Mri-Compatible PCB Materials & Routing: Low-Magnetic Design Rules and Test Plan

Key Takeaways

Magnetic Susceptibility is Critical: The primary goal of MRI-compatible PCB materials routing is minimizing magnetic susceptibility to prevent image artifacts and projectile hazards.
Surface Finish Matters: Standard finishes like HASL or standard ENIG often contain ferromagnetic nickel; Immersion Silver or OSP are preferred alternatives.
Loop Area Reduction: Routing geometry must minimize loop areas to prevent induced currents from the MRI's powerful gradient fields.
Thermal Management: MRI environments lack active air cooling (fans interfere with imaging), requiring passive thermal dissipation strategies in the PCB stackup.
Component Validation: Every resistor, capacitor, and connector must be verified as non-magnetic before the layout phase begins.
Rigorous Testing: Validation requires more than electrical testing; it involves artifact testing and heating tests inside a phantom bore.

What Designing electronics for Magnetic Resonance Imaging (MRI)-compatible PCB design means (scope & boundaries)

Designing electronics for Magnetic Resonance Imaging (MRI) environments requires a fundamental shift from standard PCB design practices. MRI-compatible PCB materials routing is not just about connecting components; it is the discipline of creating circuits that are invisible to the magnetic field while remaining immune to the massive electromagnetic interference generated by the scanner.

The scope of this process extends beyond the board substrate. It encompasses the interaction between the static magnetic field ($B_0$), the gradient fields, and the radio frequency (RF) pulses ($B_1$). A standard PCB placed in an MRI bore can become a dangerous projectile due to ferromagnetic content. Even if mechanically secured, magnetic materials distort the homogeneity of the field, causing "black hole" artifacts in the patient's image.

Furthermore, the routing geometry itself plays a safety role. The MRI gradient coils switch rapidly, creating changing magnetic flux. According to Faraday’s Law of Induction, any conductive loop on your PCB will generate a voltage. If the routing creates large loops, this induced voltage can cause signal corruption, component overheating, or even patient burns. Therefore, MRI-compatible design is a dual challenge: material science (eliminating magnetism) and geometric precision (eliminating induction loops).

At APTPCB (APTPCB PCB Factory), we emphasize that "MRI Conditional" is the industry standard target. This means the device is safe under specific conditions (e.g., 1.5T or 3T fields). Achieving this requires a holistic approach where the laminate, copper, soldermask, legend ink, and surface finish are all scrutinized for magnetic content.

Metrics that matter (how to evaluate quality)

Understanding the scope allows us to define the specific numbers and physical properties that determine if a board will survive and perform in the bore.

Metric	Why it matters	Typical range or influencing factors	How to measure
Magnetic Susceptibility ($\chi$)	Determines how much a material becomes magnetized. High $\chi$ causes severe image artifacts.	Target: $\chi \approx 0$ (diamagnetic or paramagnetic). Copper is -9.6 × 10⁻⁶ (safe). Nickel is +600 (unsafe).	Vibrating Sample Magnetometer (VSM) or Gouy balance.
Dielectric Constant (Dk)	Critical for RF coils. Inconsistent Dk alters the resonant frequency of the coil, degrading image SNR.	Range: 2.2 to 10.0. Must remain stable across the MRI frequency (64MHz for 1.5T, 128MHz for 3T).	IPC-TM-650 2.5.5.5 (Strip-line method).
Loss Tangent (Df)	High loss generates heat and reduces signal strength in receive coils.	Target: < 0.002 for high-performance RF coils.	Resonant Cavity Method.
Induced Voltage ($V_{emf}$)	Caused by gradient switching. High voltage damages sensitive pre-amps.	Dependent on Loop Area ($A$) and Slew Rate ($dB/dt$). $V = -A \times (dB/dt)$.	Simulation (SPICE) or oscilloscope measurement during gradient sequences.
Thermal Conductivity	MRI bores are enclosed spaces. Heat cannot be removed by fans (magnetic motors).	FR4: ~0.3 W/mK. Ceramic/Metal Core: 1.0–3.0+ W/mK.	ASTM D5470 (Steady-state thermal transmission).
Specific Absorption Rate (SAR)	The rate at which RF energy is absorbed by the PCB/tissue.	Limits: < 4 W/kg (Whole body). PCB copper mass affects local SAR hotspots.	FDTD (Finite-Difference Time-Domain) Simulation.

