Low Magnetic Connector Assembly

Key Takeaways

Definition: A low magnetic connector assembly utilizes materials and plating processes designed to maintain a relative magnetic permeability ($\mu_r$) as close to 1.0 as possible to prevent field distortion.
Critical Metric: The primary success metric is relative permeability, typically required to be below 1.0005 for medical imaging and quantum applications.
Material Hierarchy: Beryllium Copper (BeCu) and Phosphor Bronze are preferred over standard brass; Nickel underplating is the most common cause of magnetic failure.
Process Control: Cold working and standard soldering can induce magnetism; stress relief and specialized soldering techniques are essential.
Validation: Verification requires precise measurement using a permeability meter (Severn gauge) or Vibrating Sample Magnetometer (VSM).
Cost Factor: Expect higher costs due to specialized raw materials (non-magnetic stainless steel or copper alloys) and slower plating processes.
Application Focus: Essential for MRI systems, quantum computing hardware, and sensitive aerospace navigation sensors.

What low magnetic connector assembly really means (scope & boundaries)

To understand the engineering challenges behind a low magnetic connector assembly, we must first define the boundaries of "non-magnetic" in an electronic context. In physics, no material is truly devoid of magnetic properties; everything is either diamagnetic, paramagnetic, or ferromagnetic. In the context of PCB manufacturing at APTPCB (APTPCB PCB Factory), "low magnetic" refers to components and assemblies that exhibit negligible magnetic susceptibility in the presence of strong external magnetic fields.

Standard connectors typically use a brass substrate with a nickel underplate to prevent copper diffusion into the gold or tin finish. Nickel is ferromagnetic. In a high-field environment like an MRI machine (1.5 Tesla to 7 Tesla) or a particle accelerator, that thin layer of nickel can cause three catastrophic issues:

Mechanical Torque: The connector can physically twist or detach due to magnetic attraction.
Field Distortion: The magnetic material distorts the homogeneity of the external field, ruining imaging quality or sensor accuracy.
Passive Intermodulation (PIM): In RF systems, magnetic hysteresis can introduce non-linear signal noise.

Therefore, a true low magnetic assembly replaces ferromagnetic elements with diamagnetic or weak paramagnetic alternatives. This involves using specific copper alloys (like BeCu), eliminating nickel barriers in favor of non-magnetic diffusion barriers (like white bronze or ternary alloy plating), and strictly controlling the manufacturing environment to prevent cross-contamination from steel tools.

Metrics that matter (how to evaluate quality)

Building on the definition of low magnetism, engineers must quantify "negligible" using specific metrics to ensure the final assembly meets system requirements.

Metric	Why it matters	Typical range or influencing factors	How to measure
Relative Permeability ($\mu_r$)	Determines how much the material supports the formation of a magnetic field. This is the primary specification.	Standard: < 1.01 High-End: < 1.0005 Quantum/MRI: < 1.0001	Low-Mu Permeability Indicator (Severn Gauge) or VSM.
Magnetic Remanence (Retentivity)	Measures the magnetism remaining after an external field is removed. High remanence causes long-term drift in sensors.	Should be near zero. Influenced by cold working of the metal.	Gaussmeter or Fluxgate Magnetometer.
Magnetic Susceptibility ($\chi$)	Indicates how responsive a material is to an applied magnetic field.	Positive for paramagnetic; Negative for diamagnetic. Goal is $\chi \approx 0$.	SQUID Magnetometer (for extreme precision).
Plating Thickness	Thicker gold/silver is often needed to compensate for the lack of a hard nickel underplate.	Gold: 0.76µm - 1.27µm (30-50µin) Silver: 2.54µm - 5.08µm	X-Ray Fluorescence (XRF).
Conductivity (IACS)	Non-magnetic alloys often have lower conductivity than pure copper. Affects current rating and signal integrity.	BeCu: 20-50% IACS Phos Bronze: 15% IACS Brass: 28% IACS	Micro-ohmmeter (4-wire measurement).
Insertion/Withdrawal Force	Without nickel, the base metal is softer. Cycles-to-failure may decrease.	Varies by connector size. Influenced by lubricant and contact geometry.	Force gauge / Automated insertion tester.

Selection guidance by scenario (trade-offs)

Once the metrics are defined, the next step is selecting the right low magnetic connector assembly strategy based on the specific operational environment.

1. MRI and Medical Imaging

Requirement: Extreme field homogeneity (static field) and safety (no projectiles).
Trade-off: Must sacrifice mechanical durability for magnetic transparency.
Selection: Use Beryllium Copper contacts with direct Gold plating (no barrier) or non-magnetic White Bronze. Avoid all 300-series stainless steels unless strictly passivated and annealed, as cold working makes them magnetic.

2. Quantum Computing (Cryogenic)

Requirement: Zero magnetic noise at milli-Kelvin temperatures.
Trade-off: Thermal contraction becomes a major issue.
Selection: Requires flux free soldering quantum PCB techniques. Standard solders become superconducting or magnetic at cryogenic temps. Use high-purity Indium or specialized non-magnetic solders. Connectors must match the CTE (Coefficient of Thermal Expansion) of the substrate.

