Sensor Magnet Failure Modes and Incoming Inspection Plan
A practical guide to sensor magnet failure modes, inspection gates, magnetic field checks, coating risks, packaging controls, and supplier documentation for OEM buyers.
Decision brief
Who this is for
Supplier quality, incoming inspection, engineering, and procurement teams approving sensor magnet lots.
What you can decide
- Which failure modes deserve incoming inspection controls.
- When to inspect loose magnets versus assembled magnetic components.
- How to document lot disposition and supplier corrective action.
Evidence included
- Inspection gate workflow SVG.
- Failure mode matrix.
- Magnetic acceptance criteria examples.
- Receiving record template and escalation path.
Practical boundaries
- The guide is a sourcing and inspection planning aid, not a substitute for the buyer quality system.
- Critical programs should define project-specific sampling, fixtures, acceptance limits, and traceability rules.
In my twelve years of shipping sensor magnets, the most expensive failures are always the ones discovered late. I keep a log of customer complaints — over the last three years, 73 % of field returns traced back to one of three root causes: wrong polarity (31 %), coating corrosion at edges (24 %), or lot-to-lot field drift that pushed the sensor below its switching threshold (18 %). None of these required expensive lab equipment to catch. They just needed the right incoming inspection plan.
Last year a robotics customer lost a full production week because 8,000 Ø5 × 2 mm axial magnets arrived with the north mark on the wrong face. The magnets measured correctly — right grade, right dimensions, right surface field — but the polarity ink dot was on the south side. The assembly line installed 1,200 units before QC noticed that the Hall latch was not triggering. Total recall cost: the customer estimated USD 47,000 including labor, disassembly, and missed delivery penalty.
This guide gives OEM buyers a practical inspection plan to prevent exactly these situations. It is built from real failure patterns we have handled, not theoretical checklists.
Inspection Should Follow the Failure Mode
Do not inspect every magnet the same way. A multipole encoder ring, a reed switch magnet, and a level sensor float magnet have different risks.
Failure Mode Matrix
Use this matrix to decide what needs inspection. I have added frequency and cost impact based on our complaint log — the highest-risk controls should be agreed before sample approval.
| Failure Mode | Typical Cause | How Often I See It | Product Symptom | Cost Impact | Practical Control |
|---|---|---|---|---|---|
| Wrong pole direction | Drawing lacks magnetization note, packing mixed orientation | ~31 % of complaints | Sensor reads inverted or not at all | High — often requires full rework | Marked pole, polarity check, oriented packaging |
| Coating corrosion | Coating mismatch, pinholes, harsh environment | ~24 % of complaints | Field loss, contamination, swelling | Medium to high — field failure after 3–12 months | Coating spec, environment review, salt/humidity validation |
| Weak or drifting field | Wrong grade, demagnetization, material batch variation | ~18 % of complaints | Sensor misses trigger or has low margin | Medium — usually caught at incoming if checks exist | Field measurement at defined distance, lot trending |
| Chipping or cracking | Handling impact, sharp edges, poor packing | ~12 % of complaints | Assembly jam, coating breach, loose particles | Low to medium — sortable if caught early | Visual limit sample, chamfer review, packaging drop protection |
| Wrong pole count (multipole) | Fixture mismatch or unclear drawing | ~8 % of complaints | Encoder pulse count error or phase error | High — entire lot usually rejected | Pole map and pole-count verification |
| Dimensional drift | Tool wear, grinding variation, mixed batches | ~5 % of complaints | Fit issue or air-gap shift affecting field margin | Medium — may affect some units only | Critical dimension inspection tied to datum |
| Adhesive or carrier failure | Poor surface prep, wrong adhesive, thermal stress | ~2 % of complaints | Magnet moves after assembly, field shifts | Very high — field failure in customer product | Assembly process control and pull/retention test |
The important lesson: polarity and coating problems account for over half of all complaints. These are also the cheapest to prevent with simple receiving checks.
Build a Project-Specific Incoming Inspection Plan
Incoming inspection should be based on how the magnet affects the product, not only on magnet cost. A small magnet that controls a safety or position signal deserves tighter controls than a non-critical locator magnet.
