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Nozzle-less Difference                                                                                                                 USI’s core technology is called tCAT, a proprietary nozzle-less ultra-Thin Coating Application Technology for the thin, uniform application of a wide variety of coatings from pure solutions to high-solids suspensions and slurries. tCAT utilizes “nozzle-less” ultrasonic spray technology combined with a precision metering pump liquid delivery system, which are part of a coating system platform with a motion and positioning system for the spray head, and a transport system for the substrates to be coated.

How it Works                                                                                                         The Ultra-Spray head is an integrated assembly consisting of an ultrasonic transducer with a spray forming tip, a liquid applicator and air directors. The ultrasonic transducer vibrates at an ultrasonic frequency (> 20 kHz). The particular ultrasonic frequency is selected based upon the material to be sprayed and the coating application requirements. In general, a lower frequency ultrasonic transducer is capable of spraying a higher viscosity liquid and producing higher flow rates. The amplitude of vibration of the spray-forming tip is also set with the ultrasonic generator.  

The coating liquid is delivered to the spray-forming tip on the ultrasonic transducer by liquid applicator. The liquid is stored in a reservoir and fed to the liquid applicator at a precisely controlled flow rate by a positive displacement Precision Metering Pump (PMP). The ultrasonic vibrations of the spray-forming tip break up the liquid into small drops and propel them from the tip in the form of a spray. The spray produced with ultrasonic energy alone has a very low velocity “sheet-like” pattern.  

Air directors are used to produce air streams to shape and accelerate the ultrasonically-produced spray. The air director impinges a jet of air on tip of the spray head opposite the liquid feed side. The resulting airflow entrains and expands the ultrasonically-produced spray to produce a flat (rectilinear) pattern up to five (5) times the width of the pattern produced by the ultrasonic energy alone. When using the air directors, the spray pattern width is a function of the spray-forming tip width and tip-to-substrate distance.

This nozzle-less ultrasonic spray head produces a substantially rectangular shaped coating distribution on the substrate. The rectangular shape is ideal for producing a uniform coating on a flat surface. The actual coating distribution is uniform across about 90% of the width of the sprayed pattern and the edges are “feathered” from the applied coating thickness to zero thickness. The figure below shows the spray pattern distribution comparison between the nozzle-less spray head and a traditional spray head as well as the resultant distribution after a series of overlapping passes.

Precision Metering Pump (PMP)                                                                                                   A highly accurate liquid delivery system has been developed that incorporates a positive displacement metering pump. The liquid flow rate is controlled by micro-stepping drive to ensure that the coating material is delivered to the ultrasonic spray head at a precisely controlled flow rate.

Since some coating liquids have suspended particles that can settle this liquid delivery system has the capability to keep the coating mixed while maintaining an accurate and repeatable flow rate of the coating to the spray head.

A dual pump version is also available that allows continuous operation, without the need to wait for the single pump to refill. With this configuration, as one pump is being used to feed the spray head, the other pump is being filled from the holding reservoir. The dual pump can also be configured to recirculate and stir the coating liquid, thus ensuring that the suspended particles do not settle out in the liquid feed lines or holding reservoir.

The Coating System Platform                                                                                                     The coating system platform consists of an X-Y-Z-Ɵ-Ø gantry system for the motion and positioning of the nozzle-less ultrasonic spray head, as well as integrated control of the precision metering pump liquid delivery system, and transport mechanism for the carriers. All critical process parameters are managed by the platform software and control system. Additionally, a log of the critical process data is recorded and stored in a process log file.

Coating Thickness Uniformity                                                                                                        In general, the uniformity and thickness of the applied coating layer are directly related to the following factors:

•  Uniformity and stability of the spray pattern produced by the ultrasonic spray head
• Accuracy and stability of the coating flow rate delivered to the ultrasonic spray head
• Consistency of the speed of the ultrasonic spray head relative to the substrate
• Programmed coating recipe – number of applied layers, the application path, and the coating flow rate

The speed and motion path of the ultrasonic spray head is controlled with a closed-loop servo drive system.  The servo drive is tuned for uniform head speed so the variation of the head speed is negligible.  The liquid flow rate is determined by the rate of displacement of a piston in the pump.  The motion of the piston is controlled by a micro-stepping drive system so the variation of liquid flow rate is also negligible.

For a given coating recipe, the variation in the total amount of liquid applied to the substrate is less than ±0.1%. This can easily be verified since the system software logs the total amount of liquid applied to each substrate that is coated.   Any variations in the head speed and the liquid flow rate are accounted for in the process log

 Examples of the coating uniformity achieved with tCAT are shown below.

 Example #1 – Photoresist on Touch Panel Glass

In this example, a 10% solids formulation of photoresist is applied to the touch panel glass plate at a wet film thickness of 15 microns.  The glass plate is dried on a hotplate and the resulting dry thickness is approximately 1.5 microns.  As shown in the measured thickness plot, the thickness variation is negligible. 

Example #2 – Fuel Cell Coating

In this example, a precious metal-based catalyst is formulated at 5% solids by weight.  The required loading is 1.5 mg per square centimeter.  The loading achieved with tCAT is 1.45 to 1.55 mg per square centimeter, 99% of the time.

Example #3 – EMI Shield Coating to Components

In this example, a high-density silver slurry with a solids content of about 72% is applied to components.  The cured coating layer provides EMI shielding to the components when they are placed into circuit assemblies.  The top side layer thickness is 6 +/- 0.5 microns and the side surface thickness is 3 +/- 0.5 microns.  The repeatability of this process at different process settings is shown in the thickness variation plot.  Data provided by Henkel Electronic Materials.  

Example #4 – Anti-reflection Coating to Solar Glass

In this example, an anti-reflection coating is applied to a 1,200 mm x 600 mm glass plate.  To provide the required anti-reflection properties a uniform coating layer thickness of 120 nanometers is required.   The thickness of the dry layer was measured at 80 points across the surface of the glass.  The measured data is:

Target: 120 nm; Ave.: 120.2; Min.: 117.5 ; Max.: 122.9; S.D.: 2.1

Ultrasonic Spray Heads – CAT ILDS Head and Blade Head                                     Two basic ultrasonic spray head types are available: the Dual Mode CAT ILDS Head and the Blade Head. 

 ILDS Head 

The ILDS Head has two (2) modes of operation; wide mode and narrow mode. Each mode of operation is selected in the Prism platform software via the coating recipe and can be selected at any time without any mechanical adjustment or other intervention. The spray is expanded to a rectilinear pattern in the wide mode and focused in a narrow circular pattern in the narrow mode. The particular mode of operation is selected based upon the particular coating application requirements. The width of the spray pattern produced by the ILDS head is from approximately 2 mm to 30 mm and this head is typically used to coat smaller areas. 

 Blade Head 

The blade head is designed to produce a wide rectilinear pattern from about 25 mm to 300 mm wide. This head is typically used to coat larger areas quickly. A variety of blade head models are available to meet specific coating application requirements. 

Since the spray is produced with ultrasonic energy rather than pressure, and because a low-velocity air stream is used only to shape the spray, very little coating is wasted due to overspray with a transfer efficiency of 97% to 99%. 

 All process parameters for the Ultra-Spray head are set electronically for maximum control and flexibility, including: 

• Liquid flow rate
• Air pressure
• Head height
• Head speed
• Motion path