Requirement: Minimal interference with Earth's magnetic field sensing.
Trade-off: Vibration resistance is paramount.
Selection: Phosphor Bronze is often selected for its spring properties to resist vibration, plated with non-magnetic chemical nickel (high phosphorus content >10%) only if strictly controlled, otherwise direct gold.

4. High-Frequency RF/Microwave

Requirement: Low Passive Intermodulation (PIM).
Trade-off: Signal loss vs. magnetic performance.
Selection: Silver plating is preferred over gold for skin-depth conductivity, but silver tarnishes. The assembly must be hermetically sealed or coated. Use non-magnetic brass bodies.

5. Oil & Gas Downhole Logging

Requirement: Directional drilling sensors need magnetic transparency + high heat resistance.
Trade-off: Materials must survive 200°C+.
Selection: High-temperature plastics (PEEK) for insulators combined with thick gold-plated BeCu. Solder alloys must be high-melting-point (HMP) and lead-free/non-magnetic.

6. Laboratory Instrumentation (Hall Effect)

Requirement: Cost-effective but reliable low-mu performance.
Trade-off: Moderate permeability (< 1.01) is acceptable.
Selection: Commercial "non-magnetic" D-Subs or SMA connectors. These are cheaper but may have trace impurities. Acceptable for general lab work but not for primary standards.

From design to manufacturing (implementation checkpoints)

Selecting the right scenario is only the beginning; the execution of a low magnetic connector assembly requires a rigorous manufacturing workflow to prevent accidental magnetization.

Checkpoint	Recommendation	Risk if Ignored	Acceptance Method
1. Bill of Materials (BOM) Scrub	Explicitly specify "Non-Magnetic" for every MPN. Do not rely on generic part numbers.	Receiving standard parts with nickel underplate.	Supplier C of C review + Sample permeability test.
2. PCB Substrate Selection	Choose materials with stable dielectric constants. See PCB Materials for options like Rogers or Teflon.	Dielectric impurities can sometimes harbor magnetic particles.	Material datasheet verification.
3. Trace Layout & Routing	Avoid current loops that generate self-inductance and magnetic fields. Use twisted pair routing logic.	Self-generated fields interfering with the connector's purpose.	Design Rule Check (DRC) & Simulation.
4. Stencil Design	Ensure aperture volume accounts for non-magnetic solder paste rheology (often different from SAC305).	Poor solder joints or tombstoning.	Solder paste inspection (SPI).
5. Solder Paste Selection	Use specific non-magnetic alloys (e.g., Sn96.5/Ag3.5 or Bi58/Sn42) and ensure flux is compatible.	Standard solder often contains trace iron or nickel.	XRF analysis of paste batch.
6. Component Placement	Use ceramic or vacuum nozzles. Avoid magnetized steel tweezers or placement heads.	Transfer of magnetic residue to the component surface.	Visual inspection & Gaussmeter check of tools.
7. Reflow Profile	Tune profile to minimize voiding. Voids can create localized stress which alters magnetic properties in some alloys.	Joint failure or stress-induced magnetism.	X-Ray inspection.
8. Cleaning Process	Aggressive cleaning to remove all flux residues.	Flux residues can become capacitive or inductive over time.	Ionic contamination testing (ROSE).
9. Mechanical Assembly	Use non-magnetic screws (Titanium or Brass). Do not use standard steel screws.	The screw becomes a magnetic dipole, ruining the assembly.	Magnet test on all hardware.
10. Final Permeability Test	Test the entire assembly, not just the connector.	Solder joints or adjacent components may have introduced magnetism.	Severn Gauge (Go/No-Go test).
11. Signal Integrity Check	Verify impedance and loss.	Non-magnetic materials are often more lossy.	TDR (Time Domain Reflectometry).
12. Packaging	Use anti-static, non-magnetic packaging. Avoid staples in bags.	Magnetic debris from packaging contaminating the part.	Visual inspection.

Common mistakes (and the correct approach)

Even with a solid plan, specific pitfalls frequently compromise low magnetic connector assembly projects.

The "Stainless Steel" Myth:
- Mistake: Assuming all stainless steel is non-magnetic.
- Reality: 304 and 316 stainless steels become magnetic after cold working (machining, bending).
- Correction: Use 316L and specify full annealing after machining, or switch to Titanium or Brass.
Hidden Nickel Layers:
- Mistake: Specifying "Gold Plating" without explicitly forbidding the nickel barrier layer.
- Reality: Plating shops default to nickel underplate for durability and corrosion resistance.
- Correction: Specification must read: "Direct Gold over Copper" or "White Bronze (Tri-M3) underplate."
Tooling Contamination:
- Mistake: Using standard magnetized screwdrivers or steel tweezers during assembly.
- Reality: Iron particles transfer to the connector surface, creating "hot spots."
- Correction: Use Beryllium Copper tools, ceramic tweezers, and demagnetize all equipment daily.
Ignoring Solder Joints:
- Mistake: Using a perfect connector but soldering it with standard paste containing trace ferromagnetic impurities.
- Reality: The solder fillet volume is large enough to trigger sensitive detectors.
- Correction: Source certified non-magnetic solder and verify with PCB Manufacturing partners.
Over-Torquing:
- Mistake: Applying standard torque values to brass or BeCu hardware.
- Reality: Non-magnetic alloys are often softer than steel; threads strip or heads shear.
- Correction: Derate torque specifications by 20-40% depending on the alloy.
Neglecting Thermal EMF:
- Mistake: Ignoring the Seebeck effect in low-voltage precision circuits.
- Reality: Dissimilar metals (like Gold on BeCu) generate voltage gradients under temperature change.
- Correction: Design for thermal equilibrium and choose contact materials with low thermal EMF relative to copper.

