PCB Copper Surface Condition and Wetting Behaviour

Copper on a finished board is not flat. The surface is a series of peaks and valleys created by the lamination texture, the plating and the etching, and every soldering process has to cope with that topography. Understanding where the roughness comes from explains why some surfaces solder easily and others do not.

Where Surface Roughness Begins

The copper foil arrives with a texture that is deliberately rough on the side that will bond to the resin, because the mechanical keying is what gives the laminate its peel strength. On the outer layers the treatment is different: the copper is plated up after drilling, and the plating inherits the grain structure of the base foil and adds its own.

Etching then changes the profile again, because the etchant attacks the copper sideways as well as downwards. A fine line ends up with a trapezoidal cross-section and a surface that is rougher than the foil it came from, and the roughness increases with the thickness of the copper removed. Our etching process notes describe the trade-off between line width and profile.

Oxidation and the Shelf Life Problem

Copper oxidises in air, and the oxide that forms is not wettable by solder. The growth is slow at room temperature and rapid in warm humid conditions, which is why a bare board has a shelf life and why the finish applied at the end of fabrication is chosen partly for how long it resists oxidation.

The practical consequence is that a board stored in a warehouse for a year may solder differently from one stored for a month, even though the two are otherwise identical. The storage conditions belong in the procurement record, not only in the quality department. Our surface finish notes compare the options.

Surface Finish and What It Changes

Every finish is a compromise. Hot air solder levelling gives an excellent solderable surface but a non-planar one, which is why it is difficult to use with fine-pitch parts. Immersion gold is planar and has a long shelf life but dissolves into the solder joint, and its thickness has to be controlled to avoid embrittlement.

Immersion tin and organic coatings are planar and inexpensive, but they are less robust to handling and to multiple reflow cycles. The choice therefore depends on the component pitch, the number of thermal cycles and the time between fabrication and assembly, and on nothing else.

Wetting and Contact Angle

Wetting is described by a contact angle, and the angle is what determines whether the solder spreads across a pad or beads on it. A clean, properly finished copper surface gives a small angle and a spreading solder; a contaminated or oxidised surface gives a large angle and a joint that looks like a ball sitting on the pad.

The measurement is made by observing a defined quantity of solder on a defined pad and reporting the time to wet and the force developed. A wetting balance curve is a compact way of expressing both, and it is the basis of most solderability specifications. Our solderability notes describe the test.

Contamination and Its Sources

Contamination reaches the surface from several directions. Handling leaves oils and salts from skin. Flux residue from a previous process remains if the cleaning is incomplete. Storage adjacent to certain materials can deposit a film. Each of these reduces the surface energy and therefore the wetting.

The countermeasure is procedural rather than technical: gloves, sealed packaging with desiccant, a defined time limit between fabrication and assembly, and a check that the finishing process has not left its own residue on the surface.

Plating Grain Structure

Electroplated copper has a grain structure that depends on the additive package in the plating bath and on the current density. A bright, fine-grained deposit is harder and smoother than a matte one, and it etches differently, which is why the same artwork can produce different line profiles in two different shops.

The difference is normally invisible until a fine line is examined under a microscope, and by then the design is fixed. Where a design is close to the process limit on line width, the grain structure is worth discussing before the panel is released.

Roughness and High Frequency Loss

At high frequency, current crowds into a thin layer at the surface of the conductor, and the roughness of that surface increases the effective path length of the current. The result is a conductor loss higher than a smooth-surface calculation predicts, and the discrepancy grows with frequency.

The effect is modelled by a correction factor applied to the smooth-conductor loss, and the factor depends on the profile of the foil and on the finish. On a long high-speed channel it can account for a significant fraction of the total insertion loss. Our high frequency laminate notes describe the related material choices.

Controlling the Surface in Production

The variables that matter on the production floor are the cleanliness of the final rinse, the concentration of the finishing bath and the age of the surface before packaging. Each can be monitored, and a simple coupon that travels with the panel gives a continuous record of what the process achieved.

The record is what allows a soldering problem at the assembly house to be attributed correctly. Without it, the discussion becomes an exchange of opinions between two parties who each believe the problem belongs to the other. Our fabrication notes guidance describes how the coupon is specified.

Process Control and Verification

Reviewing the design before the data is released is far cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. 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.

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. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

FAQ

Why does a board that soldered well last month not solder now? The most likely cause is oxidation or contamination that has progressed during storage. The rate depends on humidity, temperature and packaging, so the same board can behave differently after a few weeks in a warm warehouse than after a year in a sealed bag with desiccant.

Does a rougher surface help solderability? A moderate texture gives the flux and the solder more area to act on, but the benefit is small compared with the effect of a clean, properly finished surface. Excessive roughness, particularly from a poorly controlled etching step, is a liability rather than an advantage.

What does gopcb control on the copper surface? We control the final rinse chemistry, the finish thickness, the handling and packaging, and we retain a coupon from each panel for solderability testing. Where a customer specifies a shelf life or a storage condition, we record the date of fabrication on the panel so that the two can be reconciled.