Corona Treatment Adhesion for PCB Surfaces: 5 Process Rules

Corona treatment raises the surface energy of a plastic or a coated surface by exposing it to a high-voltage discharge. The discharge breaks bonds at the surface and leaves polar groups behind, and those groups are what allow a coating, an adhesive or a label to wet and bond. On a PCB assembly the treatment is usually applied to a solder mask, a moulded connector body, a cable jacket or a potted housing before a coating or an adhesive is applied.

The effect is real but temporary. Treated surfaces age, and the energy decays back towards its original value within hours or days depending on the material and the storage conditions. That ageing behaviour is the reason a treatment cannot be treated as a permanent property of a part, and the reason the process has to be controlled at the point of use.

What the Discharge Does to the Surface

The corona is a partial discharge in air at atmospheric pressure. Electrons accelerated by the field break carbon-carbon and carbon-hydrogen bonds near the surface, and the free radicals that result react with oxygen and nitrogen in the air to form hydroxyl, carbonyl and amine groups.

Those groups are polar, so the surface energy rises and a liquid that previously beaded up now spreads. The chemistry is confined to the top few nanometres, so the bulk properties of the part are unchanged, which is exactly why the process is used on finished parts that cannot be altered in any other way.

Because the change is only at the surface, any contamination that sits on top of it hides the effect. Fingerprints, mould release and silicone oils all defeat a treatment, and the parts that fail to bond after treatment are usually the parts that were handled after cleaning.

Surface Energy, Dyne Level and Testing

Surface energy is reported in dynes per centimetre, and the target value depends on the liquid that has to wet it. A coating with a surface tension of 30 dynes will spread on a substrate of 38 dynes but will bead on one of 28 dynes. The rule of thumb is that the substrate should exceed the liquid by roughly ten dynes.

The measurement is made with dyne pens or with a test ink set: a series of liquids of known surface tension is drawn across the surface, and the highest one that holds a continuous film without breaking into droplets indicates the level. It is a shop-floor test, quick and cheap, and it belongs with the operator rather than in a laboratory.

Because the test is comparative, the pens have a shelf life and the reading has to be taken the same way each time. A test taken minutes after treatment and one taken a day later will not agree, and that difference is information rather than error.

Treatment Power, Speed and Gap

Treatment power, the distance between the electrode and the work, and the speed at which the part passes under the discharge together set the dose. As with any activated process, a low-power treatment at slow speed can equal a high-power treatment at high speed, and the result is judged by the dyne level rather than by the settings alone.

The gap between electrode and part is the most common cause of drift. A part that is thinner than the previous one passes further from the electrode and receives less energy, so a change in substrate thickness needs the gap to be re-set. Where the part has a three-dimensional shape, the dose varies across the surface, and the corners may need a second pass.

Ozone is produced by the discharge and must be extracted. Beyond the safety requirement, ozone and the heat of the discharge can oxidise the surface if the part dwells too long, and an over-treated surface can become powdery and bond worse than it did before.

Treating a PCB Assembly Without Damage

The discharge is not selective; it will treat the copper, the solder mask and the components alike. On an assembled board that means the treatment has to be limited to the area that will be bonded, and masking or a narrow nozzle is used to keep the discharge away from wire bonds, exposed contacts and sensitive parts.

Static charge also accumulates during treatment and has to be neutralised before the board moves on, or the parts that follow will attract particles. Treating the board immediately before the coating keeps the ageing effect small, and the interval between the two operations should be written into the process. The adhesion test notes show how the result is verified after coating, and the coating application guide describes the steps that follow.

Where the surface must be clean as well as activated, the sequence is cleaning first and treatment second, since the discharge will not remove oil. Surface cleanliness is covered in the contamination control guide, and the wetting measurement itself is described in the contact angle notes.

Adhesion Failures After Treatment

A treatment that is applied correctly can still fail if the coating is applied too late. The polar groups that raise the surface energy are mobile, and they rotate away from the surface or react with the air over time. The practical window between treatment and bonding is measured in hours at most, and it shortens in a warm, dusty shop where the surface collects airborne contamination.

Over-treatment is the opposite failure and it is easy to create. A part that passes under the discharge repeatedly, or one that dwells under a stationary electrode, develops a powdery surface layer. The dyne test may still show a high value because the powder is itself polar, but the layer is weak and the bond fails at the powder rather than at the interface. Reducing the number of passes is usually the correct fix.

Where a treatment is applied to a populated board, the parts that will be bonded later should be protected from the discharge, and the operator should be able to see which boards have been treated. A visible mark, a time stamp at the station or a simple batch card all prevent a board from being treated twice or skipped altogether.

Temperature and humidity in the treatment area also matter. A cold part carries a layer of condensed moisture that the discharge has to drive off before it can modify the surface, and a very dry room encourages static that later attracts particles. Both are recorded alongside the settings when a treatment is qualified.

FAQ

How long does a corona treatment last? On many plastics the effect falls to half its initial value within a day, and on some materials it is gone in a few hours. The practical rule is to treat immediately before the bonding or coating step, and to verify the dyne level on the first part of each batch.

Can a treated part be handled normally? Handling is the main way a treatment is destroyed. Gloves should be worn, parts should be held by areas that will not be bonded, and treated parts should be covered if they must be stored.

Why does a part that passed the dyne test still fail to bond? Because the dyne test measures surface energy and not chemical compatibility. A treated surface that carries an incompatible mould release, or a coating applied after the treated layer has aged, will still fail even though the level looked correct.