Pros and cons of the main application types.

The primary types of coating processes are dip coating, manual spray coating, programmable spray coating, and manual brush application. (Manual brush application will not be discussed here.)

Manual spray coating permits a variety of parts to be coated with minimal process development effort. It is conducted using a handheld spray coating booth that features self-contained material, solvent storage tanks and pumping capabilities, and variable coating pressure and flow rates. Exhaust ports minimize buildup of flammable solvents during the spray process. Exhaust is filtered to prevent coating buildup in the facility exhaust infrastructure. These filters must be changed regularly, as cured coating can build up on the filter material and reduce exhaust flow. The spray booth also contains a turntable to allow the operator to easily rotate the part undergoing coating, while minimizing handling of the uncured coating.

Drawbacks to handheld spray coating are the difficulty in changing material types and the need to mask any areas where coating is not desired. This process does not allow the operator to avoid any areas on the assembly without risking inappropriate variation in applied coating thickness, so areas where coating cannot be applied must be masked. Masking techniques can vary, but the two most common are manually applied tape and mechanical fixturing. Each has its advantages and drawbacks. Manually applied tape has a low material cost, but the application is labor-intensive and can see variation in the areas masked from assembly to assembly. Fixtures are more expensive, but ensure consistent masking. They require regular cleaning of coating buildup and must be revised if significant changes are made to the assembly design.

Changing material types in the handheld spray coating booth requires significant effort to ensure all residual material is purged from the feed lines between the storage tanks and spray gun. Residual material left in the feed lines can result in contamination of the new material, which could inhibit curing of the new material or affect its ability to adhere to the assembly.

A handheld spray coating booth also can be used for manual aerosol spray coating. This commonly occurs when spraying a small quantity of assemblies or where the risk of contamination is high from the coating used in the spray gun. This type of application leverages the advantages of the exhaust and turntable in the spray booth, while avoiding issues related to purging old material from the system. Concerns about masking remain the same across both manual spray processes.

Dip coating is well suited for high-volume applications with minimal coating type changes due to the amount of material required to fill the tank and initiate the process. Dip coaters can come with two separate tanks to permit use of two different materials without the need to remove and discard large quantities of coating.
The equipment permits changes in dip speed, tank dwell time and removal speed.

The biggest disadvantage of a dip coating process is the use of open tanks of material. To reduce the risk from flammable solvent evaporation, we use a nitrogen blanket to inert the area just above the open tank where solvent buildup can occur. A good exhaust system will evacuate any accumulated vapors. Additionally, the pot life of a coating in an open tank is significantly less than the unopened shelf life. This makes dip coating unsuitable for processes where coating is only occasionally performed.

Selective spray coating provides advantages of automating the spray coating process (low process variability, high throughput) without some of the disadvantages of manual spray coating (labor-intensive masking or expensive hard-tooled fixtures). The major disadvantage of automated spray coating is the initial programming required for each application. Once the programming task is completed, the automated process is well suited for high-volume applications regardless of product mix, as well as applications where masking isn’t a viable option.

ACI Technologies Inc. (aciusa.org) is the National Center of Excellence in Electronics Manufacturing, specializing in manufacturing services, IPC standards and manufacturing training, failure analysis and other analytical services. This column appears monthly.