PTH Barrel Cracking and Its Root Causes

A crack in the copper barrel of a plated through hole is a failure that appears after thermal cycling rather than during assembly. The barrel is a thin copper tube inside a laminate with a much higher expansion coefficient, and the strain that results is concentrated where the copper is least able to accommodate it.

Why the Barrel Cracks

The laminate expands along the axis of the hole far more than the copper does, so the barrel is stretched when the board is heated. The copper is ductile and it can accommodate a limited amount of that stretch, and the failure appears after enough cycles rather than on the first one.

The strain is highest at the surface and at the middle of the board thickness, which is why a barrel crack usually starts at the inner layer connection or at the corner where the barrel meets the land. Our hole copper notes describe how the thickness changes where the strain falls.

Plating Thickness and Its Limit

A thicker plating carries the same strain over more copper and delays the crack. The relationship is well established, which is why a minimum plating thickness is specified for a board that will see thermal cycling.

The thickness that matters is the minimum at the worst point, not the average, and the worst point is usually the middle of the barrel. A measurement taken at the surface reports a figure that the crack does not depend on. Our plating thickness notes describe how the figure is measured.

Ductility of the Deposit

An electroplated copper that is bright, hard and full of organic co-deposits has a low elongation and cracks after fewer cycles than a dull, ductile deposit of the same thickness. The additive package that produces a bright deposit is beneficial for throwing power and detrimental for ductility.

The deposit has to be specified by its elongation as well as by its thickness, and the two are determined by the bath chemistry rather than by the current alone. A change of plating supplier is a change to both.

Desmear and the Interface

A barrel that is not bonded to the inner layer lands can move relative to them, and the crack appears at the interface where there is no adhesion. Insufficient desmear leaves resin between the copper and the land, and that is the surface where the separation begins.

Excessive desmear damages the glass bundles and produces the same result from the other direction. The window is narrow on a thick board with small holes, and it is controlled by a coupon rather than by the tank clock. Our thermal cycling notes describe how the result is qualified.

Board Thickness and Hole Size

The strain in the barrel scales with the board thickness and with the expansion coefficient, and it is carried by the copper in the wall. That means a thick board with a small hole is the case that fails first, which is the same combination that is hardest to plate.

The aspect ratio therefore appears in the reliability discussion as well as in the fabrication discussion. A design that pushes the aspect ratio for density also reduces the thermal cycle life of the same hole.

Finding the Crack

The crack is found by microsectioning a sample after the thermal excursions, and by a resistance measurement on a daisy chain during the cycling. The daisy chain gives the number of cycles to failure, and the section explains where.

Both are needed, because a resistance change says that something happened and the section says what. A section taken without the resistance data usually misses the first crack, which is the one that matters.

Correcting the Process

The corrections are to raise the minimum plating thickness, to move to a more ductile chemistry, to reduce the board thickness or the aspect ratio, and to tighten the desmear window. Each has a cost and each is measureable.

The cheapest correction is usually the plating thickness, because it requires no design change. Where the design is already at the limit, the change is to the laminate expansion coefficient, which is a material choice. Our board quality notes describe how the corrected process is verified.

Process Control and Verification

On a design of this kind, thermal cycling 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.

Process Control and Verification

On a design of this kind, thermal cycling 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.

Process Control and Verification

On a design of this kind, thermal cycling 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.

FAQ

Does a crack always fail the board? It raises the resistance and it eventually opens. An intermittent open is harder to find than a clean one.

Is a thicker board always worse? For the same hole size it is, because the laminate moves more over the length of the barrel.

What does gopcb provide for barrel reliability? We provide minimum plating thickness controlled at the middle of the barrel, deposit ductility specified by elongation, a desmear window qualified by coupon, aspect ratio review at the design stage, thermal cycling with a daisy chain and sections, and records per batch.