Via Barrel Crack Analysis By Microsection
A microsection is the reference method for examining a plated hole, and the way the section is cut decides what can be concluded from it. A section through the centre of a hole shows the plating thickness at its thinnest point, the shape of the inner layer connections and the condition of the hole wall; a section that misses the centre shows a barrel that is thicker than it is and can hide a crack that runs around one side.
This article covers how the section is prepared, how a barrel crack is classified, and how the crack is traced back to the process that caused it.
Preparing The Section
The hole is first located and the panel is cut so that the hole of interest is inside a small coupon. The coupon is mounted in a fixture that holds it perpendicular to the polishing plane, and the fixture is what determines whether the final cut passes through the axis of the hole. Because the hole is small, the alignment is checked by grinding a small amount of material, polishing, and examining the result, then repeating the adjustment until the barrel appears symmetrical on both sides.
Grinding and polishing follow a sequence of progressively finer abrasives, and each step must remove the damage left by the previous one. Copper is soft and smears easily under a coarse abrasive, and a smeared layer can bridge a fine crack so that it appears closed or absent. The final polish is therefore light, and the surface is examined before any etching, because etching removes the distinction between a crack and a scratch. The plating thickness measured on the section is the true minimum over the barrel, as described under copper plating defects prevention.
Classifying A Barrel Crack
Cracks are classified by position and by extent. A crack at the surface corner, where the barrel meets the pad, is the classic thermal fatigue site. A crack at an inner layer connection sits at the junction between the barrel and the inner layer copper, and it is the more serious of the two because it removes the electrical path at the same time as the mechanical one. A crack that runs through the middle of the barrel wall, away from any interface, points at a different problem, usually a defect in the deposit itself.
Extent is described in terms of how far the crack runs around the circumference and whether it crosses the full plating thickness. A crack that does not cross the full thickness is an electrical risk only if it is expected to grow, and it is treated as a defect to monitor. A crack that crosses the full thickness, or that runs around most of the circumference, is a failure regardless of whether the resistance measurement has changed. The classification system only means something if the sampling plan is known, because a crack found in one section out of many is a different statement from one found in the first section cut.
What Causes The Crack
The mechanical cause is laminate expansion: the copper barrel and the laminate grow by different amounts in the direction of the board thickness. The laminate expands more, so the barrel is stretched as the board heats and compressed as it cools, and the interface between copper and resin is where the strain concentrates. A thinner board with a smaller hole is less severely affected; a thick board with a small hole is the worst case.
The process causes are the ones that can be corrected. Resin smear left on the wall after drilling prevents the plating from bonding to the laminate, so the barrel is mechanically detached from the hole wall and takes all the strain alone. A damaged or over-etched inner layer pad gives a connection with insufficient copper around the barrel. A plating deposit with high internal stress or a columnar grain structure cracks earlier than a ductile one. The additive chemistry that governs grain structure is described under electroplating additives for PCB, and the direction the laminate moves when it is heated is treated under PCB dimensional stability and expansion.
Reading The Rest Of The Section
A crack is only one of the features a section reveals. The same image shows the plating distribution along the barrel, which is thickest at the surface and thinnest in the middle on a poorly controlled process, and it shows the shape of the hole wall, where glass fibres that protrude into the barrel create local areas of thin plating. It shows the inner layer connection, where the copper of the inner layer should surround the barrel with a smooth fillet, and where a knife edge or a gap indicates an etch problem before lamination.
The laminate itself is worth reading as well. Voids between the resin and the glass bundles, or a separation along the interface between a copper foil and the resin, are lamination defects that a section exposes and that no electrical test detects. Taken together, the section is a record of the whole process chain at one location, and it is the reason it remains the reference method even when faster techniques are available, as discussed under the quality characteristics of a board design.
Sampling And Reporting
A section that is cut only when a problem appears is of limited value, because there is no baseline to compare against. Production practice keeps a section from each panel lot or from each significant process change, taken at a defined location on the coupon, and records the plating thickness, the inner layer condition and the presence of any crack. The record then shows the direction a process is drifting, and a drift is visible long before a failure.
Reporting should state where the sample came from, which hole and which position on the panel, so that a result can be related to the process that produced it. The plating thickness at the panel edge is usually thinner than at the centre, and a hole near the edge of a panel is a more severe test of the process than one in the middle. Without that positional information, a favourable result can be read as a statement about the whole panel rather than about one favourable hole.
From Defect To Root Cause
A section that shows a crack has identified a defect, not a root cause. The next step is to compare the location of the crack with the process history of the panel. A crack at an inner layer connection, with a knife edged pad and a gap between pad and barrel, points at the etch and oxide treatment before lamination; a crack with resin smear still visible along the hole wall points at drilling and desmear; a crack in the middle of an otherwise ductile deposit points at the plating bath.
Effective analysis changes one variable at a time and sections the same hole position on every sample, because a crack at the surface corner and a crack at an inner layer connection have different causes and different fixes. Recording the location on the panel, the hole size and the board thickness alongside every section turns a series of individual examinations into evidence, and evidence is what allows a process change to be justified rather than guessed.
FAQ
How deep should a section be ground? Only until the barrel appears symmetrical on both sides, which shows the plane passes through the axis. Grinding further removes the evidence, and grinding less leaves a section that overstates the plating thickness.
Can a crack be found without sectioning? A resistance chain test detects the electrical consequence of a crack, and X-ray can find a large separation, but neither identifies the position or the cause. The section remains the method that distinguishes a surface corner crack from an inner layer crack.
Does a small crack matter? It matters as a trend. A crack that does not cross the plating is not an immediate failure, but its presence indicates that the strain at that location is high, and the next set of conditions will extend it.
