When OEM buyers audit a wiring harness supplier, most conversations orbit around capacity, lead time, and price. But the suppliers that remain stable at scale—and the programs that avoid “mystery failures”—usually share one trait: they treat workmanship as measurable. In harness assembly, the crimp is the most common workmanship “single point of failure,” and the best way to verify crimp geometry beyond surface appearance is microsection testing (also called crimp cross-section analysis).
Microsection testing is not about perfectionism. It’s about risk control. A crimp can look clean and still be wrong inside: strands folded, voids present, improper compression, insulation contamination, barrel deformation, or incomplete conductor fill. Many of these issues won’t show up in continuity tests, and some may not show up in pull tests until the process drifts. Microsection testing makes the internal structure visible so you can qualify a process, validate tooling setup, and catch problems early—before you scale production.
This guide explains microsection testing from an OEM buyer’s point of view: what it is, what it proves, how to request it from a wiring harness supplier, how to read the report, and how to incorporate it into supplier qualification and first-article approval. If you’re preparing an RFQ package, start with Custom Wiring Harness. If you want to anchor verification expectations and evidence language, reference Tests & Inspections along with Quality Guarantee, Quality Policy, and Certificates.
What microsection testing is
Microsection testing is a destructive inspection method. A crimped terminal is cut, mounted in resin, polished, and examined under magnification. The resulting cross-section image reveals the internal geometry of the crimp: how strands are compressed, how the barrel formed, whether there are voids, whether strands are missing or damaged, and whether insulation or debris contaminated the conductor crimp. Done correctly, it is one of the most direct ways to validate that a crimp process is physically sound.
Pull tests and crimp height measurements are still valuable, but they answer different questions. Pull testing is a functional mechanical screen. Crimp height is a dimensional process control. Microsection testing is a structural truth-teller: it shows why a crimp passes or fails and whether a “pass” is robust or marginal.
A practical way to think about it is this: pull tests can tell you that a joint is strong today. Microsections can tell you whether the joint is built correctly in a way that remains stable across time, variation, and scale. For supplier qualification—especially with new terminals, new wire constructions, or new tooling—microsections reduce uncertainty.
When OEM buyers should require microsection testing
Not every harness program needs microsections on every lot. But many programs benefit from microsections at key moments—especially when the cost of a field failure is high or the design is sensitive to variation.
Microsections are most valuable when:
Your program is onboarding a new supplier or transferring production.
A new connector/terminal family is introduced, or you change plating or terminal geometry.
Wire construction changes (strand count, insulation type, OD), even if the gauge stays the same.
Your application sees vibration, repeated motion, thermal cycling, moisture, or harsh chemicals.
You have intermittent failures that are hard to reproduce and basic tests are inconclusive.
You need evidence-based supplier qualification for regulated or high-uptime environments.
If your program is automotive or mobility adjacent, microsections often align with the culture of evidence and long-life reliability. It can be helpful to connect this qualification mindset to Automotive & E-Mobility. If your program is medical-like in documentation expectations, microsection evidence and traceability language often maps well to Medical & Healthcare.
What microsections actually reveal inside a crimp
A microsection image is not just a pretty photo. It is a compact model of your process health. Below are the internal features that matter most for supplier qualification.
Strand fill and compression uniformity
You want to see consistent compression of the conductor strands with proper “fill” of the barrel. If the crimp is under-compressed, the strands may not fully pack, leaving voids and unstable contact. If over-compressed, strands may be cut or excessively deformed. Microsections show both conditions clearly.
Uniformity matters because inconsistency is a sign of unstable tooling setup, unstable stripping, or inconsistent wire construction. In production, instability becomes drift.
Voids, gaps, and internal irregularities
Voids can form from insufficient compression, incorrect wire insertion, or strand distribution issues. Voids are a reliability risk because they reduce contact area and can create micro-movement under vibration. Over time, micro-movement increases resistance and accelerates fretting corrosion.
A supplier that routinely produces void-free, consistent geometry is usually controlling the process well. A supplier whose microsections show voids may still ship harnesses that “pass today,” but they are more likely to drift under real life.
