Wire Bonding Technologies: Wedge, Ball, and Ribbon Interconnections

Role of Wire Bonding in Hybrid Assembly

Wire bonding is the primary method for making electrical interconnections between semiconductor dice and the substrate conductor pattern in hybrid microcircuits. Each wire bond must satisfy electrical (low resistance, minimal parasitic), mechanical (survive thermal cycling, vibration, shock), and thermal (compatible with temperature range and CTE mismatch) requirements.

The two dominant wire bonding technologies are thermosonic (ball) bonding using gold wire and ultrasonic (wedge) bonding using aluminum wire. Ribbon bonding, using flat wire profiles, serves power applications requiring higher current-carrying capacity.

Wire Materials

Gold (Au) Wire

Gold wire is the most widely used wire bonding material, particularly for thermosonic ball bonding. Gold offers excellent ductility (easy loop formation without work hardening), oxidation resistance, good electrical conductivity (3rd best after silver and copper), and compatibility with copper and gold pad metallisations. Palladium-doped gold wire (Pd-Au) offers improved grain structure stability and is the standard choice for high-reliability military and aerospace applications.

Aluminum (Al) Wire

Aluminum wire is used for ultrasonic wedge bonding at room temperature or mildly elevated temperatures. Pure aluminum (99.99% Al) and aluminum-1% silicon alloy (Al-1%Si) are the most common grades. Al wire is significantly lower cost than gold but has higher electrical resistivity (1.6x gold) and is incompatible with gold pad metallisations due to purple plague (AuAl2) intermetallic formation at elevated temperatures.

Copper (Cu) Wire

Copper wire offers the lowest resistivity of all common wire bonding materials (1.67 microohm-cm vs. 2.44 for gold) and is significantly lower cost than gold. However, copper is more susceptible to oxidation and requires a forming gas atmosphere during thermosonic ball bonding. Copper wire bonding is increasingly adopted in commercial and automotive applications where cost and conductivity are primary drivers.

Thermosonic Bonding (Au Wire, Ball Bond)

Thermosonic ball bonding uses heat, ultrasonic energy, and force to form a bond between gold wire and the die bond pad. The process sequence: (1) wire is fed through a ceramic capillary; (2) an electronic flame-off (EFO) spark melts the wire tip, forming a spherical ball (1.5-2.5x wire diameter); (3) the ball is brought into contact with the die bond pad at 100-180 degrees C stage temperature with ultrasonic energy (60-120 kHz, 20-80 mW) and force (15-50 grams-force); (4) the capillary moves to the substrate bond pad, forming a stitch (wedge) bond; (5) the wire is torn or clipped.

Ultrasonic Bonding (Al Wire, Wedge Bond)

Ultrasonic wedge bonding uses high-frequency ultrasonic vibration (40-120 kHz) and force without heat to form a bond between aluminum wire and the pad. The wedge tool requires the wire to be oriented at a specific angle, limiting routing flexibility compared to ball bonding. Ultrasonic wedge bonding is performed at room temperature to 75 degrees C, making it suitable for temperature-sensitive assemblies. However, Al wire is incompatible with gold pad metallisations.

Bonding Parameters

Primary wire bonding parameters: Bond force (downward force, 15-200 grams-force depending on wire size and material), Ultrasonic power (amplitude of vibration, typically 20-150 mW for Au ball bonding, 50-300 mW for Al wedge bonding), Bond time (ultrasonic duration, 5-50 ms for ball bonds, 20-100 ms for wedge bonds), and Stage temperature (100-180 degrees C for thermosonic, room temp to 75 degrees C for ultrasonic).

Too little energy produces weak bonds (stitch lift or ball lift); too much energy produces cratering (damage to semiconductor material beneath pad) or excessive ball deformation. Parameter optimisation follows Design of Experiments (DOE) methodology.

Shear and Pull Testing

Wire pull testing (MIL-STD-883 Method 2010) hooks a calibrated tool under the wire at its midpoint and pulls until the wire breaks or the bond lifts. Acceptable failure modes are wire break (good bond strength) and bond lift above minimum specified force. Unacceptable failure modes are bond lift below minimum force and cratering. Minimum pull forces for 1-mil (25 micrometre) gold wire are typically 5-8 grams-force.

Ball shear testing uses a shear tool to push the ball bond off the pad horizontally. Ball shear force is proportional to contact area and interface strength. Ball shear testing detects bond formation problems sensitive to interface quality. MIL-STD-883 Method 2011 covers ball shear as part of bond strength testing.

Failure Modes

Lift-Off

Lift-off occurs when the ball or stitch bond separates from the pad during pull testing or during thermal cycling. Root causes include: pad contamination (organic residue, oxide, moisture), insufficient ultrasonic energy or force (incomplete bond formation), pad metallisation issues (poor adhesion, insufficient thickness), and intermetallic formation problems at the interface.

Heel Crack

Heel cracks are fatigue cracks that initiate at the wire exit point from the stitch or ball bond (the "heel") and propagate through the wire. Heel cracks are caused by cyclic flexural stress at the heel during thermal cycling due to the coefficient of thermal expansion (CTE) mismatch between the die, wire, and substrate. Wire hardness, loop geometry, and bond height all affect heel stress. Fine-pitch wires (below 25 micrometres diameter) are particularly susceptible.

Cratering

Cratering is damage to the semiconductor material beneath the bond pad caused by excessive ultrasonic energy or force during bonding. It ranges from micro-cracking directly under the bond to gross cratering that exposes the underlying semiconductor. Cratering can cause immediate electrical failure or create latent defects that manifest as early field failures. Cratering is detected by decapsulating the device and inspecting the bond pad area under magnification.

Ribbon Bonding for Power Applications

Ribbon bonding uses flat wire (rectangular cross-section) instead of round wire. The rectangular geometry provides: higher current-carrying capacity (wider conductor at the neck of the loop, lower AC resistance at high frequencies), larger contact area (lower resistance at both bonds), and improved thermal dissipation (larger surface area for heat transfer).

Common ribbon dimensions are 75 x 25 micrometres (3 x 1 mil) and 125 x 50 micrometres (5 x 2 mil). Ribbon bonding uses modified thermosonic or ultrasonic equipment with ribbon feed capabilities. The wedge tool geometry is different from round wire wedge bonding. Ribbon bonding is the standard interconnect method for RF power transistors, high-power laser diodes, and automotive power modules.

Process Optimization

Wire bonding process optimisation follows a structured approach: (1) define the wire-pad metallisation combination and the reliability requirements; (2) select the wire type, diameter, and bonding method; (3) develop initial parameters using DOE with pull testing and shear testing as responses; (4) characterise the parameter window (the range of force/power/time that produces acceptable bonds); (5) perform thermal aging and thermal cycling on test vehicles to verify reliability; (6) implement SPC on critical parameters (EFO gap, stage temperature, bond force calibration) for production control.

Cleaning Considerations

Wire bond quality is highly sensitive to surface contamination. Before wire bonding, the substrate and die surfaces must be cleaned to remove: organic contamination (flux residue, handling residue), particles, and moisture. Plasma cleaning (oxygen or argon plasma) immediately before wire bonding is standard practice in high-reliability hybrid assembly. Plasma cleaning removes organic contamination and activates the surface for improved wire bond adhesion. The plasma clean must be performed immediately before bonding -- not hours before -- to avoid recontamination.

Wire Bond Reliability Comparison