Heavy Bonding Wire & Ribbon for Power Semiconductor Packaging

The topside interconnect in a power device

A power semiconductor die is soldered or sintered to a substrate on its underside. Its topside, a thin aluminium-based metallisation over the source or emitter, must also be connected to the substrate tracks, terminals or neighbouring dies. In most power modules that connection is made by heavy bonding wire or bonding ribbon.

The current path runs from the power die, through its topside metallisation, into the wire or ribbon, and onwards to the substrate copper, a lead frame or a module terminal. The interconnect therefore has three jobs. It must carry tens to hundreds of amperes with low resistance and modest self-heating; it must be bonded without damaging the thin metallisation and the active cells beneath it; and it must survive tens of thousands to millions of thermal and power cycles in service.

Heavy-wire and ribbon bonding uses ultrasonic wedge bonding: a tool presses the conductor onto the pad and vibrates it at ultrasonic frequency, typically at room temperature, so that oxide films break up and the metals form a solid-state weld. This is a different technology from the fine-wire ball bonding (gold, copper, silver or AlSi1 wire of 15–50 µm) used in ICs and LEDs, and from clip and copper-pillar interconnects used in some advanced packages.

Heavy wire vs bonding ribbon

The first decision is geometry. Heavy wire is round and specified by diameter; ribbon is rectangular and specified by width × thickness. A ribbon of the same cross-section as several wires sits flatter, needs fewer bonds and leaves a lower loop, which helps with package height and gel encapsulation.

The table below compares cross-sectional areas, calculated from geometry, to show how many round wires a ribbon replaces. Current capacity also depends on length, cooling and allowed temperature rise, so treat this only as a layout guide.

Round-wire areas: 300 µm = 0.071 mm², 400 µm = 0.126 mm², 500 µm = 0.196 mm².

In practice, layout decides more often than area. A die with a long, narrow source pad may suit two or three ribbons better than eight wires, while a die surrounded by gate pads, sensors and other wires is usually easier to connect with round wire. If you are considering a change from wire to ribbon, check the pad geometry, the clearance to neighbouring bonds and the path to the substrate before comparing material prices.

Aluminium heavy wire and ribbon

Aluminium is the established material for power-device topside bonding. It bonds readily to aluminium die metallisation at low force, it is soft enough not to crack the die, and decades of field data exist for it. Metline supplies aluminium-based heavy wire and ribbon in the following families:

Each wire is characterised by its breaking load, elongation, diameter or section tolerance and surface condition. These properties set the bonding window. Two wires of the same nominal diameter but different hardness or elongation will not bond identically, which is why the supplier datasheet must be compared line by line during a second-source evaluation.

Aluminium vs copper: a system change, not a swap

Copper is increasingly used on new high-performance modules, especially SiC, because of its lower resistivity and higher fatigue resistance. Metline supplies the aluminium route only; the comparison below is to help you decide whether an aluminium design can be kept, or whether a copper-based redesign is justified.

Moving from aluminium to copper is therefore not a same-size conductor substitution. It changes bonding force, ultrasonic power, tool wear, die metallisation and often the substrate finish. For an existing, qualified aluminium product, the lower-risk route is usually to keep aluminium and, if more life is needed, evaluate a reliability-enhanced aluminium wire or ribbon first. SiC does not automatically require copper, and ribbon is not automatically better than wire; the right answer depends on the package.

Checks before changing bonding material

Even a wire of the same alloy and diameter from a different source should be run through a short design of experiments on the production bonder. Small differences in hardness, surface oxide or winding tension can shift the optimum bond parameters.

Initial bond quality is not long-term reliability

A bond that passes pull and shear tests on day one can still fail in the field. Each power cycle heats the die and wire, and because aluminium expands about nine times more than silicon, the bond interface and the wire heel are strained on every cycle. Over thousands of cycles this causes:

• Heel cracking: fatigue cracks at the thinned section where the wire leaves the bond
• Bond lift-off: cracks that grow along the interface between wire and metallisation until the bond detaches
• Metallisation reconstruction: the thin aluminium pad roughens and degrades, increasing resistance
• Wire recrystallisation: grain growth in pure aluminium at high temperature, which speeds crack growth

Only package-level testing under the project’s own conditions can confirm a material change. The base properties of the wire are an input to that test, not a substitute for it.

From engineering sample to approved source

1. Specification reviewCompare the current material datasheet (alloy, diameter or section, breaking load, elongation, spool) with candidate products.
2. Engineering samplesBond samples on your production bonder and tool, with your devices and substrates.
3. Process and mechanical validationEstablish the process window and confirm pull, shear and cross-section results.
4. Reliability validationRun the power-cycling or thermal-cycling tests your product requires.
5. Multi-lot evaluationRepeat on several material lots to confirm lot-to-lot consistency.
6. Customer qualificationRelease under your change-control process, with the agreed certificate of conformance and lot data for each delivery.

What to provide for a quote or sample

• Current product name or datasheet, alloy and diameter, or ribbon width × thickness
• Device or module type (IGBT, MOSFET, SiC, diode; discrete or module)
• Topside metallisation and substrate finish, if known
• Bonder model, wedge tool and spool format
• The goal: second source, supply continuity, cost reduction or new design
• Reliability requirements and any customer standard that applies

For a new platform, evaluation can start with partial information; candidate matching begins first and requirements are added as validation proceeds.

Related bonding and conductor guides

Heavy aluminium (aluminum) bonding wire and ribbon: FAQ