The purpose of the electroless copper deposition process is to metallize the isolating wall of driller holes. This metallization provides an electrical connection between the sides of a panel, and to the inner layers of multilayer boards.

Before electroless copper process, the drilled PWB is mechanically and chemically cleaned by scrubbing, removing of grease, micro-etching and oxide-removing.

Then the surface of the PWB must be activated. Activating is necessary, because the copper coating must be deposited onto the isolating surface of epoxy-resin. After this stage PWB is ready to metallization, which usually means through-hole plating.

The electroless copper deposition process is based on the chemical reduction of copper initialized by the activating particles. Slow operating type of electroless copper bath is used to deposit a very thin, but continuous layer of copper. Electroless copper layers, deposited from a room temperature bath, are only ca 1 micrometer thick. The advantage of this type of bath is the careless operation. When using slow operating type of electroless copper bath, the thin coating of electroless copper is thickened by electroplating. In this way the copper coating being inside the holes is formed by a two-stage depositing process: electroless copper followed by electroplating. By using quick operating electroless copper bath, it is possible to deposit the copper coating by a single-stage process, without electroplating. When panels are removed from the electroless copper bath, they should be neutralized, rinsed, dried as soon as possible to prevent oxidation. Panels are typically dried during a mild scrubbing operation.

When inspecting panels as they are being processed along the elec­troless copper line, attention must be paid to the holes, not just the surface of the board. The surface may appear satisfactory, while little coverage is taking place inside the holes or along the edges. Panel edges are a more accurate reflection of copper deposition quality inside the holes, than the surface is. If there is a marginal condition in the electroless copper process, it will show up inside the holes first, then on the board edges, and lastly on the surface.

The dwell time in copper is usually 15-20 minutes. Electroless copper is a solution containing sodium hydroxide also. This component is hygroscopic, which means that it will pull moisture out of the air. Caustic solutions like electroless copper baths are difficult to rinse off. The only fully reliable way to rinse is to neutralize first, then rinse. Usually 1% solution of sulfuric acid or phosphoric acid is used for neutralizing. In the course of an operating shift, analysis should be conducted, involving pH check of the all copper baths.

Most of the baths require an air source which pumps bubbles through the bath at all times. The air is beneficial for two reasons: air helps stabilize the copper and retard any tendency to plate out; and air provides a small measure of solution agitation when the tank is not being used. During use, there is plenty of agitation by way of moving of panels, or by a mechanical rack agitation system. It should be remembered, that only a trickle of air is required for good bath stability: too much air agitation removes the formaldehyde (which will extinguish the deposition reaction).

According to the imaging, electroless copper process can be fol­lowed by electroplating. Electroplated copper thickness is ca 5 micro­meters. This panel plating is necessary, when PWB is microetched before pattern plating. Microetching means removing of ca 2-3 micrometers thick layer of copper.

PWB plating system for electroless deposition and pattern plating

The copper surface of the PWBs must be cleaned and then a negative mask is made onto the surface of the PWB, after electroless copper process and prior to pattern plating.

There are differences in the cleaning processes which must be used for photoresist imaging and screen printing. Dry photoresist filmed panels require microetching of the copper surface prior to electro­plating. If this is neglected, all copper would peel. The copper surface must be etched until it is matte pink. For screening resists the type of the cleaner depends on the solvent of the resist.

After through-hole plating and prior to panel plating a negative mask should be applied to the board surface. The aim of the negative mask is to form a selective covering on the copper surface according to the layout of the circuit. Negative masking means, that the required pattern of the PWB is not covered by resist. The mask formed before pattern plating must be resistive against plating baths.

For patterning (or imaging) with negative mask, the dry film photoresist method is the most popular in the PWB industry. Alternatively, the less expensive but lower resolution screen printing imaging technology can be used. Both processes are discussed in the next page.

The purpose of pattern plating is to deposit ca 25 micrometer thick copper onto the wall of the holes. The copper is coated by tin or tin-lead by electroplating. The thickness of this coating is ca 10 micrometers, and it serves as a mask during etching, and called metal etch resist. In plain cases it can be used as a solderable coating when assembling and soldering components onto the board. Some chemicals (brighteners etc.) must be added continuously to copper plating and other baths. All plating baths have a certain degree of tolerance for organic and metallic contamination, and for components.

Pattern plating is followed by stripping, i.e. removing the negative photoresist mask and etching the copper. During this process the tin coating will protect the PWB circuitry against the etchant.

Etching is the process, which actually transforms an image into a circuit. In general, conveyorized alkaline ammonia or sulfuric acid/hydrogen peroxide baths are used for etching.

Processing line for etching after negative mask stripping

Organic resists are formulated to withstand most etching solutions. They all withstand common acid etchant. Alkaline soluble screen printing inks are not formulated to work well in etchants with pH above 7.5. Vinyl screen printing resist, and solvent or semi-aqueous developing dry film photoresist work very well as etch resist. The fully aqueous de­veloping photoresists should be given more careful consideration, and therefore most commonly used for plating resist, and tin metal etch resist technology is preferred.  

The etching rate can be defined as the etching through time of a 35 µm thick copper layer. As the copper foil is etched, some amount of copper is also removed from the sidewalls of the conductors with a lower etching rate. It is the so called under­cutting. Undercut depends on etching technology, etchant, pH, etc. A high etch factor, which means low overhang, generally indicates the ability to hold fine lines with tight conductor spacing. Excessive overhang causes short circuiting, as the overhanging conductor breaks off and forms an electrical bridge between two points in the circuitry. Overhang can be eliminated by fusing (or reflow) of tin metal etch mask.

Using thin copper foils on the laminate can be very helpful in reducing undercutting, attaining fine lines with high density and in using dif­ferential etching for bare copper circuitry. Differential etching occurs when the plated copper is used as the etch resist. To do this, the plated copper must be considerably thicker than the copper foil.

Tin metal etch resist stripping by selective etching