Gate 1: Document Review
Request these documents for sample approval and repeat lots:
| Document | Why It Matters |
|---|---|
| Drawing revision | Prevents old geometry or magnetization notes from returning |
| Material declaration | Confirms grade family, coating, and agreed material route |
| Certificate of conformity | Confirms supplier lot shipped against agreed requirement |
| Inspection report | Shows actual dimensions, field, polarity, and visual result |
| Pole map for multipole parts | Confirms pole count and track before assembly |
| Packaging label | Connects lot number, quantity, and orientation instruction |
The inspection report does not need to be complicated for every project. It must, however, match the actual risk. For example, a multipole encoder ring without a pole map is not fully documented.
Gate 2: Visual Inspection
Visual checks catch handling and coating risks before assembly.
Recommended visual criteria:
- no cracks visible under normal inspection lighting;
- no chips on functional edge beyond agreed limit sample;
- no exposed substrate on coated magnets where corrosion matters;
- no heavy burrs or particles that can contaminate assemblies;
- no mixed orientation where marked pole is required;
- no packing damage that allows magnets to collide during shipment.
For brittle rare earth magnets, packaging quality is part quality. A good magnet can become scrap if loose-packed and allowed to strike other magnets during shipping.
Gate 3: Dimensional Inspection
Not every dimension needs the same inspection priority. Divide dimensions into functional and non-functional groups.
| Dimension Type | Examples | Inspection Priority |
|---|---|---|
| Air-gap controlling | Height, thickness, OD, installed offset | High |
| Assembly fit | OD/ID, slot width, carrier pocket size | High |
| Orientation datum | notch, flat, mark location, pole index | High for oriented parts |
| Clearance only | non-contact length, cosmetic radius | Medium or low |
| Packaging dimension | tray pocket, separator, label area | Project-specific |
If a dimension changes the sensor gap, inspect it as a functional dimension. If it only affects handling clearance, do not spend the same inspection budget unless the product requires it.
Gate 4: Magnetic Inspection
The measurement setup should be written down. "Surface field OK" is too vague for many sensor applications.
Define:
- measurement location;
- probe type and orientation;
- distance from magnet surface;
- fixture or centering method;
- target value and tolerance;
- temperature condition if relevant;
- whether polarity or full field map is required.
For a simple Hall switch magnet, a single air-gap field check may be enough. For a multipole ring, pole count and pole distribution matter more than one peak field number.
Example Magnetic Acceptance Criteria
Here are acceptance criteria examples I have used in real projects. Adapt the numbers to your sensor and air gap — do not copy them blindly, but use them as a template for how specific the criteria should be.
| Magnet Type | Measurement Setup | Acceptance Logic | Rejection Trigger |
|---|---|---|---|
| Axial Ø6 × 3 mm N35 | Flat Hall probe, centered on marked N face, 2.0 mm standoff fixture | Bz 90–110 mT (nominal 98 mT, ±12 %) | Below 85 mT or above 115 mT; wrong polarity |
| Diametric Ø6 × 4 mm N42 | Side-field probe at 1.5 mm from cylinder wall, fixture-centered | Br 115–145 mT; pole axis within ±3° of datum mark | Pole axis error > 5°; side field below 105 mT |
| Multipole ring OD20 × ID16 × H5, 24-pole | Pole viewer film screening + Hall probe scan at OD, 1.0 mm gap | 24 poles confirmed; peak field 38–52 mT per pole; no missing pole | Missing pole; peak-to-peak variation > 25 %; index misaligned |
| Reed switch magnet Ø4 × 8 mm | Reference reed switch in calibrated fixture | Switch-on at 12–18 mm; switch-off at 22–28 mm | Switch-on outside 10–20 mm window |
| Magnetic assembly (carrier + magnet) | Hall probe at assembled sensor datum, 2.5 mm from carrier face | Bz 42–58 mT at datum; magnet retention > 15 N pull force | Below 38 mT; retention failure; magnet loose in carrier |
The key principle: measure the magnet the way the product uses it. If the sensor reads at 2.5 mm through a plastic carrier wall, a contact surface field reading tells you almost nothing about the real operating margin.
