Copper Foil Types for PCB Laminates

The copper on a board is supplied as a foil that is laminated to the dielectric, and there are two families of foil with different manufacturing routes and different properties. One is rolled from a cast ingot and the other is electrodeposited onto a rotating drum, and the difference shows up in the grain structure, the surface roughness and the loss of a high frequency trace.

The Two Manufacturing Routes

An electrodeposited foil is grown on a drum from a copper solution. The grain grows in the direction of the plating current, so the foil has a columnar structure and the side that faced the drum is smooth while the side that faced the solution is rough.

A rolled annealed foil is cast as an ingot, rolled to thickness and annealed. The grain is elongated in the rolling direction, which gives the foil a ductility that the electrodeposited type does not have, at a higher cost.

Both are supplied with a treatment on one side that provides adhesion to the laminate and often a passivation on the other that protects it from oxidation before the circuit is imaged.

Surface Roughness and Adhesion

The bond between the foil and the resin is mechanical, not chemical. The treatment on the foil side creates a rough surface with a large area, and the resin keys into it. A smooth foil does not adhere reliably.

The roughness is described by a profile measurement, and a low profile foil is one with a reduced roughness that still provides an acceptable bond. The adhesion and the roughness pull against each other, and a low profile foil is the result of a treatment that gives both.

The adhesion is verified by a peel test, and the value depends on the resin system as much as on the foil. A foil that peels well from one laminate may peel poorly from another, so the combination should be qualified rather than the foil alone.

Roughness and Conductor Loss

At high frequency the current travels near the surface of the conductor. A rough surface makes the path longer, which raises the resistance and adds to the conductor loss.

The effect grows with frequency, because the skin depth falls and the current follows the profile more closely. For a high speed channel with a long route, the roughness can account for a significant part of the total loss.

A low profile foil reduces the effect, which is why the copper is specified together with the laminate for a high frequency design. Choosing a low loss laminate and leaving a rough foil in the stack leaves part of the improvement on the table.

Ductility and Handling

A rolled annealed foil bends without cracking, which is why it is used for a flexible circuit and for a rigid flex bend area. An electrodeposited foil of the same thickness will crack at a bend radius that the rolled foil survives.

The difference comes from the grain structure. The elongated grains in a rolled foil can accommodate strain by sliding, while the columnar grains of an electrodeposited foil have fewer paths to deform.

Ductility also matters for a rigid board that is handled roughly or that is subjected to a thermal cycle. A foil with more ductility is more tolerant of the strain at the corner of a pad, though the effect is smaller than in a flex application.

Thickness and Its Selection

The foil thickness is chosen from the current and from the impedance. A thicker copper carries more current and has a lower resistance, while a thinner copper allows a finer line and a thinner dielectric.

The thickness also affects the impedance, because a thick trace is electrically slightly wider than a thin one of the same drawn width. The stack calculation should use the actual finished thickness rather than the nominal foil figure.

The finished thickness is greater than the foil thickness because plating is added during the process. The plated thickness has to be included in the calculation, and a design that assumes the foil figure alone will have a wider trace and a lower impedance than the design intended.

The Treatment and the Process

The treatment on the foil side has to survive the lamination and the drilling. A treatment that is damaged by the press cycle will give a poor bond, and the failure appears as a blister after reflow rather than as an obvious delamination.

The treatment also affects the etching. A rough surface etches differently from a smooth one, and the undercut and the resulting line width depend on the profile. The etch factor should be established for the foil that is used rather than transferred from another.

The foil has a shelf life and it should be stored under the specified conditions. Oxidised foil does not laminate reliably, and the oxide can also affect the adhesion of the plating that follows.

Selection for the Application

A rigid digital board with a moderate data rate uses an electrodeposited foil with a standard treatment. The roughness helps the adhesion and the loss is not the limiting factor.

A high frequency board uses a low profile foil, and a flexible circuit uses a rolled annealed foil. A rigid flex board uses a rolled foil in the flex area and may use a different foil in the rigid area, which means the stack has two foils and the drawing should say which is which.

The selection should be made with the laminate rather than after it. The resin system, the treatment and the roughness form a combination that is qualified together, and a change in any of the three is a change of the combination.

Specification and Verification

The specification should state the type of foil, the thickness, the treatment and the profile. Those four items describe the foil in a way that can be checked against a datasheet and against an incoming inspection.

The incoming inspection can include a thickness measurement, a weight per unit area and a visual check for oxidation or damage. A simple weight check detects a foil that is at the wrong end of the thickness tolerance.

The peel strength after lamination and the line width after etching are the process measurements that confirm the foil behaved as expected. Both should be recorded with the material batch so that a change can be traced.

Practical Rules

Match the foil type to the application, keep the foil and the laminate as a qualified combination, and include the roughness in the loss budget. State the type, the thickness, the treatment and the profile.

Record the foil data with the build records and the laminate properties, and review the low loss material selection and the trace width calculation when a stack is specified.

FAQ

Which foil is better for a high frequency board? A low profile foil. The conductor loss grows with surface roughness because the current near the surface has to follow a longer path.

Why is rolled annealed foil used in flex circuits? Its elongated grain structure bends without cracking, so it survives a bend radius that an electrodeposited foil of the same thickness would not.

What fixes the copper adhesion? The treatment on the foil side, which creates a rough surface that the resin keys into. The adhesion is a property of the foil and resin combination, not of the foil alone.