Plating Thickness Distribution Across a Panel

Plating thickness is not uniform across a panel, and the variation is a property of the process rather than a defect. The current density is highest where the cathode geometry concentrates it, so the corners of a panel and the isolated features plate differently from the centre and the dense areas.

What Sets the Distribution

The plating current has to reach every feature through the electrolyte, and it finds the path of least resistance. Features near the bus bar and away from other copper receive more current, and features in the middle of a dense area receive less.

The distribution therefore depends on the pattern, the panel size, the anode spacing and the agitation. It is not a function of the plating time, because a longer time scales every figure including the variation. Our plating thickness notes describe how the figure is measured.

Consequences for the Hole

The barrel of a plated through hole is a deep recess, and the current density inside it is lower than on the surface. The result is a thickness that falls from the surface to the middle of the barrel, which is the opposite of what the thermal cycling requirement needs.

That is why the specification is written as a minimum at the middle of the barrel rather than as an average. A board that meets the average and misses the minimum will fail in thermal cycling rather than in electrical test. Our hole copper notes describe how that minimum is controlled.

Improving the Distribution

The available levers are the current waveform, the agitation, the anode configuration and the pattern itself. A periodic reverse waveform plates more evenly into a recess than a direct current, and agitation that reaches inside the hole does the same.

The pattern lever is the one the designer controls. Adding thieving to the sparse areas and reducing the density difference across the panel raises the minimum, which is the figure that matters. Our copper balance notes describe how that is arranged.

Measurement and Sampling

The measurement is taken on a coupon that includes holes of the same size and in a comparable position on the panel, and it is taken by microsection. A surface measurement by X-ray fluorescence reports the foil rather than the barrel.

The sampling has to cover the extremes of the panel, because the centre and the edge do not plate alike. A single coupon in the middle of the panel describes neither of them.

Specifying a Minimum

A specification written as a minimum with a nominal is enforceable and a specification written as an average is not. The shop plates to the minimum and the nominal is a consequence.

Where the design needs a specific figure for the thermal cycle life, that figure belongs in the requirement. Our board quality notes describe how the requirement is verified on the finished panel.

Interaction With the Etch

Plating adds copper to the surface as well as into the hole, and that added copper has to be etched away between the lines. A thicker plating therefore requires a longer etch, which removes more copper sideways and narrows the line.

The two requirements pull against each other, which is why a board that needs a thick barrel and a fine line is plated in two stages or etched differently. The stack-up and the process are chosen together rather than in sequence.

The record that makes the process controllable is the minimum thickness measured on the coupon, the position of the coupon and the plating parameters. With those three a drift can be separated from a pattern effect.

The trend over time shows whether the bath is drifting, and the position data shows whether the distribution is stable. Both are needed, because a bath that is drifting and a distribution that is shifting look the same in a single reading.

Process Control and Verification

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.

Can a thicker foil fix a thin barrel? No. The barrel is plated, and the foil weight does not change what happens inside the hole.

Does a smaller panel plate more evenly? It usually does, because the current has less distance to travel and the pattern difference across the panel is smaller.

What does gopcb provide for plating distribution? We provide a specification written as a minimum at the barrel centre, coupon and section measurement at the panel extremes, thieving recommendations for the pattern, waveform and agitation set for the hole size, and records that separate a bath drift from a pattern effect.

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.