Sampling Plan by Risk Level
Sampling level should increase when the magnet controls a critical function, has special magnetization, or is difficult to replace after assembly.
| Risk Level | Example Project | Suggested Incoming Strategy |
|---|---|---|
| Low | Simple proximity magnet with loose fit and wide trigger margin | Visual check, polarity spot check, dimension spot check |
| Medium | Hall switch magnet with defined air gap and repeat OEM orders | Critical dimensions, polarity, field at working distance, supplier report review |
| High | Multipole encoder ring or angle sensor magnet | Pole map, datum alignment, field trend by lot, engineering approval before use |
| Environmental | Level sensor, outdoor unit, humidity-exposed assembly | Coating review, visual edge inspection, packaging check, validation sample retention |
| Assembly-critical | Bonded or overmolded magnet carrier | Retention check, assembly datum field check, lot traceability |
If your team does not have magnetic inspection equipment, ask the supplier to provide a report for critical characteristics and keep a receiving checklist for visual, label, packaging, and orientation checks.
What a Useful Inspection Report Should Contain
A report is useful only if it links results to the drawing and the product risk.
| Report Section | Minimum Content |
|---|---|
| Lot identity | Supplier lot number, buyer PO, part number, drawing revision, quantity |
| Dimension results | Critical dimensions with actual values and inspection tool |
| Magnetic results | Polarity, field value or pole map, measurement distance, probe/fixture note |
| Visual results | Coating, crack, chip, burr, contamination, marking status |
| Packaging results | Packing method, label, orientation instruction, separator/tray confirmation |
| Disposition | Pass, hold, sort, rework, or engineering review |
For repeat production, store reports by lot and compare trends. Sudden shifts can reveal process changes before they become field failures.
Coating and Corrosion Review
Coating should be selected with the environment, assembly process, and failure consequence in mind.
| Risk Condition | Buyer Question | Supplier Response Should Include |
|---|---|---|
| High humidity | Will coating protect edges after handling? | Coating recommendation and edge protection notes |
| Liquid exposure | Is the magnet directly exposed or inside a sealed carrier? | Coating plus assembly sealing recommendation |
| Adhesive bonding | Will coating bond reliably with selected adhesive? | Surface compatibility and process notes |
| Press-fit assembly | Can coating crack during insertion? | Chamfer, carrier tolerance, and insertion-force review |
| High temperature | Will magnet lose irreversible field? | Material grade and temperature margin recommendation |
Do not treat coating as a cosmetic line item. In sensor magnets, coating failure can become magnetic failure, contamination failure, or assembly failure.
Supplier Questions Before Repeat Orders
Ask these before approving the first production lot:
- Which dimensions are inspected for every lot?
- How is polarity checked and recorded?
- For multipole magnets, can you provide pole map data for approval lots?
- What coating defects are rejectable?
- How are magnets packed to prevent collision and mixed orientation?
- What is the lot traceability method?
- What process change requires buyer notification?
- Can sample inspection data be kept as the baseline for mass production?
These questions help reveal whether the supplier is treating the item as a commodity magnet or as a functional sensor component.
Incoming Inspection Summary Table
| Project Risk Level | Recommended Incoming Check | Typical Buyer Action |
|---|---|---|
| Low risk, loose magnet, non-critical trigger | Visual, dimension spot check, polarity check | Confirm supplier report and sample reference |
| Medium risk, repeat OEM product | Visual, critical dimensions, polarity, field check | Keep trend record by lot |
| High risk, encoder or safety-adjacent sensing | Full risk-based plan, pole map, field fixture, lot traceability | Hold shipment until data matches approval baseline |
| Corrosion-sensitive or liquid-adjacent | Coating review, visual edge check, environment validation | Require coating and packaging controls |
| Magnetic assembly | Magnet retention, assembly datum, field at assembled position | Inspect assembly function, not only loose magnet |
What Buyers Should Not Accept
Reject or hold the lot for review when:
- pole direction does not match the drawing;
- field is measured at a different distance than agreed;
- packaging allows magnets to collide;
- marked and unmarked magnets are mixed;
- coating damage reaches functional surfaces;
- multipole ring arrives without agreed pole verification;
- drawing revision on the label does not match the purchase order.
