SiC Module Signal and Power Terminals: Swiss Machining and Silver Sintering

Introduction to SiC Module Terminals

Terminals are the electrical interface between the SiC power module and the external power circuit. They must carry high currents (50–800 A for power terminals), maintain signal integrity for gate drive signals, withstand extreme thermal cycling (−40°C to +200°C), and provide reliable mechanical connection through hundreds of mating cycles. The manufacturing precision of these terminals directly affects the electrical performance and long-term reliability of the entire power module.

SiC module terminals are divided into two functional categories: power terminals that carry the main load current, and signal terminals (also called control pins or gate pins) that carry gate drive and sense signals. Power terminals are typically larger structures designed for high current and low resistance, while signal terminals are smaller pins optimized for reliable low-current connections. The manufacturing processes for each type differ significantly in their precision requirements, material choices, and assembly methods.

Swiss-Type CNC Machining of Signal Terminals

Signal terminals for SiC modules are precision-machined pins that connect the gate drive signals from the module housing to the DBC substrate bond pads. These pins typically have diameters of 0.5–1.5 mm and lengths of 10–30 mm, with extremely tight dimensional tolerances to ensure proper alignment with both the DBC wire bond pads and the connector system.

Swiss-type automatic lathes are the preferred manufacturing method for signal pins due to their ability to produce complex geometries with tight tolerances in a single operation. The Swiss machining process supports the workpiece close to the cutting tool, eliminating deflection and enabling the production of long, slender pins with exceptional straightness.

The material selection for signal pins is critical for both electrical performance and mechanical reliability. Copper alloys with high electrical conductivity, such as C19210 (1.2% Fe, 0.03% P) or C19400 (2.3% Fe, 0.03% P), are common choices that offer conductivities of 60–90% IACS combined with yield strengths of 250–400 MPa. For signal pins, the combination of adequate conductivity and spring properties for press-fit retention is essential.

Copper Power Terminal Machining

Power terminals for SiC modules must handle currents ranging from 50 A to over 800 A, requiring conductor cross-sections of 10–200 mm² of copper. The manufacturing of these terminals involves a combination of stamping for the basic shape and CNC machining for critical features such as contact surfaces, mounting holes, and alignment features.

The stamping process for copper power terminals uses progressive dies that blank, form, and coin the terminal shape from copper strip. The typical copper thickness for power terminals is 0.5–3.0 mm, depending on the current rating. For high-current terminals, the current density is typically limited to 5–10 A/mm², requiring substantial conductor cross-sections that drive the terminal dimensions.

After stamping, CNC machining is performed on critical surfaces that require tolerances that cannot be achieved through stamping alone. These include the external connection surfaces where busbars attach, the internal connection surfaces for ultrasonic wire bonding, and any threaded or press-fit features. The machining operations are typically performed on a 4-axis or 5-axis CNC mill with the terminal held in a custom fixture.

Press-Fit Terminal Design and Force Control

Many SiC module terminal designs use press-fit technology, where the terminal pin is inserted into a plated through-hole in the PCB or busbar with controlled interference that creates a gas-tight electrical connection without soldering. The press-fit zone of the terminal must be manufactured to precise dimensional tolerances to ensure the correct insertion force and retention force.

The press-fit zone geometry—typically a needle-eye or compliant pin shape—is designed to elastically deform during insertion and maintain contact pressure against the plated hole wall through springback. The critical dimensions include the pin width, the slot width (for compliant designs), and the edge radius at the contact surfaces.

The insertion force must be controlled within the specified range to avoid damaging the PCB plated through-hole (excessive force) or creating an unreliable connection (insufficient force). Process control includes periodic force-displacement monitoring during press-fit assembly to verify consistency.

Silver Sintering for Terminal Attachment

Silver sintering has become the preferred attachment method for SiC module terminals, replacing traditional soldering and welding approaches. Sintered silver joints offer superior thermal and electrical conductivity, higher temperature capability (melting point of silver is 961°C), and significantly better thermal cycling reliability compared to solder joints.

The silver sintering process for terminal attachment uses a silver paste containing micron-sized silver particles (0.5–5 µm diameter) suspended in a solvent system. The paste is applied to the terminal attachment area on the DBC substrate, typically at the thick copper pads reserved for terminal connections. The terminal is placed on the paste, and the assembly is heated under pressure to sinter the silver particles into a dense, porous-free joint.

The sintering parameters for terminal attachment are typically 250–300°C temperature with 5–30 MPa applied pressure for 3–10 minutes. The pressure is essential for achieving high-density (80–95% of bulk silver) joints with low porosity. The resulting silver sinter joint has a thermal conductivity of 200–240 W/m·K and an electrical resistivity of approximately 3 µΩ·cm, both approaching the properties of bulk silver.

Selective Plating for Contact Surfaces

The external contact surfaces of both signal and power terminals require selective application of surface finishes to ensure low contact resistance and corrosion resistance over the module's lifetime. Selective plating processes apply precious metal coatings only to the functional contact areas, minimizing material costs while providing the required surface properties.

For signal terminal contacts, a gold finish of 0.5–1.5 µm over nickel (3–5 µm) is the standard specification. This provides low and stable contact resistance through multiple mating cycles while preventing oxidation of the base copper. The gold is applied selectively to the contact portion of the pin through masked electroplating or brush plating.

For power terminal contact surfaces, the choice of finish depends on the mating connector type. Silver plating (5–10 µm) is common for bolted connections due to its low cost and high conductivity, while tin plating (5–15 µm) is used for press-fit connections. For demanding environments, gold flash (0.1–0.3 µm) over nickel is specified for power contacts requiring extremely low and stable resistance.

Terminal Material Selection Guide

The material selection for SiC module terminals must balance electrical conductivity, mechanical strength, thermal expansion compatibility, and solderability or sinterability. Copper alloys are the preferred material family, with specific alloys selected based on the terminal's current rating and mechanical requirements.

For power terminals carrying more than 100 A, oxygen-free copper (C10200) provides the highest conductivity at 101% IACS and is readily available in strip and rod forms. However, pure copper has relatively low strength and can deform under high clamping forces or during thermal cycling. Precipitation-hardened copper alloys offer higher strength with acceptable conductivity loss.

For terminals requiring nickel or silver sintering attachment, the copper alloy must form a reliable sinter joint without excessive oxide formation. OFHC copper is the preferred choice for silver sintering because its purity ensures consistent sinter bond formation.

Conclusion

The manufacturing of SiC module terminals requires precision across multiple technologies. Swiss-type CNC machining produces the miniature signal pins with tolerances of ±5 µm, while stamped copper power terminals are refined through CNC machining of critical surfaces. Silver sintering provides a high-reliability attachment method capable of withstanding SiC operating temperatures, and selective plating ensures reliable electrical contacts throughout the module's service life.