90Ω Micro Coaxial Cable for MIPI Interface - Micro Coaxial Cable factory-(FRS)

In high-resolution camera modules, displays, and embedded vision systems, the MIPI (Mobile Industry Processor Interface) has become the de facto standard for camera (CSI-2) and display (DSI) interconnects. As resolutions, frame rates, and channel counts increase, the humble micro coaxial cablehas evolved from a simple wiring component into a critical high-speed interconnect that determines image quality, system stability, and EMI compliance.

This article provides an in-depth look at the role of the 90Ω micro coaxial cablein MIPI applications, its electrical and mechanical characteristics, and practical guidelines for selection and design.

Why 90Ω for MIPI?

MIPI D-PHY and C-PHY specifications define the differential impedance of the transmission line, not a single-ended value. The differential impedance of a 90Ω micro coaxial cable is the correct target for a 100Ω differential MIPI pair.

• Differential Impedance: The impedance measured between the two conductors of a differential pair.
• Single-Ended Impedance: The impedance from one conductor to ground.

For a well-designed differential pair, the relationship is approximately:

Differential Impedance ≈ 2 × Single-Ended Impedance

Thus, a 100Ω differential target is achieved with a 50Ω single-ended impedance. A 90Ω differential cable is effectively a 45Ω single-ended cable, which is a standard and manufacturable value.

Industry Practice: Many MIPI cable and connector vendors specify their products as “90Ω differential” or “100Ω differential” to match the D-PHY standard. The slight difference between 90Ω and 100Ω is often within the tolerance of the overall channel, especially when considering PCB traces and connector transitions. The key is ensuring the entire channel is impedance-controlled and consistent.

MIPI Speeds and the Need for Micro Coax

MIPI specifications define the following per-lane data rates:

• D-PHY: 1.5 Gbps per lane in HS mode (with multi-lane aggregation up to several Gbps total).
• C-PHY: Up to 6 Gbps per lane (using a 3-phase encoding scheme).

At these multi-gigabit speeds, even small impedance mismatches, dielectric loss, or crosstalk can cause reflections and degrade the eye diagram, leading to image artifacts like sparkles, flickering, or frame drops.

Why Micro Coax is Essential:

• Precise Impedance Control: The coaxial structure (center conductor, dielectric, shield, jacket) allows for tight control over single-ended impedance (typically 45–50Ω), ensuring the differential impedance is close to the 90–100Ω target.
• Superior Shielding: Each micro coax pair is individually shielded, drastically reducing crosstalk between adjacent MIPI lanes and protecting against external EMI/RFI.
• Controlled Loss: High-frequency dielectric materials (like FEP or PTFE) minimize insertion loss, which is critical for maintaining signal integrity over longer cable lengths.
• Mechanical Flexibility: With outer diameters as small as 0.3–0.5 mm, micro coax can be routed through tight spaces and bent without significant performance degradation, unlike stiff FPC/ribbon cables.

Anatomy of a 90Ω Micro Coaxial Cable

A typical 90Ω micro coaxial cable consists of four main layers:

1. Center Conductor: Usually silver-plated copper (SPC) or bare copper. A larger diameter lowers resistance but increases capacitance, so a balance is needed for the target impedance.
2. Dielectric Insulation: This layer defines the single-ended impedance. Materials like foamed PTFE or FEP are common for their low dielectric constant (Dk) and low loss. The dielectric thickness is tightly controlled.
3. Shielding Layer: Typically a combination of a thin aluminum foil and a tinned copper braid. This dual-layer shield provides >90% coverage for excellent EMI protection and low radiation.
4. Outer Jacket: A flexible material like PVC or polyurethane that protects the cable from mechanical stress and environmental factors. The choice of jacket affects flexibility, temperature range, and flame retardancy.

Matching the 90Ω Micro Coax to MIPI Connectors

The performance of the entire MIPI channel is only as good as its weakest link. The transition from PCB trace to connector to cable must be seamless.

• Connector Impedance: Choose connectors explicitly specified for 90Ω or 100Ω differential impedance. Popular MIPI micro-coax connectors include I-PEX (e.g., CABLINE® series), Hirose, JAE, and others. Mismatched connectors can cause significant reflections.
• Connector Geometry: The pad size, via stub, and routing on the PCB must be designed to match the connector’s impedance profile. Poor matching at the connector interface is a common cause of signal integrity issues.
• Cable-to-Connector Termination: The shield must be securely grounded to the connector’s backshell, and the center conductor must be precisely crimped or soldered. Inconsistent termination can create impedance discontinuities and skew between channels.

Channel Length, Loss Budget, and Practical Limits

The maximum reliable length of a 90Ω micro coaxial cable for MIPI depends on several factors:

• Data Rate per Lane: Higher speeds (e.g., 4–6 Gbps) have less tolerance for loss.
• Number of Lanes: More lanes increase overall channel loss and crosstalk.
• Cable Construction: Materials and shielding quality directly impact loss and EMI performance.

General Guidelines:

• For 1.5 Gbps D-PHY, a well-designed micro-coax cable can often reach 150 mmwithout significant signal degradation.
• For 2.5–4 Gbps, lengths are typically limited to 50–100 mm.
• Some specialized systems, like certain embedded vision platforms, use high-quality micro-coax to achieve up to 1 meterfor 4-lane MIPI, but this requires careful design and often involves trade-offs.

Loss Budget Analysis: Engineers should perform a channel loss budget that accounts for the PCB trace, connector, and cable losses. If the loss at the target frequency is too high, options include:

• Shortening the cable.
• Reducing the data rate or number of lanes.
• Using a lower-loss cable construction.
• Adding active components like a repeater or retimer.

Micro Coax vs. FPC/Ribbon Cables for MIPI

While FPC/ribbon cables are cheaper and easier to mass-produce, they fall short for high-performance MIPI applications.

🛠️ Design and Manufacturing Considerations

1. PCB Layout: Maintain symmetry in differential pairs and minimize stubs. Ensure the impedance of the PCB traces matches the cable and connector (typically 45–50Ω single-ended).
2. Grounding: Use a solid ground plane near the high-speed signals. Connect the shields of all micro-coax cables to a common ground point on the PCB to avoid ground loops.
3. Channel-to-Channel Matching: For multi-lane MIPI, ensure the electrical length of each lane is matched to within a few millimeters to prevent skew, which can cause color misalignment or image tearing.
4. Manufacturing Quality: The quality of the cable assembly process is paramount. Variations in shielding termination, crimping, and soldering can ruin the impedance match. Work with vendors who have proven experience in high-speed MIPI cable assemblies.

Applications of 90Ω Micro Coax in MIPI Systems

• Mobile Devices: Smartphones, tablets, and AR/VR headsets use micro-coax to connect high-resolution cameras and displays to application processors in ultra-compact spaces.
• Automotive: In-vehicle cameras for ADAS and surround-view systems rely on shielded micro-coax to maintain signal integrity in harsh electrical environments.
• Industrial & Embedded Vision: Machine vision cameras and inspection systems use micro-coax to carry MIPI signals between sensors and processing units, often in robotic or moving assemblies.
• Edge AI & SBCs: Platforms like the NVIDIA Jetson Orin use micro-coax to connect multiple MIPI cameras, where signal integrity is critical for AI inference.

Key Takeaways for Your MIPI Design

When specifying a 90Ω micro coaxial cablefor your MIPI interface, focus on the entire channel, not just the cable itself. Ensure that the PCB, connectors, terminations, and cable are all designed and specified as a system to meet your performance targets for data rate, length, and EMI.