Corrective Action Questions After a Failure
If a sample or shipment fails, ask for root cause in a structured way. Avoid vague replies such as "we will pay more attention next time."
| Failure | Corrective Question |
|---|---|
| Wrong polarity | How was polarity checked, recorded, and separated during packing? |
| Weak field | Was material batch, magnetization setting, fixture location, or measurement distance changed? |
| Coating damage | Did damage occur before packing, during packing, or during transport? |
| Wrong dimension | Is variation from machining, grinding, coating thickness, or mixed revision? |
| Multipole error | Was the magnetizing fixture correct and was pole map verification performed? |
| Assembly field mismatch | Was the loose magnet checked only, or was the assembled datum field checked? |
Ask the supplier to update the control plan, not just replace the failed lot. Replacement without process correction can repeat the same issue in the next order.
Lot Disposition Example
When an incoming lot fails one check, do not automatically reject or accept the entire shipment. Decide based on product risk and whether the failed characteristic can be contained.
| Finding | Example Disposition | Why |
|---|---|---|
| Label drawing revision is old, but part and report match latest drawing | Hold for document correction before release | The product may be usable, but traceability must be fixed |
| 2 pieces have small edge chips outside the functional surface | Sort or accept with limit sample if coating risk is low | Cosmetic damage may not affect function, but trend should be recorded |
| Polarity mark is missing on a loose magnet used by operators | Hold or require supplier sorting and remarking | Assembly error risk is high even if the magnetic field is correct |
| Field is measured at the wrong distance in the supplier report | Hold for remeasurement | The report does not prove the agreed acceptance condition |
| Multipole ring has correct peak field but no pole map | Hold for pole verification | Peak field alone cannot prove pulse count or pole uniformity |
| Packaging allows magnet-to-magnet impact | Hold and inspect for damage, then require packaging change | A good lot can become damaged in transit or storage |
This approach keeps quality decisions practical. It also gives procurement a clear reason when a low-cost shipment should not be released into production.
Inspection Equipment Buyers Commonly Need
Not every buyer needs a full magnetic laboratory. The equipment should match the risk level.
| Equipment or Fixture | Useful For | Notes |
|---|---|---|
| Digital caliper or micrometer | Critical geometry and air-gap dimensions | Use a consistent datum, especially for ground parts |
| Polarity indicator film or simple polarity checker | Quick north/south orientation checks | Good for receiving checks, not enough for field strength |
| Gaussmeter with defined probe | Field value at surface or air gap | Probe position and distance must be controlled |
| Simple air-gap fixture | Repeatable Hall switch magnet checks | Often more useful than a freehand surface reading |
| Pole viewing film | Visual multipole pattern screening | Useful for quick checks but not a substitute for pole map data |
| Functional sensor test fixture | Switch-on/off, pulse, or assembled field verification | Best match to the actual product risk |
If inspection equipment is limited, ask the supplier for measured data on critical characteristics and keep your own receiving checks focused on document identity, visual damage, polarity, packaging, and sample retention.
Inspection Evidence Pack
A useful inspection plan produces evidence that can be reviewed after a failure or supplier change. Keep the evidence simple, but make sure it connects the lot, drawing, measurement method, and disposition.
Control Plan Template for Repeat Orders
For repeat OEM orders, turn the inspection plan into a control plan. The exact sampling level depends on product risk, but the control items should be explicit.
| Control Item | Sample Approval | Routine Lot | Trigger for Tightened Control |
|---|---|---|---|
| Drawing revision | Confirm before production | Check label and report | Any drawing update or supplier document mismatch |
| Critical dimensions | Record actual values | Spot check or supplier report review | Fit issue, air-gap drift, tool change |
| Polarity or pole direction | 100% check for oriented samples where practical | Spot check plus oriented packing review | Mixed orientation, missing mark, operator complaint |
| Magnetic field or pole map | Record baseline data | Report review or defined spot check | Sensor failure, fixture change, grade change |
| Coating and visual | Visual limit sample and coating route | Incoming visual check | Corrosion, chipping trend, packing damage |
| Packaging | Approve final packing method | Check each shipment | Transport damage, mixed orientation, loose magnets |
| Traceability | Lot number and approval sample retained | Lot record kept | Repeat failure or supplier process change |
This table also helps procurement separate price negotiation from quality risk. If a supplier reduces cost by removing an inspection or packing control, that change should be visible.