Selection guidance by scenario (trade-offs)

Once you know the metrics, you must apply them to real-world situations where cost, flexibility, and signal integrity often conflict.

1. High-Field RF Receive Coils (3T - 7T)

Scenario: The PCB acts as the antenna receiving the faint NMR signal from the patient. Trade-off: Signal Integrity vs. Cost. Guidance: Standard FR4 is too lossy. You must use PTFE-based or ceramic-filled hydrocarbon laminates (like Rogers PCB). These materials offer low Dk and Df, ensuring the coil stays tuned. Routing Focus: Precise impedance matching is critical. Traces must be extremely smooth to minimize skin effect losses at high frequencies.

2. In-Bore Patient Monitoring (ECG/SpO2)

Scenario: Electronics placed directly on the patient inside the scanner. Trade-off: Safety vs. Size. Guidance: Use High-Tg FR4 to withstand potential heating. The priority here is MRI-compatible PCB materials routing that eliminates loops. Use flexible substrates to conform to the body, reducing the risk of pressure points. Routing Focus: Star-grounding is mandatory. Differential pairs must be tightly coupled to reject common-mode noise from the gradients.

3. Gradient Coil Drivers (Power Electronics)

Scenario: High-power boards located in the equipment room, driving the magnets. Trade-off: Thermal Management vs. Isolation. Guidance: These are not in the bore, so magnetism is less critical, but they handle massive currents. Heavy copper PCBs are required. Routing Focus: Wide traces to handle current. High-voltage isolation gaps (creepage/clearance) are essential to prevent arcing during rapid switching.

4. Implantable Medical Devices (Pacemakers/Neurostimulators)

Scenario: Devices inside the body that must be MRI-conditional. Trade-off: Miniaturization vs. Reliability. Guidance: HDI (High Density Interconnect) technology is required. Use biocompatible materials if the PCB housing is not hermetic. Routing Focus: Extreme miniaturization. Any long trace acts as an antenna that can heat up the lead tip, burning tissue. Routing usually involves specific filtering components at the entry point.

5. Flexible Coil Arrays

Scenario: "Blanket" coils that wrap around a knee or shoulder. Trade-off: Durability vs. Flexibility. Guidance: Flex PCB using Polyimide. Avoid adhesive-based coverlays if possible to reduce dielectric loss. Routing Focus: Hatched ground planes (cross-hatching) instead of solid copper pours. Solid copper creates stiff points and large eddy current loops; hatching retains flexibility and breaks up eddy currents.

6. Intercom and Communication Systems

Scenario: Audio systems allowing the tech to speak to the patient. Trade-off: Audio Clarity vs. RF Noise. Guidance: Standard FR4 is acceptable, but shielding is paramount. Routing Focus: Audio lines must be routed as twisted pairs on the PCB (differential routing) and shielded by ground pours stitched with vias to prevent the MRI RF pulses from rectifying into audible noise.

From design to manufacturing (implementation checkpoints)

Selecting the right scenario is useless if the execution fails during fabrication, so a strict checkpoint system is necessary.

1. Schematic Verification (BOM Scrub)

Recommendation: Review every line item. Risk: A single ferromagnetic capacitor can ruin the board. Acceptance: Supplier datasheets must explicitly state "Non-Magnetic" or "Passivated Copper/Tin termination" (no Nickel barrier).

2. Stackup Design

Recommendation: Balanced copper distribution. Risk: Warpage. In an MRI coil, warpage changes the capacitance and detunes the coil. Acceptance: Symmetrical stackup verified by Impedance Calculator.

3. Surface Finish Selection

Recommendation: Immersion Silver (ImAg) or OSP (Organic Solderability Preservative). Risk: Standard ENIG contains a Nickel layer (phosphorus content varies, but it is magnetic). ENEPIG is also risky. Acceptance: Specify "No Nickel" on the fabrication drawing.

4. Routing Geometry (Loop Check)

Recommendation: Minimize the area between signal and return path. Risk: Large loops = High Induced Voltage = Artifacts. Acceptance: Visual inspection of Gerber files. Ensure ground returns run directly under signal traces.

