Barrel Crack: Plated Through Hole Reliability Under Thermal Cycling

A plated through hole is a copper tube in a laminate that expands more than the copper does. Every thermal cycle loads the barrel at its two ends, and the crack that forms is the classic board level failure.

The Load on the Barrel

The laminate expands through its thickness by a factor of ten or more compared with the copper in the same direction. The barrel is restrained by the pads at both ends, so the expansion is converted into a strain along the wall.

The strain is concentrated at the corner where the barrel meets the pad, which is where the crack is always found. Our <a href="https://www.gopcba.com/pcb-plating-thickness-guide/” title=”plating”>plating notes describe the wall thickness that resists it.

What Sets the Life

The thickness of the board, the expansion of the laminate above its glass transition, the copper ductility and the barrel wall thickness all contribute.

The expansion above the transition is much higher than below it, so the peak temperature of the cycle matters more than the range. Our laminate notes describe the property that governs it.

The Role of the Aspect Ratio

A long barrel in a thick board accumulates more strain for the same expansion, and it is also harder to plate uniformly. Both effects work against the reliability.

The ratio also determines whether the plating reaches the middle of the barrel at the specified thickness. Our aspect ratio notes describe the limit and how it is verified.

Barrel Cracks and Their Detection

The crack starts as a partial circumferential break and it grows with cycles. It becomes an open only at the end, so an electrical test at room temperature may pass a barrel that is nearly broken.

The detection is by cross section on a coupon or by a resistance measurement taken at temperature during a thermal cycling test. Our test coupon notes describe the coupon structure.

Design and Process Measures

A material with a higher glass transition and a lower expansion through the thickness reduces the strain. A thicker copper deposit in the barrel increases the life.

The pad and the connection also matter: a barrel connected to a plane is restrained more than one on a signal net, so the relief affects the reliability as well as the soldering. Our thermal relief notes describe the trade.

Assembly Effects

Each reflow and each rework exposes the barrel to a cycle, and the damage accumulates. The number of thermal excursions a board can accept should be limited and recorded.

The assembly also heats the board unevenly, which adds a local gradient on top of the global one. Our rework notes describe the local exposure.

Verification

The verification is the cross section of a coupon from the same panel and a thermal cycling test with the resistance monitored at temperature.

Where the design uses a high aspect ratio or a material close to its limit, the test should be run rather than the margin assumed. Our thermal cycling notes describe how the test is arranged.

Process Control and Verification

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. 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. 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.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Verification and Records

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

FAQ

Does a thicker plating always help? It increases the life and it also reduces the finished hole diameter, which changes the component fit and the assembly.

Is a barrel crack visible on the surface? No. It forms inside the hole and it is found by cross section or by measuring the resistance at temperature.

What does gopcb provide for through hole reliability? We provide material selection with the expansion through the thickness, plating thickness control at the specified aspect ratio, cross section verification on a coupon, monitored thermal cycling at the peak temperature the product will see, and limits on the number of reflows and reworks per board.