Inner Layer Oxide Treatment Before Lamination

Before the layers of a multilayer board are pressed together, the copper on the inner layers is treated so that the resin can bond to it. Copper alone does not bond well to epoxy: the surface is smooth and chemically unreactive, and a bond made directly to it fails at the interface when the board is heated.

This article explains what the treatment does, how the two common chemistries differ, and how the process is controlled.

Why The Treatment Is Needed

The bond between the resin and the inner layer copper has to survive the press, the drilling, the plating and the thermal excursions of assembly. It is a mechanical and chemical bond, and it depends on the surface having a large area and a chemical affinity for the resin. A bare copper surface that has been cleaned offers neither, so the treatment converts the surface into a roughened oxide that the resin can wet and grip.

The oxide also protects the copper during the interval between the treatment and the lamination, which may be days. An untreated copper surface oxidises unevenly, and an uneven oxide is a weak layer. The treated surface is a controlled oxide, and its thickness and its structure are what the process controls. The adhesion it provides is one of the reasons a multilayer board holds together at all, and the lamination stage it feeds is described under multilayer prototype requirements.

Black Oxide And Brown Oxide

Black oxide is the traditional treatment, producing a deep, dense, columnar oxide. It gives the strongest bond and the most reliable result, and it is the reference against which the alternatives are judged. Its disadvantage is the chemistry, which is aggressive and difficult to control, and the handling, since the oxide is friable and can be damaged by contact before lamination.

Brown oxide is a modified process that produces a thinner, less friable oxide with a lighter colour. It is easier to handle and less prone to damage, at the cost of a slightly weaker bond in the most demanding cases. A third option is a chemical treatment that forms a thin organometallic layer rather than an oxide, which gives a good bond with a very thin layer and avoids the friability entirely. Each is chosen for the combination of bond strength, handling and cost that the product needs. The defects that appear when the treatment goes wrong are related to the plating defects described under copper plating defects prevention.

Pink Ring And Its Causes

Pink ring is the appearance of a ring of pink copper around a drilled hole in a sectioned board, where the oxide has been removed and the bare copper exposed. It is seen on the inner layer connection, and it indicates that the treatment has been attacked, usually by the acid in the plating line or by the desmear, which has dissolved the oxide around the hole.

The name comes from the colour of the exposed copper, which is lighter than the black or brown oxide around it. A narrow ring is normal and accepted, because some attack at the hole wall is inevitable. A wide ring is a defect, because the resin to copper bond has been lost over that area, and the inner layer connection then relies only on the plating around the barrel. The measurement is the width of the ring on a section, and the acceptance limit is stated in the workmanship standard the product is built to.

Controlling The Process

The parameters of the treatment are the chemistry, the concentration, the temperature and the immersion time, and the result is judged by the appearance, the weight gain and the adhesion. The weight of oxide per unit area is a direct measure of the amount formed, and it is used on a coupon rather than on a production panel. The adhesion is checked by a peel test, in which a piece of the copper is lifted and the force measured, or by laminating a test panel and pulling it apart.

Two process controls prevent most of the trouble. The first is the interval between the treatment and the lamination, because the oxide continues to change with time and a surface left too long is not the surface the press receives. The second is the handling, since the oxide can be rubbed off by contact, and a panel that has been handled roughly has bare copper in the areas that were touched, which laminates as a poor bond. Both are recorded in the traveller rather than left to the operator. The way the layers are then arranged and balanced is treated under balanced stackup and odd layer count.

Adhesion And Its Measurement

Adhesion is measured after lamination rather than before, because the bond is a property of the pair. The standard check is a peel test on a coupon in which a strip of copper is lifted from the laminate and the force required is measured, and the value is compared with a specification. A second check is a thermal stress test, in which the laminated panel is floated on molten solder and then examined for delamination, which exposes a bond that is adequate at room temperature and inadequate when hot.

A third measurement is the appearance of the section at an inner layer connection after drilling, which shows both the ring and the quality of the laminate. The three together give a picture of the interface that no single test provides, and the combination is why the treatment is qualified with a coupon programme rather than with an inspection.

Design And Process Interactions

The oxide treatment itself does not change with the design, but the requirements around it do. A board with a high layer count has more interfaces and therefore more opportunities for a weak bond; a board with large copper areas has more area to treat and a greater chance of an uneven result. A board with a fine inner layer pattern is more sensitive to the attack that produces a wide pink ring, because the connection area is small to begin with.

Where the product is built to a demanding specification, the treatment process and its interval are part of the qualification rather than a shop detail. The fabricator should be asked which treatment it uses, how the result is measured and what interval is allowed between treatment and lamination, because those three answers determine whether the inner layer connections will hold under thermal cycling.

FAQ

Is black oxide always better than brown oxide? It gives a stronger bond, which matters on a demanding product or a large copper area. Brown oxide is easier to handle and adequate for many builds, so the choice depends on the requirement rather than on a general rule.

What is an acceptable pink ring? The limit comes from the workmanship standard the product is built to. A narrow ring around the hole is normal, and a wide ring indicates that the bond has been lost over that area and is treated as a defect.

Can the treatment be skipped if the resin has good adhesion? Not reliably. The bond to bare copper depends on cleanliness and on surface roughness that the lamination process alone does not produce, so the treatment is used on every multilayer build.