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
On a design of this kind, plating is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
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.
Process Control and Verification
On a design of this kind, plating is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
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.
Process Control and Verification
On a design of this kind, plating is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
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.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
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.
