Fluid sprinkling – spraying – and diffusing – Processes
Reexamination Certificate
1999-10-29
2001-08-07
Douglas, Lisa Ann (Department: 3752)
Fluid sprinkling, spraying, and diffusing
Processes
C239S290000, C239S423000
Reexamination Certificate
active
06270019
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to apparatus for dispensing liquid or viscous materials and, more particularly, to an apparatus and method for dispensing droplets of liquid or viscous material onto a surface of a substrate.
BACKGROUND OF THE INVENTION
Liquid dispensing systems have become an integral part of the electronics manufacturing process for depositing underfill, encapsulants, solder fluxes, surface mount adhesives, conformal coatings and other materials onto a substrate, such as a printed circuit board. Each liquid dispensing system used in the electronics manufacturing process has a particular dispensing characteristic that is determined in large measure by the desired liquid dispense pattern on the substrate, the liquid flow rate and/or liquid viscosity of the dispensed material, and the desired electronic component assembly throughput through the dispensing system.
For example, in the assembly of ball gate arrays (BGA's) and other electronic components onto a ceramic or FR-4 substrate, the component must be soldered onto the substrate to form the necessary electrical interconnections. As each component occupies a predetermined area on the substrate, the liquid dispensing system must have the capability to dispense liquid or viscous material in a controlled manner within the selected component areas. Typically, the liquid dispenser is mounted on a movable platform to provide automated and accurate movement of the liquid dispenser in three dimensions relative to the substrate with the aid of a machine vision system.
Prior to the component soldering process for establishing the electrical interconnections, it is often necessary or at least desirable to dispense a layer of solder flux onto a substrate within rectangular areas associated with each component. To provide this capability, liquid material dispensers have been developed in the past that use filled syringes or reservoirs of solder flux, and dispensing valves to dispense droplets of flux material onto the substrate in a controlled manner with up to 25,000 to 40,000 dots of fluid per hour for a typical dispenser platform. These liquid dispensers, known as “dot jetting” dispensers, are programmed to dispense an array of liquid or viscous material droplets within each selected rectangular area which are then allowed to flow into contact with each other to form a generally rectangular thin layer of flux within the component area. “Dot jetting” dispensers are also used to dispense multiple droplets of adhesive onto a substrate to adhesively bond an electronic component to the substrate at a desired location.
In the “dot jet” process, each droplet of material is formed by rapidly closing a valve against a valve seat to impart kinetic energy to the fluid material within the dispenser. Closing of the valve causes the material to break off from the nozzle of the dispenser from which it is being dispensed and propel toward the substrate and onto the substrate as a droplet. Due to the high velocity imparted to form the jet or droplet of viscous material, the jet's tail may break into smaller droplets forming satellite portions associated with the main droplet. These satellite portions have a tendency to stray or deviate from the dispensing axis and cause problems when they hit the substrate outside of the area of the main droplet. The satellite portions may fall within areas of the substrate where it is undesirable to have a coating of viscous or liquid material, and may adversely affect the uniformity of the material coating applied to the substrate. As a result, the outlet of the “dot jetting” dispenser must typically be positioned relatively close to the substrate, such as in a range of about 1.5 mm to about 2.0 mm, to limit how far the satellite portions may deviate from the dispensing axis.
However, it will be appreciated that any reduction in the height of the nozzle tip relative to the substrate has a tendency to also reduce the size of droplet that may be dispensed onto the substrate since the droplet's ability to expand in flight toward the substrate is affected. This results in the need for more droplets of material to be dispensed to cover a given area of the substrate, and also results in increased dispense cycle times and reduced throughput of components through the dispenser.
Thus, there is a need for a liquid dispensing system that dispenses droplets of liquid or viscous material in a more accurate and repeatable manner. There is also a need for a liquid dispensing system that improves control of the dot shape and application of the dots toward a substrate. There is also a need for a liquid dispensing system that reduces dispense cycle times and improves throughput of components through the dispenser over known dispensing systems.
SUMMARY OF THE INVENTION
The present invention overcomes the foregoing and other shortcomings and drawbacks of liquid dispensing systems and methods heretofore known for forming and dispensing droplets of liquid or viscous material onto a substrate. While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention.
The liquid dispensing system of the present invention is particularly adapted for dispensing a droplet of liquid or viscous material, such as solder flux or adhesive, onto a substrate. The dispensing system includes a nozzle assembly that is configured and adapted to dispense a droplet of liquid material, and also to discharge a sheath of pressurized air that substantially surrounds the dispensed droplet and any satellite portions associated therewith during flight toward the substrate. The sheath of pressurized air is operable to contain the satellite portions within the discharged sheath to prevent them from falling onto areas of the substrate where it is undesirable to have a coating of liquid or viscous material. In this way, the height of the dispenser outlet can be increased, such as to about 2.00 mm or greater, without adversely affecting the dispensed pattern of dots.
More particularly, the nozzle assembly includes a liquid dispensing nozzle body that is adapted to be connected to a dispenser body of a liquid dispenser. The liquid dispensing nozzle body includes a liquid discharge passageway in fluid communication with the liquid dispenser for dispensing the droplet of liquid or viscous material toward the substrate. An air discharge body is operatively connected to the liquid dispensing nozzle body and includes an air discharge orifice for discharging the sheath of pressurized air. The air discharge orifice is configured and located relative to the liquid discharge passageway to discharge the sheath of pressurized air so that the sheath substantially surrounds the dispensed droplet and any satellite portions associated therewith during flight toward the substrate to contain the satellite portions within the discharged sheath.
In one aspect of the present invention, the liquid discharge passageway and the air discharge orifice are co-axially aligned with one another. The air discharge orifice may be an annular orifice that surrounds the liquid discharge passageway. The air discharge body includes a plenum connected to the air discharge orifice. The air discharge body may have multiple radially directed vanes disposed in the plenum for guiding pressurized air flow through the plenum.
The present invention also contemplates a method for dispensing a droplet of liquid or viscous material onto a substrate. The method generally involves dispensing a droplet of material from a liquid discharge passageway that tends to separate into a droplet and at least one satellite portion during flight toward a substrate. Pressurized air is discharged as a sheath from an air discharge passageway so that the droplet and the satellite portion associated therewith are substantially surrounde
Douglas Lisa Ann
Nordson Corporation
Wood Herron & Evans LLP
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