A microsection answers questions a visual inspection cannot: is the plated copper in the barrel thick enough, is there a void under the solder, did the resin fill between layers, is the intermetallic layer at the joint the right thickness. All of these are through-thickness measurements, so all of them require a cut and a polish.
The difficulty is material contrast. A PCB stacks copper, glass-reinforced epoxy, solder mask and solder — materials with very different hardness in the same field. Preparation that suits copper destroys the resin, and preparation that preserves the resin smears the copper.
Cut through the feature you need to measure, not beside it. For a plated through hole, the cut must pass through the barrel axis; a cut tangent to the hole shows a partial section and produces a false copper thickness. For a solder joint, cut through the joint centreline.
For a board with many holes, cut a strip containing three or four representative holes rather than one — plating thickness varies across a panel, and one hole is not evidence.
Cold-mount in epoxy. Hot mounting presses at 150 °C and above, which can relax the resin, move the copper, and in the case of assembled boards, reflow solder. A vacuum-assisted cold mount is better still on porous laminate because it fills the voids that would otherwise appear as false porosity.
Orient the section vertically in the mould. A tilted section makes every layer read thicker than it is.
Copper is soft and ductile; the surrounding epoxy is not. Aggressive grinding pushes copper across the resin, closing real gaps and turning a thin plating into an apparently thick, continuous layer. Three rules prevent it:
Diamond suspension at 3 µm then 1 µm on a short-nap cloth gives a flat, smear-free surface. For copper thickness measurement, a light etch (ammonia-based or dilute ferric chloride, a few seconds) darkens the copper and sharpens the boundary between plating and laminate.
Do not over-etch. A few seconds too long removes the very interface you are trying to measure and opens the resin around the glass bundles.
| Feature | Measured at | Typical acceptance |
|---|---|---|
| Barrel copper thickness | 200× – 500× | 25 µm minimum for Class 2, per IPC-6012 |
| Surface copper | 200× – 500× | Per drawing; watch for knee thinning |
| Solder void area | 50× – 200× | Usually < 25% of joint area |
| Intermetallic layer | 500× – 1000× | 1 – 4 µm; too thick indicates thermal damage |
| Resin fill / void between layers | 50× – 200× | No void spanning the weave |
| Dielectric thickness | 100× – 200× | Per stack-up drawing |
A darkfield-capable metallographic microscope is the right instrument: darkfield makes the copper-to-resin boundary and fine voids stand out, while brightfield handles layer thickness and grain structure. A 300 mm stage covers a mounted strip of several holes without re-clamping. Add a cut-off saw, a cold mounting kit and a grinder-polisher; a dedicated PCB cutter with a thin diamond blade reduces delamination at the cut edge.
Tell us the material, the feature you need to see and the magnification range. We will recommend a microscope and quote it.
