expansion and higher strength are why copper interconnects survive many more power cycles, provided the chip has a copper-compatible top metallisation.

Aluminium ribbon for power modules and battery packs

Aluminium ribbon, usually 99.99 % or 99.999 % pure and sometimes micro-alloyed, is the workhorse interconnect in IGBT, SiC MOSFET and diode power modules. Typical bonds link chip emitters or sources to the DBC or AMB substrate, and the substrate to terminals. It bonds at room temperature by ultrasound alone, so no heated stage is needed, and it is fully compatible with the aluminium top metallisation used on most power chips.

In battery packs, wide aluminium ribbon connects cylindrical or prismatic cells to busbars, and in some designs the ribbon also acts as a fusible link. Its low density keeps pack weight down and its softness protects cell terminals during bonding.

Our aluminium bonding ribbon guide covers purity grades, current capacity by size, bonding parameters and power-cycling qualification in detail.

Gold ribbon for RF and microwave circuits

Gold ribbon is the established interconnect in RF, microwave and millimetre-wave modules, hybrid circuits and optoelectronic packages. At high frequency, current is confined to a thin surface layer: the skin depth in gold is under 1 µm at 10 GHz (about 0.8 µm, calculated from its resistivity). A wide, thin ribbon offers far more surface than a round wire of the same mass, so its resistance and inductance are lower, and its flat shape gives more predictable impedance between a chip and a transmission line.

Gold does not oxidise, so it bonds reliably to gold-plated pads and stays stable for decades in hermetic and non-hermetic packages. Gold ribbon is usually bonded thermosonically, with the substrate heated to moderate temperatures, often in the range of about 100 – 150 °C, to help bond formation. Typical sizes are very thin, in the order of 13 – 50 µm (0.5 – 2 mil) thick and from about 50 µm to over 1 mm wide.

• Watch gold-aluminium contacts: gold ribbon on aluminium pads forms Au-Al intermetallics that grow with temperature and time and can weaken the bond; gold pads are preferred.
• Keep loops short and flat: loop length directly adds inductance in RF paths, so ribbon is often bonded with a very low, controlled loop.
• Specify hardness and elongation: soft, annealed gold ribbon bonds more consistently and is less likely to damage GaAs or GaN chips.

Copper and silver bonding ribbon

Copper ribbon carries more current than aluminium of the same size and has higher thermal conductivity, so it spreads heat away from hot spots on the chip. Its lower expansion and higher strength give much longer power-cycling life. The trade-offs are hardness and oxidation: copper needs higher bond force and ultrasonic energy, a chip with a copper-compatible top layer (for example a copper metallisation or a bonded copper buffer) to avoid cratering the die, and sometimes an inert or forming-gas atmosphere to keep surfaces clean. Copper ribbon is used in next-generation SiC power modules, high-current discrete packages and battery connections.

Silver ribbon has the highest electrical and thermal conductivity of any metal and is softer than copper, so it bonds more easily. It is used in selected high-power and high-frequency applications where its performance justifies the cost. Designers should consider silver's tendency to tarnish in sulphur-containing atmospheres and to migrate under bias in humid conditions, and protect or encapsulate the assembly accordingly.

Ribbon versus wire, clip and flip-chip

Ribbon is not a universal answer. Fine-pitch, high-I/O chips still need fine wire ball bonding or flip-chip. Ribbon must run in a straight line between pads, so routing is less flexible than with wire. It needs deep-access wedge bonding heads and ribbon-specific tools and cutters; a ball bonder cannot simply be converted. Wide ribbon also gives shorter spool lengths, which means more spool changes on high-throughput lines. Where these limits are acceptable, one ribbon can replace several parallel wires, reducing bond count and cycle time.

Specifications, tolerances and quality checks

Bond quality is then confirmed by the packaging house through pull and shear tests (MIL-STD-883 Method 2011, DVS 2811), cross-sections, and reliability tests such as temperature cycling and, for automotive power modules, power cycling to AQG 324. Lock the qualified ribbon specification and treat any change of grade, temper or source as a requalification.

How to order aluminium bonding ribbon from Metline

1. Send the specificationAluminium grade (4N, 5N or micro-alloyed), width × thickness, tolerances, breaking load and elongation, spool type and length.
2. Tell us the processBonder model, tool type, chip and pad metallisation, and whether bonding is ultrasonic or thermosonic.
3. Engineering review and samplesWe confirm the specification and arrange sample spools for qualification on your line.
4. Volume supplyLots with certificates of conformance, packed to protect surfaces and windings, shipped worldwide on agreed terms.

Metline supplies aluminium bonding ribbon to specification for semiconductor packaging lines, power-module makers and battery-pack assemblers. Supply is set up for production programmes with defined specifications rather than retail spools. Gold, copper and silver ribbon are described above to support material selection; Metline does not supply them.

Related bonding materials and guides

Bonding ribbon: frequently asked questions