Dispensing – Automatic control – Constant weight – volume or pressure control by output
Reexamination Certificate
1999-04-23
2001-01-16
Derakshani, Philippe (Department: 3754)
Dispensing
Automatic control
Constant weight, volume or pressure control by output
C222S061000, C222S077000
Reexamination Certificate
active
06173864
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to automated equipment used for dispensing viscous materials. More particularly, the invention relates to systems and methods for dispensing accurate amounts of viscous material, such as during printed circuit (PC) board assembly.
BACKGROUND OF THE INVENTION
In certain fields, such as the field of PC board assembly, small amounts of viscous materials must be applied accurately to a substrate. Such viscous materials have a viscosity greater than about 50 centipoise and include, for example, adhesives, solder flux, solder paste, solder mask, grease, oil, encapsulants, potting compounds, epoxies, dye attach pastes, silicones and RTV.
Manufacturers of PC boards use processes including screening, pin transfer and various other dispensing systems and methods for applying viscous materials to a PC board. Dispensing systems may include either a syringe dispenser or an ON/OFF dispenser valve. Syringe dispensers are used as contact dispensers actuated with pneumatic mechanisms, electro-mechanical mechanisms or positive displacement valves. The tip of a syringe dispenser is placed very close to the substrate, for example, at a distance of 0.005 inches for a very small droplet and a distance of about 0.060 inches for a larger droplet. The viscous material is pushed out of the syringe tip and contacts the substrate while it is still connected to the syringe tip. Syringe style contact dispensers have certain desirable attributes. However, one of the main drawbacks with syringe dispensers relates to their relatively slow operating speed. That is, it is difficult to dispense more than about ten dots of viscous material per second with a syringe dispenser. Stringing of viscous material away from the syringe tip is also a problem that reduces accuracy and quality.
Dispenser valves, which can operate as non-contact ON/OFF dispensers, exhibit certain advantages relative to other dispensing systems and methods. For example, dispenser valves can more quickly dispense a number of minute drops of viscous material on a PC board. This is generally due to elimination of z-axis movement which is necessary with contact dispensers. Also, dispenser valves can reduce or eliminate stringing problems and other problems associated with contact dispensing methods since the dispenser nozzle may be maintained at a greater distance from the substrate. Dispenser valves also allow a greater amount of viscous material to be cleanly and accurately dispensed at a single location by discharging more than one drop at the location.
Despite the various improvements in this field, including those pertaining to contact and non-contact dispensers, additional improvements are necessary to more accurately control the dispensed amount of viscous material. In this regard, variations in the amount of dispensed viscous material can occur due to variations in the characteristics of the viscous material itself or, for example, other machine or dispensing system tolerances. In other words, although the various operating parameters of the dispensing system may be maintained constant, variations may occur in the amount of dispensed material due to other outside factors. For at least these reasons, it would be desirable to provide systems and methods for more accurately controlling the amount of viscous material discharged from a dispenser.
SUMMARY OF INVENTION
In accordance with the present invention, a system is provided which accurately dispenses controlled amounts of viscous material by feeding information on the dispensed amount of viscous material back to one of various control devices which operate a viscous material dispenser. The system can more specifically comprise a pressurized supply of viscous material, a dispenser connected in fluid communication with the pressurized supply and a feedback control operatively connected with the pressurized supply. The feedback control includes a viscous material measuring device operative to measure a dispensed amount of the viscous material. A memory device stores a value representing a control amount of the viscous material. A comparing device compares the dispensed amount with the control amount and produces a correcting output signal which may then be used directly or indirectly to adjust the dispensed amount. While the preferred embodiment of this invention involves the use of at least one non-contact dispenser, various aspects of the invention will also benefit contact dispensing systems.
Three manners are presently contemplated for using the correcting output signal to adjust the dispensed amount of viscous material. First, the output signal may be used by a first transducer to change the pressure of the pressurized supply of viscous material. Second, the output signal may be used by a second transducer to adjust fluid pressure supplied for operating the dispenser. Third, the output signal may be used to change the duration that the dispenser is maintained in an open position to dispense the viscous material. These manners of adjustment may be used individually or in any combination and order of two or more. In the preferred embodiment, each of these manners of adjusting the amount of dispensed viscous material is used in succession within an integrated feedback control system. That is, the feedback control first changes the pressure of the pressurized supply of viscous material within predefined limits. If this change reaches one of the predefined limits and the dispensed amount of material does not adequately match the control amount, then the second manner of adjustment is used to correct the dispensed amount. If this still does not produce the desired amount of dispensed material, then the system reverts to the third manner of adjusting the dispensed amount as described above.
The pressurized supply connected with the dispenser more specifically includes a syringe containing the viscous material and operated by a pressurized fluid which forces viscous material from the syringe into the dispenser. The first transducer may be a voltage to pressure transducer which converts a voltage signal representing the desired correction into a pressure of the pressurized fluid used to operate the syringe. The dispenser may be a known ON/OFF non-contact dispenser valve used for dispensing small amounts of viscous material as drops. The dispenser valve may be operated by a conventional solenoid-operated valve which supplies a fluid pressure, such as air pressure, to the non-contact dispenser valve. The feedback control system may be used to adjust the fluid pressure and/or the ON time of the control valve such that the non-contact dispenser valve dispenses an amount of viscous material which is closer to the control amount than the dispensed amount. The viscous material measuring device may include, for example, a weigh scale or any other suitable measuring device, such as a vision system which measures dot diameter, area or volume.
Various methods of controlling an amount of viscous material discharged from a dispenser may be practiced in accordance with this invention. For example, a method according to the invention may include discharging a first amount of the viscous material from the dispenser, measuring the first amount of viscous material, comparing the first amount to a commanded or desired control amount stored in a memory device, and adjusting the pressure of the pressurized supply to correct for a difference between the first amount and the control amount. Another method which may be used alternatively or in conjunction with the previously described method involves discharging a first amount of the viscous material from a dispenser valve, measuring the first amount of viscous material, comparing the first amount to a commanded control amount stored in a memory device, and adjusting a fluid operating pressure used to actuate the dispenser valve to correct for a difference between the first amount and the commanded control amount. An alternative or additional method which may be practiced in accordance with the invention in
Babiarz Alec J.
Byers John P.
Lewis Alan R.
Reighard Michael A.
Suriawidjaja Floriana
Derakshani Philippe
Nordson Corporation
Wood Herron & Evans L.L.P.
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