SMTA International Conference Proceedings


Author: Stuart Erickson
Company: Ultrasonic Systems, Inc.
Date Published: 9/24/2006   Conference: SMTA International

Abstract: Precise control of coating deposition is critical to the application of conformal coatings to selected areas of printed circuit board assemblies. The coating must be applied in a defined pattern and film thickness to ensure that there is adequate coating present on areas to be coated such as soldered connections and no coating on other areas such as electrical connectors. In many cases, the areas to be coated are immediately adjacent to areas where coating cannot be applied (no-coat areas). Automated coating application techniques are increasingly being utilized to replace the less controllable hand spray and dipping techniques for conformal coating application. Some new coating application methods include air-atomized spray, film coating and ultrasonic atomized spray.

In recent years, automated systems have been developed for the application of coatings with the goal of eliminating the masking and de-masking process. These systems typically consist of a coating applicator for large areas, a coating applicator for small areas, a motion and positioning mechanism for the coating applicators and a system controller.

Typically spray valves or film coating applicators are used for coating the large areas on the substrate quickly to minimize the time required for coating application. However, typical large area coating applicators deposit the coating pattern with irregular edges. The coating cannot be applied close to the no-coat areas with these applicators due to the irregular edge of the coating pattern. In order to eliminate masking another applicator is required to coat the smaller areas immediately adjacent to the no-coat areas. These types of applicators have been effective in certain circumstances but do not produce a uniform coating and do not completely eliminate the requirement for masking. Limitations of dispensing applicators include imprecise flow rate control, difficulty producing short coating line segments, heavy coating deposition at the start and end of a coating segment and susceptibility to uneven or warped substrate surfaces. The coating flow rate from a dispensing valve is set with a manual screw to adjust the stroke of a piston that is connected to a needle. The distance that the needle moves from the seat controls the effective orifice size at the nozzle tip and thus the flow rate of the coating. This manual adjustment is subjective and will necessarily produce different results each time the valve is adjusted. The needle and seat arrangement of dispensing valves produces discontinuities as the flow starts and stops. This flow behavior coupled with the head motion profile tends to produce heavy spots at the start and end of a coating segment and makes it difficult to create a short coating segment. Programming techniques can somewhat overcome this effect, but the process can be tedious and difficult to repeat.

To apply coatings to very small areas or in straight lines with a dispensing valve, an external dispensing needle is required. To achieve optimal results, the outlet of the needle must be very close to the substrate, typically within 1 mm. If the substrate is uneven or slightly warped coating skips may result due to the changing distance between the needle and substrate. Additionally, the needle has a tendency to contact previously applied coating and pull it along with it causing skips and smears. The needle is also subject to being damaged if it comes into contact with the substrate.

An automated method for the precise application of conformal coatings has been developed that utilizes the combination of a dual mode “nozzle-less” ultrasonic spray head, a precision digital dispensing head and a precision XY- Z-?-Ø motion control platform. The ultrasonic spray head uses ultrasonic energy to break the liquid into small drops to form the spray, but the liquid does not pass through the ultrasonic device. The liquid is applied externally, to a solid surface, vibrating at an ultrasonic frequency (> 20 kHz). Directed air streams are used to expand or focus the ultrasonically produced spray, providing two distinct spray patterns: 1) narrow mode with a pattern width of approximately 5 mm at a distance of 25 mm between the tip of the head and the substrate and 2) wide mode with a pattern width in the range of 3 to 25 mm, proportional to the distance between the tip of the head and the substrate. The digital dispensing head uses a micro flow solenoid valve and a streaming nozzle that produces a pattern width of 1 mm from a distance of 5 mm to 15 mm between the nozzle and the substrate.

This paper considers a particular advancement in precision application of conformal coating using the combination of Ultra-Spray technology with integrated fluid delivery applicator, the Micro-Line digital dispensing head and precision X-Y-Z-?-Ø motion control platform.

Key words: spray coating, conformal coating, ultrasonic spray, printed circuit board assemblies, selective coating.

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