5. Trace Width and Thickness

Recommendation: Account for skin depth at MRI frequencies (64MHz/128MHz). Risk: Excessive resistance leads to signal loss. Acceptance: Calculate trace width for target impedance and current carrying capacity.

6. Solder Mask and Legend Ink

Recommendation: Use standard LPI mask, but verify pigment composition. Risk: Some black or red pigments contain iron oxide or carbon black (conductive). Acceptance: Use White or Yellow non-conductive inks, or omit silkscreen entirely on sensitive RF areas.

7. Vias and Plating

Recommendation: Copper-filled or resin-filled vias. Risk: Magnetic via barrel plating (rare, but possible in non-standard processes). Acceptance: Certify the plating bath chemistry is 100% Copper.

8. Fabrication Cleaning

Recommendation: Ionic contamination removal. Risk: Residues can become conductive under high RF power. Acceptance: Ionic cleanliness test (ROSE test).

9. Assembly Tooling

Recommendation: Use non-magnetic tweezers and reflow pallets. Risk: Magnetized tools can transfer magnetism to components or damage sensitive parts. Acceptance: Gaussmeter check of assembly line tools.

10. Final De-gaussing (Optional)

Recommendation: De-gauss the finished assembly if minor residual magnetism is suspected. Risk: Ineffective if the material itself is ferromagnetic. Acceptance: Residual field measurement < 0.5 Gauss.

Common mistakes (and the correct approach)

Even with a rigorous checklist, designers often fall into specific traps that compromise MRI-compatible PCB materials routing.

Using Standard ENIG:
- Mistake: Assuming Gold is safe. The underlying Nickel barrier is ferromagnetic.
- Correction: Use Immersion Silver, Immersion Tin, or OSP. If Gold is required for wire bonding, use "Soft Gold" without a Nickel underlayer (direct plating), though this is technically difficult. See PCB Surface Finishes for details on non-magnetic options.
Ignoring Component Terminations:
- Mistake: Buying "Ceramic Capacitors" without checking the leads. Most standard MLCCs use a Nickel barrier to prevent leaching.
- Correction: Source specialized "Non-Magnetic Series" capacitors which use Silver-Palladium or Copper terminations.
Solid Ground Planes in Gradient Fields:
- Mistake: Using a solid copper pour for grounding in a high-gradient zone. This creates massive eddy currents, heating the board and fighting the MRI gradient (Lenz's Law).
- Correction: Use "Hatched" or "Meshed" ground planes to break up large eddy current loops while maintaining electrical continuity.
Right-Angle Routing:
- Mistake: Using 90-degree corners in RF traces.
- Correction: Use 45-degree miters or curved routing. Sharp corners cause impedance discontinuities and can act as emission points for RF noise.
Neglecting Connector Materials:
- Mistake: Designing a perfect board but using a standard D-Sub or USB connector with a steel shell.
- Correction: Specify connectors with brass, beryllium copper, or plastic shells. Use non-magnetic screws (Titanium or Brass).
Overlooking Thermal Expansion:
- Mistake: Ignoring the CTE (Coefficient of Thermal Expansion) mismatch between the PCB and rigid components during MRI operational heating.
- Correction: Use materials with matched CTE or compliant leads to absorb stress.

FAQ

To clarify the nuances of avoiding these mistakes, here are answers to the most frequent questions we receive at APTPCB.

Q: Can I use standard FR4 for MRI PCBs? A: Yes, for digital or low-frequency analog circuits inside the bore, provided the copper cladding and finish are non-magnetic. For high-performance RF receive coils, FR4 is too lossy; use PTFE or ceramic-filled laminates.

Q: Is "Low-Phosphorus" Nickel safe for MRI? A: Generally, no. While high-phosphorus nickel (>10%) is less magnetic, it can still exhibit magnetic properties after thermal cycles (reflow). It is safer to avoid Nickel entirely.

Q: How do I test if my PCB is MRI compatible? A: The gold standard is ASTM F2052 (Force test) and ASTM F2119 (Artifact test). A quick bench test involves suspending the board on a string and bringing a strong rare-earth magnet near it. If it moves, it fails.

