Selective Coating Robot Path Programming

Selective coating applies material only where it is wanted, using a robot or a gantry to move a dispense valve along a programmed path. It avoids the masking that spraying and dipping require, and it puts the coating exactly on the areas the design identified. The whole process rests on the accuracy of the path and the consistency of the dispense, and both of them drift in ways that are invisible until the coating is inspected.

Planning the Path

The path has to cover the areas that need protection while avoiding the areas that must stay clear. That means the programmer works from a coating drawing that shows both, with tolerances for the boundary. Where the boundary tolerance is tight, the path must be taught with more points and the valve must be able to start and stop cleanly, because a slow-acting valve leaves a trail of material at each transition.

Coverage also depends on the shape of the board rather than only on the drawing. A tall component creates a shadow that the dispense pattern must account for, and a recessed area may need a different nozzle angle or a slower pass. Programming on the actual assembly, with the real component heights, produces a path that works; programming from a CAD model alone usually requires correction at the machine. Coating process selection determines how much of this the method can tolerate.

Dispense Parameters

The dispense parameters are the flow rate, the valve open time, the travel speed and the nozzle height, and they combine to determine the film that is laid down. Increasing speed thins the film unless the flow is increased to match, and increasing the height spreads the material and blurs the boundary. Each parameter has a working range, and the range narrows as the required boundary tolerance tightens.

The valve itself needs calibration and maintenance. Wear in the seat changes the flow, dried material changes the response time and a partially blocked nozzle reduces the deposit without changing anything the machine can see. A periodic check of the deposited weight on a sample, or a measurement of the coating thickness after cure, is what keeps the parameters meaningful. Coating application practice provides the baseline for those checks.

Viscosity and Material Drift

Material viscosity is the variable that most often defeats a well-programmed path. It changes with temperature, with the age of the material and with the time it has spent in the system. A coating that flows correctly in the morning may be thicker in a warm afternoon, producing a narrower and thinner film from the same program, and the operator will attribute the change to the machine.

Controlling the material temperature at the dispense head, and checking the viscosity or the flow rate against the supplier’s range on a schedule, keeps the program valid. Where the material is supplied in a syringe or a cartridge, the lot number and the expiry should be recorded, because a change of lot is a change of process until it is verified. Masking decisions interact here, since a change in film thickness changes the behaviour at the mask boundary.

Coverage Verification

Verification has to answer two questions: whether the coating is where it should be, and whether it is thick enough. Ultraviolet inspection, where the material contains a tracer, shows the coverage quickly and reveals the missed areas that a visual check under normal light cannot see. Thickness measurement on a witness piece or by an eddy current gauge confirms that the film is within the specified range.

The checks should be performed at the start of a build, at defined intervals and after any change to the program or the material. Where a gap is found, the response is to correct the path or the parameters and to re-verify rather than to touch up the board, because a touch-up hides the process problem. Coating inspection describes the acceptance criteria that the verification is measured against.

Curing Consistency

Curing must be matched to the material and the film thickness. A selective coating that is thicker in one area than another needs a cure schedule that reaches the thickest region, which may over-cure the thin areas. Where the material is moisture cured, the surrounding humidity matters and should be recorded, because a dry winter week can leave a film that is soft when it should be hard.

The cure should be verified rather than assumed. A solvent rub, a hardness test or an adhesion test on a witness piece confirms that the film has reached its intended state, and the results should be recorded with the cure parameters. Where curing is performed in an oven, the load and the position within the oven affect the result, which is the same lesson that applies to any batch thermal process.

Program Control and Change Management

A coating program is a process parameter and should be controlled like one. It should have a revision, a backup and a record of the verification that was performed when it was released. Where the program lives only on the machine, a software update or a hardware change can lose it, and the product then depends on somebody’s memory of the correct path.

Changes to the product are the most common reason for a program to become invalid. A moved component, a new connector or a change in the keep-out areas all require the path to be updated and re-verified, and the change should be triggered by the engineering change process rather than discovered at the machine. Linking the program revision to the product revision is the simplest way to make that automatic.

Acceptance and Its Evidence

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

FAQ

Can a coating program be developed from CAD alone? It can be started that way, but it should be verified on the real assembly, because component heights and board variation change the coverage.

Why does coverage change during a shift? Usually because of material viscosity drift with temperature, or because the valve has begun to clog. Check both before re-programming.

How is coverage verified? By ultraviolet inspection where a tracer is present, plus thickness measurement on a witness piece or the assembly.

What should be recorded? Program revision, material lot, viscosity or flow check, cure parameters and the result of the coverage and thickness verification.