Strand damage, missing strands, and nicks
Strand damage often begins before crimping—during stripping. A crimp may still pass pull if enough strands remain, but the damaged joint can become fatigue-sensitive. Microsections can reveal broken, nicked, or missing strands and help you trace root causes back to stripping method, strip length, or tool wear.
This is one reason wire definition belongs in your BOM discipline. A wire with a different strand count or strand hardness can behave differently under stripping and crimping. To keep terminology consistent across your documentation, align wire language to Cable Wiring Materials.
Insulation contamination in the conductor crimp
If insulation is caught in the conductor crimp, the crimp may be mechanically adequate but electrically unstable. Insulation contamination reduces metallic contact and can create resistance drift with thermal cycling. It is a common root cause of “intermittent” issues in the field.
Microsections make this defect obvious. Visual inspection sometimes misses it, especially when the crimp looks tidy externally.
Barrel deformation and geometry problems
Microsections show whether the barrel formed correctly. Deformation can be caused by incorrect die geometry, incorrect applicator setup, or a mismatch between terminal and wire construction. Barrel cracking, asymmetry, or improper curl shapes are warnings that the process window is narrow.
For OEM qualification, these signals matter because they indicate whether the supplier has a robust process or a process that only works when everything is perfect.
Microsection testing process
OEM buyers don’t need to run microsections themselves, but you should know what a competent supplier workflow looks like so you can evaluate the evidence.
A typical microsection workflow includes:
Selecting representative samples (often multiple crimps from different positions or operators).
Cutting at the correct cross-section location (so the image is comparable).
Mounting the crimp in resin to stabilize it.
Grinding and polishing to a clear finish without smearing metal.
Imaging under controlled magnification and lighting.
Documenting measurements and observations against defined acceptance criteria.
If any of these steps are sloppy, the image can be misleading. Over-polishing can smear copper; poor mounting can tear strands; inconsistent cut location can make a good crimp look bad or vice versa. A mature harness supplier understands this and documents their method. That maturity should align with their broader operational discipline as described on Assembly Capabilities and their quality commitments on Quality Guarantee.
How OEM buyers should request microsection evidence
If you want microsections to strengthen supplier qualification, you must request them in a way that yields comparable evidence across suppliers. “Provide microsections” is not enough. You need to define the intent: which terminals, which wires, which conditions, and what the report must include.
A buyer-friendly request can be simple and still effective. You can require microsections for:
The highest-risk terminals (power circuits, safety circuits, high-vibration circuits).
Any new terminal family or wire construction.
First-article approval or process validation samples.
You can then specify the report deliverables: labeled cross-section images, identification of terminal P/N and wire construction, measured values where relevant, and a pass/fail statement against agreed criteria.
To keep supplier responses consistent, route your requirement through the same submission path as your RFQ package via Custom Wiring Harness and align verification expectations with Tests & Inspections.
What a good microsection report should include
A microsection report is useful only when it’s interpretable. For supplier qualification, the report should be clear enough that a buyer can understand what was tested and what the results mean, even if the buyer isn’t an inspection specialist.
At minimum, a good report should include:
Clear identification of the terminal (manufacturer + part number), wire (gauge + construction), and connector system.
Cross-section images with labels showing which crimp is being evaluated.
Magnification level or scale.
Notes describing key observations (strand fill, voids, strand damage, insulation intrusion).
Any measurements the supplier uses as acceptance criteria.
A pass/fail conclusion tied to the agreed criteria and the correct revision.
If the supplier provides only images without context, that’s not evidence; it’s decoration. If the supplier provides conclusions without images, that’s not evidence either. The value is in the combination: structured context plus visual proof.
Acceptance criteria: how to avoid “opinion-based” qualification
Microsection interpretation becomes messy when acceptance criteria are vague. Buyers sometimes fear specifying criteria because they don’t want to “tell the factory how to do their job.” The trick is to specify outcomes and references, not micromanagement.
A practical approach is to anchor acceptance criteria to terminal manufacturer guidance and internal workmanship standards. You can require that microsections demonstrate consistent conductor compression without strand damage, minimal voids, no insulation contamination, and correct barrel formation, aligned with terminal manufacturer recommendations for the specified wire construction.
