SiC Module Housing: Injection Molding and Insert Molding Precision

Introduction to SiC Module Housings

The housing of a SiC power module serves multiple critical functions: it provides electrical isolation between the internal circuitry and external environment, positions and retains the signal and power terminals with precision, seals the module interior against moisture and contaminants, and provides structural integrity for handling and mounting. The housing must maintain these functions across a demanding temperature range of −40°C to +200°C while withstanding the mechanical stresses of module assembly and field installation.

The materials and manufacturing processes used for SiC module housings are fundamentally different from those for standard silicon IGBT modules due to the higher operating temperatures of SiC devices. While standard silicone gel encapsulation can tolerate 150–175°C, SiC modules operating at 200–250°C junction temperatures require housing and encapsulation systems that can withstand these elevated conditions without degradation.

Material Selection: PPS vs PBT for High-Temperature Housings

Polyphenylene sulfide (PPS) and polybutylene terephthalate (PBT) are the two most common thermoplastic materials used for SiC module housings. Both offer excellent electrical insulation properties, good dimensional stability, and resistance to chemicals and moisture. However, their thermal performance characteristics differ significantly.

PPS is a high-performance engineering thermoplastic with a continuous service temperature of 220–240°C and a melting point of 285°C. These properties make PPS the preferred choice for SiC module housings that must withstand the elevated temperatures near the ceramic substrate and terminals. PPS also exhibits exceptional chemical resistance, low moisture absorption (0.02–0.05%), and intrinsic flame retardancy (UL94 V-0 without additives).

PBT offers a lower cost alternative with good mechanical properties and a continuous service temperature of 120–140°C. The melting point of PBT is 225°C. For SiC modules with more moderate temperature requirements, or for housings that are positioned farther from the heat-generating devices, PBT provides adequate performance at approximately 30–50% lower material cost than PPS.

Glass fiber reinforcement is universally used in housing materials to improve mechanical strength, dimensional stability, and thermal performance. Typical glass fiber loadings are 30–40% by weight for PPS and 20–30% for PBT. The glass fibers also help match the CTE of the housing material more closely to the terminal materials, reducing stress at the terminal-molding interface during thermal cycling.

Injection Molding Process Control

The injection molding of SiC module housings requires tight process control to achieve the dimensional accuracy and material properties demanded by power module assembly. The key process parameters include melt temperature, mold temperature, injection speed, packing pressure, and cooling time, all of which must be optimized for the specific housing geometry and material.

For PPS with 40% glass fiber reinforcement, the recommended melt temperature range is 310–340°C, with mold temperature maintained at 140–160°C. The high mold temperature is essential for achieving the crystallinity level (typically 50–65%) that provides the optimal balance of mechanical strength, chemical resistance, and dimensional stability. Mold temperatures below 120°C result in amorphous skin layers on the part surface that have inferior mechanical and chemical properties.

The injection speed for PPS housing molds is typically set to 50–150 mm/s, with the speed profile optimized to prevent jetting or flow marks in thin-wall sections. The packing pressure of 60–100 MPa ensures complete cavity filling and minimizes sink marks at the terminal slot features. The cooling time is typically 15–30 seconds for housing wall thicknesses of 1.5–3.0 mm, giving total cycle times of 30–60 seconds per shot.

Injection Mold Design for Housing Precision

The injection mold design is the single most influential factor in achieving the dimensional accuracy of SiC module housings. The mold must produce features with tolerances of ±0.05 mm or better across an operating temperature range of 25–160°C, while withstanding the abrasive effects of glass-filled thermoplastic at high injection pressures.

The mold construction for PPS and PBT housings uses hardened tool steel (H13 or A2) for cavity and core inserts, with surface nitriding or PVD coating to resist abrasive wear from the glass fiber reinforcement. The annual mold wear rate for 40% glass-filled PPS is approximately 0.005–0.015 mm per 100,000 cycles, requiring periodic refurbishment to maintain dimensional specifications.

The gate location and type are selected to minimize weld lines in critical areas such as the terminal slots and sealing surfaces. A fan gate at the center of the housing produces radial flow that fills all cavities simultaneously, reducing the formation of visible weld lines.

