Rolled Annealed and Electrodeposited Copper Foil

Copper foil is made by two processes, and the foil that results is different in ways that matter to a flexible circuit and hardly at all to a rigid one. The difference is in the grain structure, and it shows up as ductility rather than as conductivity.

How the Two Foils Are Made

An electrodeposited foil is grown onto a rotating drum from a bath, and its grain structure is columnar with a direction through the thickness. A rolled annealed foil is cast and then rolled to thickness and annealed, and its grains are elongated in the rolling direction.

The two are not interchangeable in a dynamic bend. Our multilayer flex notes describe where the bend requirement appears.

Points to Confirm at First Article

On a design of this kind, ductility is the item that decides how the rest of the board is arranged. Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. 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 first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Documented Process Control

On a design of this kind, ductility is the item that decides how the rest of the board is arranged. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

Inspection and Traceability Notes

On a design of this kind, ductility is the item that decides how the rest of the board is arranged. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it. 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.

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

What to Record and Why

On a design of this kind, ductility is the item that decides how the rest of the board is arranged. A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process.

Ductility and Bend Life

A rolled annealed foil bends further before it cracks, because the grain boundaries are oriented along the bend rather than across it. A deposited foil cracks earlier at the same thickness and the same radius.

That is why a dynamic flexible circuit uses a rolled foil and a static one can use either. The bend axis relative to the rolling direction also matters, which is why the foil orientation is specified on the drawing for a critical flex. Our laminate properties notes describe the substrate side of the same choice.

Both foils carry a treatment on the side that bonds to the resin, and the treatment is what gives the adhesion. A rolled foil is often supplied with a lower profile treatment because its surface is smoother, and a smoother surface adheres less well without a chemical treatment.

The adhesion is therefore a property of the treatment as much as of the foil, and a foils change is a lamination change. Our hole copper notes describe how the plating builds on the same surface.

A rolled foil etches differently from a deposited one, because the grain structure changes how the etchant attacks the sidewall. The sideways removal is similar and the sidewall profile is not.

That difference shows up in the line width tolerance on a fine pattern. A design that has been characterised with one foil and is built with the other can measure a different width at the same artwork.

A lower profile foil reduces conductor loss at high frequency, and a rolled foil is smoother than a deposited one at the same weight. That is the second reason a high frequency design sometimes specifies a rolled foil on a rigid board.

The trade is adhesion, which is the same trade that a smooth foil presents everywhere. Both sides of it are measured rather than assumed.

A rolled annealed foil costs more than a deposited one, and it is available in a smaller range of weights. That limits its use to the designs that need it and makes substitution a design change rather than a purchasing decision.

The availability matters for a long product life, because a foil that is discontinued forces a re-qualification of the lamination. Our board quality notes describe how the change is verified when it happens.

The drawing should name the foil type where the bending or the loss requirement depends on it, together with the rolling direction where a dynamic bend is involved. Where the requirement is not stated, the shop supplies its own foil and the choice is made for the process.

Stating the type without the direction is half a specification for a flex circuit, because the bend life depends on both. The two are written together or not at all.

Is a rolled foil necessary for a flexible circuit? It is for a circuit that bends repeatedly, and it is optional for one that is folded once during assembly.

Does a rolled foil conduct better? The conductivity of copper is the same, and the smoother surface reduces loss at high frequency.

What does gopcb provide for foil selection? We provide foil type and treatment matched to the bend requirement, rolling direction defined for a dynamic bend, lamination qualified with the foil that will be used, line width characterised for the foil, and records that tie the result to the material lot.