FAQ

Q: What is the difference between "non-magnetic" and "low magnetic"? A: "Non-magnetic" is a theoretical ideal. "Low magnetic" is an engineering specification, usually defined as having a relative permeability ($\mu_r$) less than 1.0005 or 1.01 depending on the application.

Q: Can I use standard SAC305 solder? A: Generally, yes, as Tin, Silver, and Copper are non-magnetic. However, commercial pastes may contain trace iron contaminants. For critical applications like quantum computing, specialized flux free soldering quantum PCB processes or certified high-purity alloys are recommended.

Q: Why is Beryllium Copper (BeCu) preferred over Brass? A: BeCu offers superior spring memory and fatigue resistance compared to brass, which is crucial since we cannot use a hard nickel underplate to stiffen the contact.

Q: How do I test if a connector is truly low magnetic? A: The quickest field test is using a strong rare-earth magnet to see if there is any attraction. For certification, a Severn Gauge (permeability indicator) is used to measure the specific $\mu_r$ value.

Q: Is low magnetic assembly more expensive? A: Yes. The raw materials (special alloys), the non-standard plating processes (slow deposition gold or white bronze), and the rigorous testing requirements typically increase costs by 30% to 100% compared to standard connectors.

Q: Can APTPCB manufacture PCBs with embedded non-magnetic components? A: Yes, APTPCB specializes in complex assembly requirements. You can submit your design via our Quote page, ensuring you specify the magnetic permeability requirements in the notes.

Q: Does the PCB substrate material affect magnetism? A: Most standard FR4 is non-magnetic. However, some black soldermasks contain carbon or iron-oxide pigments that can be slightly magnetic. It is safer to use clear or green soldermask, or high-frequency materials like Rogers.

Q: What is White Bronze plating? A: It is a copper-tin-zinc alloy (tri-metal) that is non-magnetic, corrosion-resistant, and acts as a good diffusion barrier, replacing nickel.

PCB Materials: Explore substrates suitable for high-frequency and low-loss applications.
PCB Manufacturing: Learn about our assembly capabilities and quality control standards.

Glossary (key terms)

Term	Definition
Permeability ($\mu$)	A measure of the ability of a material to support the formation of a magnetic field within itself.
Relative Permeability ($\mu_r$)	The ratio of a material's permeability to the permeability of free space ($\mu_0$). $\mu_r=1$ is vacuum.
Diamagnetic	Materials (like Copper, Gold, Silver) that create an opposing magnetic field, effectively repelling external fields slightly ($\mu_r < 1$).
Paramagnetic	Materials (like Aluminum, Platinum) that are weakly attracted to magnetic fields ($\mu_r > 1$).
Ferromagnetic	Materials (like Iron, Nickel, Cobalt) that are strongly attracted to magnets and can retain magnetism.
Retentivity	The ability of a material to stay magnetized after the external magnetic field is removed.
BeCu (Beryllium Copper)	A copper alloy with 0.5-3% beryllium, known for high strength and non-magnetic properties.
Passivation	A chemical process for stainless steel that removes free iron from the surface to improve corrosion resistance and reduce surface magnetism.
Severn Gauge	A test instrument used to measure the magnetic permeability of materials by comparing them to calibrated standards.
Diffusion Barrier	A plating layer (usually Nickel, but White Bronze in low-mag apps) preventing the base metal from migrating into the surface finish.
Tesla (T)	The SI unit of magnetic flux density. MRI machines typically operate at 1.5T or 3T.
Cold Working	Deforming metal at room temperature (bending, stamping), which can alter the crystal structure and induce magnetism in stainless steel.

Conclusion (next steps)

Successfully deploying a low magnetic connector assembly requires more than just buying the right part; it demands a holistic approach covering material science, PCB design, and strictly controlled manufacturing hygiene. From avoiding nickel underplates to utilizing flux free soldering quantum PCB techniques for ultra-sensitive environments, every detail counts in reducing magnetic interference.

If you are designing for MRI, aerospace, or quantum applications, do not leave the magnetic properties of your assembly to chance.

Ready to validate your design? When submitting your data to APTPCB for a DFM review or quote, please ensure you provide:

Target Permeability: (e.g., $\mu_r < 1.0005$).
Plating Specifications: Explicitly state "No Nickel" or specify the required barrier layer.
Testing Requirements: Define if 100% testing or batch sampling is required.
Application Context: (e.g., Cryogenic, High-Vibration) to help us suggest the best solder alloys.

Visit our Contact page today to discuss your low magnetic requirements with our engineering team.