How thick is the copper on the hole wall? Is each inner layer actually joined to the barrel? Are there voids in the dielectric after lamination? Electrical test and visual inspection cannot answer these. The only way is to cut the hole lengthwise, grind and polish it, and look at it under a metallurgical microscope. That is a microsection, also called cross-section analysis.
Its limits are just as clear. A microsection is destructive, and each one shows only the plane that was cut. A passing section proves that those holes passed in that plane. Before reading the result, you need to know where the section was taken and what it contains.
What a microsection can and cannot show
| Area | A microsection shows | A microsection cannot show |
|---|---|---|
| Hole copper | Barrel plating thickness (minimum and average), plating voids, nodules, folds | Continuity of every hole on the board |
| Hole-to-inner-layer joint | Inner-layer separation (ICD), smear residue, etchback or negative etchback | Holes that were not sectioned |
| Surface copper | Wrap plating continuity, total surface copper | Opens and shorts in the circuitry |
| Lamination | Laminate voids, delamination, resin recession (after thermal stress), dielectric thickness | The impedance value itself (needs TDR) |
| Registration | Inner-layer annular ring, layer-to-layer shift | Outline and hole-position tolerance |
| Blind / microvias | Bond between the microvia base and target pad, fill dimple | Microvias elsewhere in the lot |
Drilled hole-wall quality (roughness, nailheading) is also visible in a section. The drilling-stage checks are covered in PCB drilling hole quality inspection.
How a microsection is prepared
Preparation follows IPC-TM-650 2.1.1, in roughly five steps:
- Sampling: cut a small piece containing the target holes from a coupon or board, with enough margin that cutting heat and stress do not damage the area of interest.
- Mounting: encapsulate the sample in resin, usually under vacuum so resin fills the holes and the barrel copper is not dragged during grinding.
- Grinding: grind through progressively finer grits until the plane reaches the hole centerline.
- Polishing: remove grinding scratches, which could otherwise be read as cracks.
- Micro-etching: when needed, a light etch reveals the interfaces and grain structure, so electroless copper, electroplated copper and inner-layer foil can be told apart.
Reaching the centerline is what makes the readings valid. If the plane misses the center, it cuts the barrel at an angle, the copper looks thicker than it is and the hole looks smaller. Check this first when reading any section report.
What to read in a section report
Work through a report or photo in this order:
- Barrel copper thickness: the report should give the minimum, not only the average. Thickness varies between the hole center and the knee, and in high-aspect-ratio holes the center is usually thinnest. See high-layer-count PCB drilling, aspect ratio and throwing power.
- Plating voids: record how many, where, and whether they go through the plating. Wedge voids tend to sit at the junction of barrel and inner-layer copper.
- Inner-layer connection: look for a separation line between each inner pad and the barrel. An ICD can still conduct at room temperature, so electrical test misses it; it opens when heated.
- Wrap copper: confirm that surface copper wraps continuously into the barrel at the hole knee.
- Dielectric and lamination: measure the critical dielectric layers and check for voids or delamination.
- Section plane: both barrel walls should look symmetric and the hole diameter close to the drilled size, which confirms the centerline was reached.
Acceptance is judged against the clauses of IPC-6012 for the specified class (Class 2 or Class 3), with IPC-A-600 as the visual reference. The classes differ on minimum barrel copper, void count and location, and etchback. State the class on the order instead of writing only "microsection required".
Coupon sampling vs. production-board sampling
| Sample | Advantages | Limits |
|---|---|---|
| Test coupon (attached at the panel edge) | Does not destroy a deliverable board; laminated, drilled and plated with the boards; hole sizes and layer structure can be designed in | Sits at the panel edge, where plating distribution may differ from the board center; represents only the holes designed into it |
| Production board | Shows the actual holes in the product | Destroys a deliverable board; shows only the chosen locations |
Coupon structures can follow the test patterns in the IPC-2221 appendix. Two requirements belong in the design input: the coupon should include the smallest hole size and the highest aspect ratio on the board, and if the board has blind vias, buried vias or back-drilling, the coupon should include the same structures. Otherwise a passing coupon says nothing about them.
Production-board sections are normally used for first-article approval, failure analysis, or locations a coupon cannot represent, such as via-in-pad under a BGA.
Before and after thermal stress
An as-plated section hides some defects. IPC-TM-650 2.6.8 defines the thermal stress test for plated through-holes: the sample is baked, floated on molten solder, then sectioned. Comparing sections before and after stress shows:
- whether barrel copper cracks under Z-axis expansion, especially at the hole center and the knee;
- whether inner-layer connections separate;
- whether delamination, blistering or resin recession appears in the dielectric.
Resin recession appears between barrel and dielectric after stress and has to be distinguished from true delamination. How laminate Z-axis expansion loads the barrel is covered in High Tg FR-4: Tg, Td and Z-axis CTE.
When to call for microsections on the order
Specify microsection requirements on the order or drawing when:
- IPC-6012 Class 3 applies, or the product has long-term reliability requirements;
- the board has high-aspect-ratio through-holes, high thickness or a high layer count;
- the build is HDI with blind, buried or stacked vias, or back-drilling;
- the board is heavy copper;
- it is the first article with a new supplier, or the first lot after a laminate or process change.
When the plating stage adds sections on its own, see PCB electroplating quality inspection and release.
How to write it on the order
Turn "microsection required" into an acceptance condition with at least:
- Standard and class: for example, "accept to IPC-6012 Class 3".
- Sample type: coupon, production board, or both; which hole types the coupon must contain.
- Quantity and location: how many per lot or per panel, and whether board center and edge are covered.
- Thermal stress: whether sections are taken after IPC-TM-650 2.6.8 thermal stress.
- Report content: readings and photos for minimum barrel copper, voids, inner-layer connection and dielectric thickness.
- Sample retention: whether mounted sections ship with the goods or are kept on file.
How SKPCB handles it
SKPCB's inspection system includes microsection analysis and copper-thickness measurement. Same-lot test coupons are used for trace width, registration, microsection and thermal stress checks, and special builds and abnormal lots get additional sections. The exact inspection set and sampling level are confirmed per order and product class, and acceptance can follow IPC-6012 Class II or Class III. See Quality control and Multilayer PCB; to agree a section plan for a specific stack-up, send your data through Quote.
Summary
A microsection is the only direct view of barrel copper thickness, inner-layer connection and the inside of the laminate, but it represents only the plane that was cut. Specify the standard class, sample type, thermal stress and report content on the order, and the section result becomes something you can accept against.
FAQ
Q: How does microsectioning differ from X-ray inspection?
A: X-ray is non-destructive and suits solder joints, internal registration and larger voids. A microsection is destructive and measures barrel copper, reveals inner-layer separation and finds small plating defects.
Q: Why should barrel copper be reported as a minimum?
A: Plating thickness varies along the hole wall, and reliability is set by the thinnest point. An average alone can hide a thin hole center.
Q: If the board passes electrical test, is a microsection still needed?
A: It depends on the product class. Inner-layer separation can still conduct at room temperature and pass electrical test; sections, especially after thermal stress, find it.
Q: Does a passing coupon mean the whole board passes?
A: Only for the hole types designed into the coupon. The coupon should contain the smallest hole, the highest aspect ratio and any blind or buried structures on the board.
Q: Is resin recession after thermal stress a reject?
A: Judge it against the clauses of the IPC-6012 class on the order. Resin recession must be distinguished from delamination and barrel cracks.
Q: Does SKPCB section every lot?
A: Same-lot test coupons are used for microsection and thermal stress checks, and special builds and abnormal lots get additional sections. The exact set and sampling are confirmed per order and product class.