If your program uses IPC/WHMA-A-620 style workmanship thinking, you can reflect that mindset without citing obscure clauses. The point is to translate it into observable criteria that the supplier can show. Your internal site pages such as Tests & Inspections can act as the public-facing bridge between “we care about workmanship” and “here is how we verify it.”
Sampling strategy: microsections that actually reduce risk
One microsection sample can confirm that a supplier can produce one good crimp. It cannot confirm that the process is stable. A smarter sampling strategy is to treat microsections as a process validation tool.
For qualification, buyers often request multiple microsections across:
Different crimp positions on the harness (to capture variation).
Different operators or shifts (to capture human variation).
Different tooling setups if changeovers occur.
You do not need huge sample counts. You need representative coverage. A small set of well-chosen samples often reveals whether a supplier’s process is robust or “held together by experience.”
If the program is fast-moving and you want speed, microsection sampling can be limited to first article and major changes. If the program is high-risk, microsection sampling can become a periodic audit. Either way, your supplier should be able to propose a sampling plan that reflects risk, not convenience.
How microsection findings connect to corrective action
Microsections are valuable not only for pass/fail decisions but for diagnosis. When a microsection shows a defect, it often points to a root cause path:
Voids and low compression often point to under-crimping, incorrect crimp height, or wire/terminal mismatch.
Strand damage often points to stripping method, blade wear, or incorrect strip length.
Insulation contamination often points to strip length control or insertion technique.
Asymmetry often points to tooling misalignment or die wear.
A supplier that can map findings to corrective actions is a supplier with process maturity. That maturity is one of the strongest B2B selection signals you can look for because it predicts stability over time.
This is also where buyer trust increases: you’re not just accepting a harness; you’re validating that the supplier can detect and correct process drift.
Common “microsection surprises” in harness supplier audits
OEM buyers are often surprised by how frequently microsections reveal hidden issues even when external appearance is “fine.” A few classic surprises:
The crimp looks neat, but strands are folded and not fully captured.
Pull tests pass, but insulation contamination reduces true metallic contact.
Wire gauge matches, but strand count mismatch changes fill and creates voids.
Tooling produces asymmetric barrel geometry that is not obvious externally.
Insulation support is inconsistent, creating fatigue risk in motion systems.
If your harness will live in a dynamic environment—robotics, automation, motion platforms—these issues matter because micro-movement and repeated flex amplify any marginal joint. That’s why it’s often useful to connect supplier qualification thinking to application hubs like Industrial & Robotics or control-focused pages like Control Wire Harness.
FAQ
What is microsection testing for wiring harness suppliers?
Microsection testing is a destructive crimp inspection method where a crimp is cut and polished to reveal internal geometry. It validates strand compression, voids, insulation contamination, and barrel formation.
Is microsection testing better than pull testing?
They serve different purposes. Pull tests screen mechanical retention; microsections reveal internal structure and process robustness. Many OEM qualification programs use both.
When should OEM buyers require microsections?
During supplier onboarding, first-article approval, new terminal or wire changes, harsh-environment programs, or when investigating intermittent failures.
What should be included in a microsection report?
Terminal P/N and wire construction, labeled cross-section images with scale, key observations, any measurements used, and a pass/fail conclusion tied to agreed criteria.
Can microsections prevent intermittent field failures?
They can reduce risk by exposing hidden defects that pass basic tests. Combined with controlled crimp height and verification, microsections help stabilize long-term performance.
If your program depends on long-life reliability, microsection testing is one of the most efficient ways to qualify a wiring harness supplier’s crimp process. It turns internal geometry from guesswork into visible evidence, so you can scale production with fewer surprises.
To request a quote or qualification review, submit your BOM + drawing + pinout/circuit list through Custom Wiring Harness. If you want an engineering-first review—microsection scope, sample plan, and acceptance criteria—reach out via Contact. We’ll align verification expectations using Tests & Inspections and document a practical evidence package for supplier approval.