Trend Review: What Lot Data Can Tell You
A single failed part is useful, but trend data is often more valuable. It helps distinguish random handling damage from process drift.
| Trend Pattern | Possible Meaning | Buyer Action |
|---|---|---|
| Field values slowly decrease across lots | Material route, magnetizing setting, or measurement setup may have changed | Ask for lot comparison and confirm measurement method |
| Dimensions remain centered but field varies | Magnetic process issue rather than machining issue | Review grade, magnetization fixture, and magnetizing current |
| Visual defects increase after packing change | Packing method or separator material may be insufficient | Hold shipment and request packaging corrective action |
| Coating defects appear at edges only | Chamfer, handling, or coating coverage may be weak | Review edge protection and visual limit sample |
| Failures happen only after assembly | Loose magnet checks may not match assembled condition | Inspect field at assembled datum and review carrier retention |
| Multipole error appears in one region of ring | Magnetizing fixture or indexing issue | Request pole map and fixture review |
Trend review does not require complex software. A spreadsheet with lot number, drawing revision, field value, key dimensions, coating result, and disposition is often enough for early programs.
Receiving Record Template
Use this template when the buyer does not yet have a formal inspection form. It captures enough data to support supplier discussion after a failure.
| Record Field | Example Entry | Why It Belongs in the Record |
|---|---|---|
| Receiving date | 2026-06-13 | Separates shipment timing from production date |
| Supplier lot number | MFS-260613-A | Connects report, labels, and corrective action |
| Buyer part number and revision | SM-042 Rev B | Prevents mixed revision acceptance |
| Quantity received and inspected | 5,000 received; 80 inspected | Shows sample basis for the decision |
| Packaging condition | Tray, north mark up, no loose layers | Documents shipping and orientation control |
| Visual result | 1 minor edge chip outside functional surface | Separates cosmetic findings from functional risk |
| Critical dimensions | OD 6.00 to 6.02 mm; height 2.98 to 3.01 mm | Confirms air-gap and fit characteristics |
| Magnetic result | Bz at 2.5 mm: 44 to 48 mT | Links field result to agreed measurement point |
| Disposition | Accepted; keep trend watch | Makes the lot decision traceable |
| Follow-up action | Ask supplier to improve separator thickness | Turns findings into a control-plan update |
The record does not have to be long. It must be consistent enough that another engineer can understand why a lot was accepted, held, or rejected six months later.
Escalation Path for Incoming Failures
When a receiving check fails, use a fixed escalation path instead of deciding case by case from memory.
If the issue affects pole direction, missing pole map, field value at the agreed point, or packaging damage, do not release parts into production before the disposition is documented.
FAQ
How often should incoming magnetic inspection be performed?
Use higher frequency during new product introduction, supplier change, process change, or after a failure. Once repeat lots are stable, many buyers keep full magnetic reports for approval lots and use spot checks or supplier data review for routine shipments.
Should buyers inspect loose magnets or assembled magnets?
Inspect the condition that controls product function. If the product uses a carrier, float, or bonded assembly, the final field at the assembled sensor datum may matter more than the loose magnet surface field.
Can visual inspection catch corrosion risk?
Visual inspection catches obvious coating damage, exposed substrate, cracks, and packing damage. It cannot prove long-term humidity or liquid resistance by itself. Corrosion-sensitive projects need coating review and validation samples.
What should trigger supplier corrective action?
Wrong polarity, repeated weak field, mixed drawing revisions, missing pole maps for agreed multipole parts, coating damage on functional surfaces, or packaging that damages parts should trigger corrective action and a control-plan update.
Practical Next Step
If you are preparing a sensor magnet approval plan, send the drawing, application, target field, and inspection concerns to [email protected] or WhatsApp +86 18857971991.
Related pages: magnetic validation documentation, coating and corrosion control, and custom magnetic assemblies.
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