Microsection Sample Preparation and Reading Quality
A microsection is the only way to see inside a plated hole, and it is also one of the easiest places to produce a false result. The section that is examined is a slice a few micrometres thick, and what it shows depends on where the cut was made, how the sample was ground and polished, and how the image is read. This article covers the whole chain, from selecting the sample to interpreting the measurement, and the artefacts that appear when a step is rushed.
What a Microsection Is Asked to Show
A section answers a specific question: the copper thickness in the barrel, the presence of a crack or a void, the interface between the plating and the laminate, the integrity of an inner layer connection or the shape of a solder joint. The question determines where the cut is made and at what magnification it is examined.
Because the section is destructive and slow, it is normally used for qualification, for process verification and for a failure investigation. Treating it as a routine inspection tool on every lot is expensive and unnecessary when non destructive methods can carry the daily load.
Sample Selection and Location
The hole that is sectioned has to be representative, and the least plated hole in the batch is the one that matters. Plating thickness falls towards the centre of a large panel and in the holes with the highest aspect ratio, so a sample taken near the edge of the panel usually looks better than the worst case in the lot.
The location should also be recorded. A section through a single hole tells you about that hole; a section that follows the plating distribution across the panel tells you about the process. Our aspect ratio guide explains how the depth to diameter relationship drives that distribution.
Mounting and Orientation
The sample is mounted in a casting resin before grinding, and the mounting has to hold the edge being examined flat and supported. A poorly supported hole can round over during polishing, and the copper then appears thinner and the wall appears curved.
Orientation matters as much. A section through the centre of a hole shows the true barrel, while an off centre cut shows a chord that appears thicker and can hide a defect on the opposite side. For a crack investigation, the plane of the cut should be across the suspected crack direction.
Grinding and Polishing Sequence
Preparation steps down through progressively finer abrasives, and each step has to remove the damage introduced by the previous one. Skipping a step leaves scratches and deformed metal that appear as artefacts in the final image, and the deformation can smear copper across a genuine gap.
The final polish is usually done with a fine suspension on a soft cloth, and the time and the load matter as much as the abrasive. A section that is polished too aggressively develops relief, where the soft resin wears away faster than the metal, and the edges of the copper become rounded.
Smear, Pull-Out and Preparation Artifacts
Resin smear is the most common artefact in a plated hole section. Polishing pushes softened resin across the wall, and if the smear is not removed before plating it becomes a separation between the copper and the laminate. In a finished board, the section can also create smear that did not exist, which is why an unetched comparison is useful.
Pull-out is the loss of a fibre or a particle during polishing, leaving a cavity that looks like a void. Scratching, rounding, relief and contamination from a dirty cloth are the others. The rule is that a defect seen in only one section should be confirmed before a lot is rejected on it.
Etching for Structure
Etching the polished surface reveals the grain structure and the interfaces, which is useful for examining the plating and the intermetallic layers. The etch has to be matched to the material and controlled in time, because an over etched surface loses the fine detail and an under etched one shows nothing.
Etching also changes the apparent thickness at the edges, so the thickness measurement should be taken either before etching or on a matching unetched sample. Our hole copper notes describe what the measurement is meant to capture in the barrel.
Measurement Practice and Calibration
Thickness is measured on the polished face with a calibrated scale, either an eyepiece reticle or an image analysis system. The calibration should be verified against a stage micrometer before a series of measurements, and the magnification should be recorded because it sets the uncertainty.
The measurement position should also follow a rule: a minimum thickness reading should be the minimum in the barrel, while an average should state how many points were used and where. A single reading from the thickest part of the wall is a number rather than a result. Our plating thickness guide covers how the acceptance figures are defined.
Reading the Result Without Over-Reading It
A section shows one plane through one hole, and the temptation is to generalise from it. A crack that appears in a single section may be a preparation artefact, while a wall that looks uniform may hide a defect on another plane. The result should be reported with the sample size and the location.
The interpretation should also separate the process question from the product question. A thin wall is a process finding, while a crack that will grow in service is a reliability finding, and they lead to different actions. Our quality guide describes how a section finding is classified at gopcb, and our hole types notes describe the different barrels a section may contain.
Documentation and Repeatability
Every section should be recorded with the sample identification, the location, the mounting orientation, the preparation sequence, the magnification and the measurement. That record is what allows a second laboratory to reproduce the result, and it is what makes a comparison between two builds meaningful.
The most valuable use of the technique is over time. A series of sections taken the same way on successive lots shows the drift in the process before it produces a reject, which is a far better return than the occasional emergency section after a failure.
Additional Considerations for This Build
Practical attention to sample preparation pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating sample preparation explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, sample preparation is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
FAQ
How many holes should be sectioned? Enough to represent the distribution, which usually means several holes across the panel rather than a single worst case. The number should be stated in the procedure rather than chosen at the time.
Can a void be confirmed from one section? Not reliably, because an off centre cut can create the appearance of a void. Confirming with a second cut or with X-ray is the standard practice before a decision is made.
Does etching change the thickness measurement? It can remove material at the surface and round the edges, so the measurement should be taken on an unetched face or on a matched sample prepared without etching.
