Barrel Cracks in Plated Holes: Causes and Detection

A barrel crack is a fracture in the copper that lines a plated through hole, running around the barrel or along its length, and it is the most common way a plated hole fails. The crack may be present when the board leaves the fabricator or it may open later under thermal cycling, and the two cases have different causes and a different response.

What a Barrel Crack Is

The barrel is a thin copper tube, typically twenty to twenty five microns thick, that connects the pads on different layers. When the board expands and contracts, the laminate moves much more than the copper does, so the barrel is stretched and compressed with every temperature change it sees.

If the copper can deform plastically, it survives; if it cannot, it fractures. A crack that runs around the barrel separates the layers it was meant to connect, and a crack that runs vertically tears the wall, and both produce an intermittent connection before they produce a permanent one. The resistance change while the crack is narrow is small, so a board can pass every test and still be a reliability risk.

Where the Crack Starts

Cracks usually begin where the barrel meets the surface pad or an inner layer land, because that is where the geometry changes and the stress concentrates. A sharp corner at the knee of the hole, a rough wall or a thin deposit at the transition point all give the crack a place to start. Barrel cracks that begin at the surface are often found first at the corner of a pad rather than in the middle of the hole.

Inside the hole the copper is thinnest at the middle of the barrel on a thick board, because plating throws more material at the surface than into the centre. That thin middle section is where a vertical crack appears on a high aspect ratio hole.

Plating Ductility and Chemistry

The mechanical property that matters most is plating ductility, which describes how far the copper can stretch before it breaks. A ductile deposit accommodates the movement of the laminate, while a brittle one fractures even under modest cycling.

Ductility is set by the plating chemistry. High concentrations of organic additives, excessive brightener and contamination from the resist or the mask all produce a harder, more brittle deposit, and the effect is invisible on a thickness measurement. Our plating thickness notes describe how the deposit is built. A bath that is out of balance produces a deposit that measures correctly and behaves badly, which is the difficult combination.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/PTH-copper-plating7-1.png" alt="Microsection showing a barrel crack in a plated through hole” />

Thermal Stress in the Press and the Oven

The first thermal stress a plated hole meets is the lamination cycle, where the panel is pressed at a temperature high enough to soften the resin. A barrel that is already marginal can crack before the board is even drilled, imaged or tested for the final time.

Assembly adds a second cycle at a higher temperature, and rework adds more. A board that survives one reflow may fail the second, which is why the number of thermal cycles allowed for a panel is usually limited and recorded rather than left to chance. Tracking those cycles is a discipline that is easiest to establish at the start of a project.

Aspect Ratio and Hole Geometry

Aspect ratio, the ratio of board thickness to hole diameter, determines how difficult the barrel is to plate and how much movement it has to absorb. A tall thin hole is harder to plate evenly and concentrates more strain in the middle of the barrel.

The practical limit depends on the plating process and on the reliability class required. Where the design calls for a thick board and a small hole, the plating has to be qualified on a coupon rather than assumed, and the drill programme may need to be adjusted to keep the wall smooth. Our aspect ratio guide sets out the limits. Where the design sits close to that limit, relaxing the hole size is often the cheapest change available to the designer.

Microsection and Detection

Detection requires a microsection, because a cracked barrel is not visible from the surface and often passes electrical test while the crack is still narrow. The section is taken through a sample of holes from each panel and examined under magnification for cracks, voids and thin plating.

A thermal stress test is used to make latent cracks visible. The sample is floated on solder or cycled in an oven and then sectioned, and the resulting inspection shows whether the plating can survive the assembly process. Our test coupon notes describe how that sample is organised. The coupon should represent the smallest hole and the thickest section of the product panel, because those are the hardest features to plate.

Assembly Effects and Field Failures

At assembly, a cracked barrel shows up as an intermittent fault that changes when the board is flexed or heated. It can also appear as a hole that fails in-circuit test on one pass and passes on the next, which is the classic signature of a partially open connection.

In the field the crack opens slowly under power cycling, and the failure looks like a component that has stopped working rather than a board defect. That is why barrels are one of the first places to look when a product fails after a period of service. Our hole copper notes describe the copper that carries the current. A section taken from a failed unit usually settles the question in minutes, provided the board has not been stripped of its components first.

Process Controls That Prevent Cracks

Prevention starts with the drilling. A rough or smeared wall gives a poor surface for the plating to bond to, so desmear, hole cleaning and drill parameter control all contribute to the result even though they are not plating steps.

The plating itself is controlled by chemistry analysis, current density and the additive balance, and the results are verified by a ductility test on a coupon. A deposit that is thick but brittle is worse than a thinner ductile one, which is why the specification sets a ductility requirement rather than a thickness alone. Ductility is measured on a foil sample taken from the plating bath rather than on the finished board.

Specifying and Accepting Plated Holes

The drawing should state the required copper thickness in the barrel, the acceptance class for the hole, and the thermal stress or ductility evidence expected with the lot. Without that, the fabricator supplies a deposit that meets a dimensional requirement and may not meet the mechanical one.

Acceptance should be based on a section from each lot rather than on a certificate alone, and it should record what was found rather than simply that the lot passed. That record is what allows a field failure to be investigated years later, and it is what distinguishes a controlled process from a lucky one. Repeating the section at intervals through a long run shows whether the process is stable or slowly drifting.

FAQ

Can a cracked barrel be repaired? Not reliably. Filling the hole with solder or a conductive epoxy may restore continuity for a while, but the mechanical damage remains and the joint continues to move with temperature.

Does a thicker plating always help? Only up to a point. Beyond a certain thickness the deposit becomes more brittle and the stress it carries increases, so a balanced ductile layer at the specified thickness performs better than a heavy one.

How does gopcb control barrel quality? We control drilling and desmear, analyse the plating chemistry, verify copper thickness and ductility on coupons from each lot, run a thermal stress test where the customer requires it, and keep the section results with the job file.