Microsection Analysis of Plated Holes
What a Microsection Shows
A microsection is a cross section through a plated hole or a via, prepared by encapsulating a sample, grinding and polishing it to a plane, and examining it under a microscope. It shows the copper plating on the barrel wall, the thickness of the surface copper, the interfaces between the layers, the laminate and the glass fibres, and any defect such as a crack, a void, a separation or an incomplete plating. It is a destructive test, so it is performed on a coupon or on a sample, and it is the most direct evidence of the quality of the plating and the lamination.
Preparing the Sample
The preparation decides the result. The sample has to be taken from a position that represents the product, which usually means a coupon on the panel rather than a corner of a production board. It is encapsulated in a resin whose properties match the sample, so that the grinding does not round the edges or pull the copper. The grinding and polishing use progressively finer abrasives, and the final step uses a polish that does not smear the copper. A section that is over heated during the mounting or over polished will show artefacts, such as a smear that looks like a crack or a rounded edge that looks like a thin plating. Where the result is surprising, the preparation should be checked before the process is blamed.
What the Section Reveals
The plating thickness is the primary measurement, taken at several points around the barrel and at the surface, including the knee where the barrel meets the surface pad, which is the most stressed point. The section also reveals the plating’s quality: a ductile, coherent copper against a cracked or brittle one, and the presence of voids or inclusions. The laminate is examined for separation between the layers, for resin recession, for fibre protrusion at the hole wall and for the condition of the glass bundles. The barrel to pad interface is checked for a crack or a separation, which is the classic failure of a plated hole under thermal cycling.
The Knee and the Barrel
The knee of the barrel, where it transitions to the surface, is the location where most failures begin, because the geometry concentrates the strain and the plating is thinner there. A section that shows adequate copper in the middle of the barrel and a thin knee has identified the weak point. The barrel wall’s thickness also varies around the circumference, so a single measurement can be misleading; the standard practice is to measure at several positions and to report the minimum. Where the minimum is close to the specification limit, the process is marginal even if the average looks comfortable.
Voids, Cracks and Separations
The defects that matter are recognisable. A void in the plating is a discontinuity in the copper along the barrel, which reduces the current path and can open under thermal cycling. A crack is a separation in the copper, often at the knee or where the plating crosses a layer boundary. A separation between the copper and the pad, or between the layers of the laminate, indicates a lamination or a plating adhesion problem. Resin recession, where the resin has pulled back from the hole wall leaving the glass fibres exposed, is a common finding and it affects the plating’s adhesion. Each of these has a cause in the drilling, the desmear, the plating or the lamination, and the section is the evidence that points to it.
Coupons and Their Design
The coupon should be designed to represent the product and to test the process. It should contain plated holes of the smallest diameter used on the product, since the small holes are the hardest to plate, and it should include the highest aspect ratio, because the plating’s uniformity depends on the ratio of the board’s thickness to the hole’s diameter. The coupon should be positioned on the panel where the process conditions are representative, and it should be identified so that the section can be traced to the panel and the lot. A coupon that is easy to plate and does not represent the product gives a comfortable result that means very little.
Interpreting and Acting
A microsection result is used for acceptance and for improvement. Where the measurement is within the specification and the structure is sound, the process is accepted for the lot. Where a defect is found, the section should be supplemented with the process data: the drilling parameters, the desmear, the plating current and the lamination cycle. A defect that appears in one location on a section is a local event, while one that appears around the barrel is a process problem. The result should be recorded with the image so that a later comparison can be made, because the trend across lots is often more informative than any single section.
FAQ
What is a microsection for? To measure the plating and examine the structure of a plated hole, via or laminate in cross section.
Why measure at the knee? Because the barrel to pad transition concentrates the strain and the plating is thinner there.
Why measure several points around the barrel? The thickness varies around the circumference, so the minimum matters more than the average.
What defects does it reveal? Thin or voided plating, cracks, barrel to pad separations, laminate separation and resin recession.
Why is a small hole on the coupon important? Small holes and high aspect ratios are the hardest to plate, so they reveal the process limit.
Conclusion
A microsection is the most direct evidence of plating and lamination quality, so prepare the sample carefully, measure the minimum and record the image. Compare across lots. Fabrication quality belongs to PCB manufacturing, the laminate and finish selection is part of PCB capabilities, and the acceptance criteria sit within quality management. Coupon design for a new product is set during prototype PCB assembly in 2026.