Insert Molding of Terminals in the Housing

Insert molding is the process of placing pre-machined terminals into the injection mold cavity and molding the housing material around them. This process creates a permanent, hermetic bond between the terminal and the housing material, eliminating the need for secondary assembly operations while providing the positional accuracy required for automated module assembly.

The terminal positioning within the mold is critical because the finished assembly must align the terminals with the DBC substrate wire bond pads and the external connector system simultaneously. The positional tolerance for signal terminal locations is typically ±0.10 mm in all three axes, with power terminal positions held to ±0.15 mm.

The terminal insertion and retention features are designed into the terminal geometry. Features such as knurling, undercuts, or through-holes in the terminal shank create mechanical interlocking between the metal terminal and the thermoplastic housing. Without these features, the CTE mismatch between metal (16–18 ppm/K for copper) and plastic (15–25 ppm/K for PPS) would cause the terminal-housing interface to fail during thermal cycling.

Terminal Slot Accuracy and Post-Molding Machining

The terminal slots in the housing must provide precise guidance and retention for the terminals while maintaining the electrical creepage and clearance distances required by the module's voltage rating. For 1,200 V SiC modules, the minimum creepage distance between terminals is typically 5–8 mm, depending on the pollution degree and housing material CTI rating.

The as-molded accuracy of terminal slot positions is typically ±0.15 mm, which may not meet the requirements for high-precision assemblies. For demanding applications, post-molding CNC machining of critical slot surfaces can improve positional accuracy to ±0.03 mm. This machining is typically performed on a 3-axis CNC mill with the housing held in a custom vacuum fixture.

Sealing Surface Machining

The sealing surface of the SiC module housing is where the cover or encapsulation system seals the module interior. This surface must be flat and smooth to ensure an effective seal with the silicone gel encapsulation or the rigid cover adhesive bond. The typical flatness requirement for the sealing surface is 0.05 mm over the entire housing perimeter.

Post-molding machining of the sealing surface is almost always required to achieve this flatness. The sealing surface is machined on a CNC mill with a fly cutter or face mill, removing 0.1–0.3 mm of material from the as-molded surface. The machining parameters for PPS are typically 200–400 m/min cutting speed with feed rates of 0.05–0.10 mm/rev for finishing cuts.

Warpage Control During Injection Molding

Warpage is one of the most common defects in injection-molded SiC module housings, particularly for housings made from glass-filled PPS. The anisotropic shrinkage caused by fiber orientation during mold filling creates differential stresses that can distort the housing out of the cavity shape after ejection.

The primary cause of warpage in PPS housings is the orientation of glass fibers in the flow direction during mold filling. Fibers align parallel to the flow direction in the skin layer and perpendicular in the core, creating different shrinkage rates in the flow and cross-flow directions. The shrinkage differential can be 0.2–0.5% between the two directions, sufficient to warp the housing by 0.1–0.3 mm.

Post-mold annealing is the most effective method for correcting warpage in PPS housings. The housing is placed in a fixture that maintains its flatness while being heated to 180°C (below the PPS heat deflection temperature) for 1–2 hours, allowing the internal stresses to relax without causing additional distortion.

Quality Inspection Methods

The inspection of SiC module housings involves dimensional measurement of critical features including terminal positions, slot dimensions, and sealing surface flatness. Coordinate measuring machines (CMM) with touch probes provide the accuracy needed for dimensional verification on sampled parts. For higher-volume production, in-line vision inspection systems with resolutions of 5–10 µm per pixel provide 100% inspection of terminal position and presence.

Conclusion

The manufacturing of SiC module housings through injection molding and insert molding is a precision process requiring careful material selection, mold design, and process control. PPS with glass fiber reinforcement provides the high-temperature performance needed for SiC applications, while terminal overmolding creates a robust assembly without secondary operations. Post-molding machining of the sealing surface ensures the flatness required for reliable module encapsulation, and rigorous dimensional inspection verifies the positional accuracy needed for automated assembly with DBC substrate bond pads.