Microsection Analysis Guide

A microsection analysis is the process of cutting a coupon out of a board, potting it, grinding and polishing it, and looking at the result under a microscope. It is the only method that shows what is inside a plated hole and between the layers, and it is the reason a process problem can be proved rather than suspected.

What a Microsection Shows

The section shows the barrel of a plated hole, the copper on each layer, the resin and the glass, and the interfaces between them.

It also shows the plating thickness at every point around the barrel, which is the measurement that matters most for a press fit hole or a via that carries current.

The section reveals defects that no other method can see: a void in the copper, a crack at the corner of a barrel, a delamination between the layers and a separation at the pad.

It is a destructive test, so it is performed on a coupon or on a sample board rather than on a product.

The section also shows the hole wall, and a rough wall with a wavy profile indicates a worn drill or a feed rate that was too high. That is a process observation that would otherwise require a separate measurement.

How the Sample Is Prepared

A coupon is cut with a saw or an abrasive cutter, and the cut has to be far enough from the area of interest that the cutting damage does not affect it.

The sample is potted in a resin that fills the holes, so that the barrel is supported during grinding and does not smear. A void in a barrel that is not supported is filled with copper debris and disappears.

The sample is then ground and polished in stages, with progressively finer abrasives, until the surface is mirror smooth. Each stage removes the damage from the previous one.

The polishing has to be done in the plane of interest, and for a hole the section must pass through the axis. A section that is slightly off axis shows an oval barrel and gives a thickness reading that is too high on one side and too low on the other.

The polished surface is examined directly, or after a light etch that makes the intermetallic layers and the grain structure visible. The preparation is the largest source of error in the whole method.

Measuring Plating Thickness

The thickness is measured on the barrel wall at several points, usually at the top, the middle and the bottom of the hole. The minimum value is the one that is compared with the specification.

The measurement is made with a calibrated microscope scale or with an image analysis system. The calibration has to be checked with a known standard, because a scale error of a few per cent is enough to turn a pass into a fail.

The thickness at the corner of the barrel is the critical value in most specifications, and it is also the hardest to measure. The corner is where the plating is thinnest and where a crack starts.

The plating thickness requirement should be quoted as a minimum in the barrel, so that the measurement has a defined reference point.

Voids, Cracks and Delamination

A void appears as a dark area inside the copper, and it is the result of an air bubble or an organic inclusion in the deposit. A void in the barrel wall is a stress riser and a path for corrosion.

A crack appears as a dark line, usually at the corner between the barrel and the pad. A barrel crack that starts at the corner and grows around the wall is the classic thermal fatigue failure.

A delamination appears as a separation between two layers, and it is often found along the interface between the resin and the glass or between the copper and the resin.

A pad lifting from the laminate is another form of the same problem, and it is visible in a section as a gap under the copper.

Intermetallic Layers

Where solder meets copper, an intermetallic layer forms at the interface. It is a necessary part of a good joint and a problem when it grows too thick.

The copper grain structure is also visible after the etch, and it is a useful indicator of the plating process. A fine equiaxed grain suggests a well controlled bath, while a columnar or a banded structure suggests a bath that is out of balance and a deposit that may be brittle.

The layer is visible in a section after a light etch, and its thickness is a measure of the thermal history of the joint. A layer that is several microns thick indicates that the joint was at temperature for too long.

A layer that is too thick is brittle, so a joint with a thick intermetallic layer fails under thermal cycling while the same joint with a thin layer survives.

The same effect appears at the interface between a gold finish and the solder, where the gold dissolves into the joint and the joint becomes brittle if the gold layer is thick.

<img src="https://www.gopcba.com/wp-content/uploads/2024/11/ad2ffa604430c189be1ed276092a644.webp" alt="Polished cross section of a multilayer board coupon” />

Acceptance Criteria

The acceptance criteria come from the standard that the product is built to, and they cover the barrel thickness, the voids, the cracks and the delamination.

The criteria are written as limits that can be measured, so the section has to be prepared with a defined magnification and a defined measurement method. A section that is quoted without the method cannot be compared with the standard.

A single section is a sample of one location, so a barrel that passes in one hole does not prove the panel. The number of holes to be sectioned should be part of the inspection plan.

The result should be recorded with the panel and with the coupon, and the quality record should show the measurement rather than a conclusion.

Practical Rules

Prepare the sample properly, with a potted and supported barrel, because the preparation error is larger than the measurement error in most cases.

Measure the thickness at the corner of the barrel as well as the wall, and compare the minimum with the specification.

Look for the defects that only a section can show: a void, a crack, a delamination and a thick intermetallic layer.

Record the magnification, the method and the result, and keep the photograph with the panel documentation for the fabrication record.

Process Control and Verification

On a design of this kind, voids is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. 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.

FAQ

Why is a microsection destructive? Because the board has to be cut and ground to expose the internal structure. It is therefore done on a coupon or on a sample rather than on a product.

What is the most common preparation error? An unsupported or smeared barrel, which fills a void with debris and hides the defect. Potting in a resin before grinding prevents it.

How many holes should be sectioned? Enough to be representative, and the number should be stated in the inspection plan. A single section proves very little about a panel.