Q: What is the best way to route differential pairs for MRI? A: Route them tightly coupled. Any gap between the positive and negative trace creates a loop area that can pick up gradient noise. Twisted pair wiring is preferred for off-board connections.

Q: Can I use vias in MRI coil loops? A: Minimize them. Vias add inductance and resistance, which lowers the Q-factor of the coil. If necessary, ensure they are thoroughly plated and consider filling them.

Q: Does the color of the solder mask matter? A: Yes. Some black pigments use carbon (conductive) or iron oxide. Green, Blue, or White are typically safer, but always verify the ink datasheet.

Q: What is the difference between MRI Safe and MRI Conditional? A: "MRI Safe" means the item is non-conducting, non-metallic, and non-magnetic (e.g., a plastic rod). Almost all PCBs are "MRI Conditional," meaning they are safe only within specific field strengths (e.g., 1.5T or 3T) and usage guidelines.

Q: How does APTPCB handle MRI PCB orders? A: We review the BOM and Gerber files specifically for magnetic risks. We can source non-magnetic laminates and apply specific finishes like OSP or Immersion Silver to ensure compliance.

For further exploration beyond these answers, utilize these resources to refine your design.

Medical PCB Solutions: Deep dive into reliability standards for medical electronics (ISO 13485).
Rogers PCB Materials: Technical data on low-loss laminates essential for RF coils.
Impedance Calculator: Verify your trace widths for controlled impedance routing.
Flex PCB Capabilities: Explore options for conformal coils and wearable MRI tech.

Glossary (key terms)

To ensure we are speaking the same language regarding tools and pages, here are the essential terms.

Term	Definition
Artifact	A distortion in the MRI image caused by magnetic susceptibility mismatch or RF interference.
B0 Field	The main static magnetic field of the MRI scanner (measured in Tesla).
B1 Field	The RF field generated by the transmit coils to excite the protons.
Diamagnetic	Materials that are slightly repelled by a magnetic field (e.g., Copper, Water). Safe for MRI.
Eddy Current	Electric current induced in a conductor by a changing magnetic field. Causes heating and opposes the gradient field.
Ferromagnetic	Materials strongly attracted to magnets (e.g., Iron, Nickel, Cobalt). Dangerous in MRI.
Gradient Coils	Coils that generate spatially varying magnetic fields to localize the signal.
Larmor Frequency	The resonant frequency of protons at a specific B0 field (approx. 42.58 MHz per Tesla).
Paramagnetic	Materials slightly attracted to a magnetic field (e.g., Aluminum, Platinum). Usually acceptable in small amounts.
Phantom	A fluid-filled object used to simulate a human body for testing MRI image quality and SAR.
Q-Factor	Quality factor of a coil; indicates efficiency. Higher Q means better signal-to-noise ratio.
Quench	Sudden loss of superconductivity in the MRI magnet, releasing helium and collapsing the B0 field.
SAR (Specific Absorption Rate)	Measure of RF energy absorbed by the body (Watts/kg).
Susceptibility ($\chi$)	The degree to which a material becomes magnetized in an applied magnetic field.

Conclusion (next steps)

Mastering MRI-compatible PCB materials routing is a prerequisite for entering the high-stakes world of medical imaging. It requires a disciplined approach to excluding ferromagnetic materials—from the nickel in surface finishes to the pigments in the silkscreen—and a geometric strategy that renders the board invisible to gradient induction.

When you are ready to move from prototype to production, your manufacturing partner must understand these unique constraints. A standard PCB fab house might inadvertently substitute a magnetic component or finish, ruining the safety profile of the device.

APTPCB specializes in the rigorous demands of medical electronics. When submitting your design for a DFM review or quote, please provide:

Gerber Files with clear outline and routing paths.
Stackup Specifications indicating specific laminate requirements (e.g., Rogers, Teflon, or High-Tg FR4).
Surface Finish Requirement explicitly stating "Non-Magnetic / No Nickel."
BOM highlighting critical non-magnetic components.
Test Requirements (e.g., Ionic cleanliness levels).

By aligning your design intent with our manufacturing capabilities, we ensure your product is safe, reliable, and ready for the bore.