Hole Wall Copper Thickness: Measuring the Minimum That Matters
Hole wall copper carries the current between the layers of a board and provides the mechanical support that keeps a plated barrel intact through thermal cycling. It is specified as a minimum rather than an average, because a barrel fails where it is thinnest, and the thinnest point is not where a casual measurement looks. Getting the measurement right is therefore as important as getting the plating right.
The measurement has three parts: where the section is taken, where the reading is taken on the section, and how the reading is made. A laboratory that is consistent in all three produces numbers that can be compared between lots and between suppliers. A laboratory that is not can produce a passing report on a board that will crack.
Why the Minimum Matters
A plated barrel expands and contracts with the laminate during a thermal cycle, and the copper has to strain with it. The strain is concentrated where the copper is thinnest, and a barrel with an average of 25 um and a minimum of 12 um will fail at the thin point rather than at the average. That is why standards specify a minimum thickness and why the reported minimum is the number that decides acceptance.
The minimum also reflects the process more honestly than the average does. Plating thickness varies with the current density, and the current density varies with the position in the hole, the aspect ratio and the state of the bath. An average hides all of that variation in a single number, while a minimum and a range show the process behaviour.
What the Specification Requires
Typical requirements for a general purpose board call for a minimum of 20 to 25 um of barrel plating on the wall, measured after plating, with a higher value for a board that will see severe thermal cycling or that carries a heavy current. The requirement applies to the finished board, after the plating, the etching and the surface finish, because each of those steps removes a small amount of copper.
The specification should state the measurement method as well as the value. Whether the copper is measured on a plated coupon or on a production hole, whether the etch has been completed, and how the measurement points are distributed all change the number. A requirement that names a value without a method is a requirement that cannot be enforced.
Where the Copper Is Thinnest
Copper is thinnest at the middle of a barrel, because the current density falls with distance from the surface and the solution exchange is poorest where the hole is deepest. It is also thin at the junction with the surface copper when the hole has a sharp corner, and at the point where the barrel meets a laminate defect. On a high aspect ratio hole the middle can be half the thickness of the end.
The position around the circumference varies as well, because the current distribution depends on the pattern and on the position of the hole on the panel. Plating thickness distribution across a panel is covered in the notes on plating current density. A measurement taken only on the side facing the anode will report a value that no other side of the same hole achieves.
Measuring on a Microsection
The reference method is a microsection cut through the axis of the hole. The cut has to pass through the centre, because a cut that is off axis makes the barrel appear thicker than it is by lengthening the path through the copper. The section is mounted, ground and polished so that the copper edges are sharp, and the measurement is taken with a calibrated scale at a defined magnification.
At least four points around the barrel should be measured, at the top, the bottom and both sides, and the minimum reported. On a thick panel a set of points at the middle of the barrel and at both ends is more informative than a set taken at one depth. The preparation itself is the subject of the notes on barrel microsection.
Measurement Points and Reporting
The report should carry the individual readings, the minimum, the average and the location of the minimum. The location is what makes the number actionable: a minimum at the middle of the barrel points to throwing power or to solution exchange, while a minimum at the surface points to the entry of the hole or to a drilling defect.
Magnification and the calibration of the scale belong in the report as well. An uncalibrated scale introduces a systematic error that is invisible in the individual readings, and two laboratories using different magnifications can disagree about the same feature without either being wrong about their own work.
Non-Destructive Methods
Microsection destroys the board, so it is applied to coupons or to a sample of production panels. Non-destructive methods estimate the thickness from a different quantity: a resistance measurement across a plated hole uses the known geometry to calculate the average cross section, and an eddy current or an X-ray fluorescence method measures the copper on a surface rather than in a hole.
Each of those methods reports an average and cannot see the minimum. They are useful for monitoring a stable process and for detecting a gross change, and they are not a substitute for a section when the question is whether a barrel meets a minimum requirement. A resistance measurement that passes while the section fails is the classic sign of a barrel that is thin in the middle and thick at the ends.
The Effect of Aspect Ratio and Current
Aspect ratio is the ratio of the board thickness to the hole diameter, and it sets the difficulty of the plating. As the ratio rises, the solution exchange in the middle of the hole worsens, the current density falls and the copper thins. Above about 8 to 1 the process needs better agitation, a longer plating time, a pulse current or a modified chemistry to hold a minimum.
Agitation and current waveform are the two practical levers. Air and solution agitation renew the electrolyte where the hole is deepest, and a reverse pulse waveform improves the distribution by thinning the diffusion layer at the surface and favouring the deposit deeper in the hole. The mechanism behind that improvement is described in the notes on hole wall condition and plating adhesion.
Coupons and Sampling
A plating coupon carries holes of the same diameters as the production panel and is plated with it, so it experiences the same current distribution and the same chemistry. The coupon should sit in the process edge in a position that represents the panel rather than the corner, and it should be marked so that it can be traced to the panel it travelled with.
Sampling should cover the positions that are hardest to plate: the highest aspect ratio hole, the hole nearest the panel edge and the hole in the densest area of the pattern. A single coupon from the middle of the panel reports a number that the rest of the panel may not meet. The copper thickness coupon practice is described in the notes on copper thickness coupons.
Process Control and Records
The controls that keep the minimum inside its window are the bath analysis, the current density and rectifier calibration, the agitation, the temperature, the aspect ratio of the design and the plating time. Each is recorded per lot, and the measurement result is attached to the record together with the coupon identity.
Where a minimum reading falls below the requirement, the report should identify the position where it occurred, because that position points to the cause. A trend of the minimum over several lots, plotted beside the bath analysis, distinguishes a chemistry change from a hardware change, and it is the record that prevents a recurrence rather than only detecting it.
FAQ
Why not specify an average thickness? An average can be met while a thin point fails under thermal cycling. The minimum is the number that the barrel actually depends on.
Can a coupon substitute for a production hole? A coupon plated with the panel experiences the same process, and it cannot represent a hole in a position the coupon does not occupy.
Does the surface finish change the measurement? Yes. Etching and finishing remove copper, so the measurement should be taken at the point in the sequence that the specification names.
