Busbar plating mask: bare contact zones

Why a mask is a design call

Selective plating is common on busbars because different areas of the same bar need different surfaces. A joint face may need bare copper for a low resistance contact, a weld zone may need to stay unplated, and the rest of the bar may need tin or nickel for corrosion resistance. The mask is what separates those zones during the plating process.

A mask is not a piece of tape added at the end. It is a process step that changes the geometry of the finished part. The edge of the mask leaves a step, a ramp or a ridge, and that feature can sit under a joint, interfere with a lug or trap cleaning fluid. The drawing has to define the zone, the transition and the inspection, not just the coating.

The cost of masking is real. Every masked area adds labour, and a complex mask can double the handling time on a small batch. Mask only where the function needs it, because a blanket call to keep the whole bar bare usually means the finish has not been thought through.

Which zones stay bare

The most common bare zone is the joint face that carries current to a lug, another bar or a terminal. That face needs a known surface, a known flatness and a known contact resistance. Plating can be acceptable there, but only if the plating is part of the joint design and the thickness is controlled. Bare copper is easier to specify when the joint is assembled with a contact paste or a Belleville washer.

Welded and brazed zones usually stay bare. Plating changes the resistance and the surface condition, and a weld schedule qualified on bare copper may not transfer to a plated face. If the weld is made after plating, the plating around the weld is damaged and needs a touch-up or a mask.

Threads are a separate case. A plated thread can be used if it is gauged after plating, but a masked thread keeps the fit predictable. Earth pads, test points and areas that will be soldered also deserve their own call. Each zone should have a dimension, a finish and a reason.

Masking methods

The method follows the shape and the volume. Tape is quick for flat faces and simple outlines, but it can lift at an edge or leave adhesive behind. Plugs and caps suit holes and threads, provided the fit is tight enough to stop solution creep. Lacquer suits complex shapes and small batches, but it needs a cure and a clean removal. Wax and stop-off compounds are useful for local areas, with the same contamination risk.

Rack shielding is a different approach. The part is held so that the area to be kept bare sits in a low current density region or behind a shield. It avoids consumables but is harder to control and is usually reserved for a dedicated line. The choice should be made with the plater, because the same mask can behave differently in an alkaline or an acid bath.

The plating sequence with a mask

The sequence starts before the part reaches the bath. The zones are marked on the drawing, the part is degreased and activated, and the mask is applied to a clean surface. A mask applied over oil or oxide traps contamination and produces a patchy deposit at the boundary.

After plating, the part is rinsed and neutralised with the mask still in place. Removing the mask before the rinse can leave solution under the edge, which shows up later as a stain or a corroded line. The mask is then removed, the part is dried and the boundary is inspected.

The order matters for the finish as well. If the part is bent after plating, the mask line bends with it and the coating can crack. If the part is bent before plating, the mask has to follow the formed shape. The process route should say which operation comes first.

The transition zone

Plating does not stop abruptly at the edge of a mask. Current density falls near the mask, so the coating thins over a distance that depends on the bath, the current and the geometry. A practical transition can be one to three millimetres. If the mask line sits inside the contact area, part of the joint face has a thin coating and part has none, which is the worst case for contact resistance.

The design rule is to keep the mask line at least the transition distance away from the contact area. If the joint face is twenty millimetres wide, a mask line two or three millimetres outside the face gives the plater room without thinning the contact zone.

A step at the mask line can also hold the joint off the mating face. A coating of five to ten micrometres is thin, but a ridge of plating at the mask edge can be thicker. If the joint relies on full face contact, specify that the transition sits outside the seated area, or that the step is removed by a light facing operation.

Inspecting a masked part

Inspection has two parts: the coating and the boundary. The coating is checked for thickness, adhesion and coverage. The boundary is checked for position, for a ragged edge and for any plating that has crept into the bare zone. A visual check under magnification is usually enough for the edge, provided the acceptance criteria are written down.

Thickness is measured on the plated area and, where the joint is critical, near the mask line. A single reading in the middle of a large plated face can miss a thin band at the edge. Adhesion is checked by a bend or a tape pull on a sample, and the result is recorded with the batch.

The bare zone itself needs a check for contamination. Adhesive residue, lacquer or wax can remain after stripping, and a contaminated contact face raises resistance. A solvent wipe and a visual check are a practical production control. For a critical joint, measure the contact resistance on a sample before the parts are packed.

• Measure thickness near the boundary as well as in the middle.
• Look for plating creep into the bare zone.
• Confirm that no adhesive, lacquer or wax remains on the contact face.
• Check that the mask step does not sit under a joint face.

What to put on the drawing and the RFQ

Show each bare zone with a dimension from a datum, a finish for the plated area, and a note on the transition. Say whether the zone is a contact face, a weld zone, a thread or a test point, because the plater will treat each one differently. Add the inspection method and the acceptance for the boundary.

The RFQ should include the annual volume, the bar size, the masking method if one is preferred, and the cleaning requirement after stripping. If the part is bent after plating, say so. If the joint face must be flat after plating, say that too, because a mask step can affect flatness even when the coating is thin.

A first article should include a photo of the boundary, the thickness readings and the result of the adhesion check. That record is more useful than a certificate that says the part was plated to a standard without saying where the mask sat.