Advancements in Fluid Dispensing
Electronic devices are growing in demand and shrinking in size, requiring manufacturing and packaging technologies to dispense new varieties of materials in microscopic amounts with ever-increasing speed and accuracy. As a result, fluid or liquid dispensing has grown from the early use of a small plunger manually pushing fluid out of a syringe to today’s sophisticated computer controlled devices. Traditional fluid dispensing methods are hampered by slow speed. Noncontact jet dispensing offers speed, precision and a lower cost of ownership. While the best method is ultimately determined by the material and application at hand, jetting provides a superior technology and a wide process window.
Time/Pressure Dispensing (TPD) methods use a syringe and a pneumatic valve. The TPD Syringe method utilizes controlled and timed air pressure, producing a measured shot of air that forces fluid out of a syringe, through the dispensing needle, and onto the work surface. Due to its simplicity and low cost, this method is widely used in bench top applications. To achieve more repeatable results, TPD valve methods incorporate a pneumatic valve to better control the volume of fluid being dispensed. For low to medium viscosity fluids, a Needle valve or a Diaphragm valve is recommended. An automated controller opens a piston/needle assembly or diaphragm, which in turn opens a path for the fluid to flow through. When the air signal stops, the piston returns to the original position and closes the valve. Time and pressure can be precisely controlled to dispense dots or continuous beads of fluid. Higher viscosity fluids often require a Spool Valve, where the piston drives down to open the fluid path. When the liquid dispenser cycle is completed, the spring of internal compression lifts the piston up to close the valve, creating a natural back-suction feature that prevents drips.
A common weakness of time/pressure dispensing is its susceptibility to fluctuations in the dispensed fluid‘s viscosity. If the fluid becomes thicker or thinner, pressure must be adjusted accordingly to maintain repeatability. One way to avoid this problem is to use a Semi-Positive Displacement Method such as a Rotary or Auger pump. The auger pump uses a rotary feed screw (auger) that rotates inside a fluid chamber to push fluid from the auger inlet to the dispense tip. A better way to control repeatability of the dispense process is with a True Positive Displacement Method. As the name suggests, pushing down in a displacement chamber pumps an identical volume of fluid out of a matching dispensing chamber. Displacement pumps vary by the type of mechanical action used, such as the Progressive Cavity Pump, Piston Pump, and Peristaltic Pump.
Jet dispensing is common in SMT applications such as silver epoxy for die bonding, adhesive dispensing for component assembly and encapsulant under fill for flip chips. Advancements in jet technology and system design continue to expand the range of jet-able materials, such as jetting silicone phosphor in LED assemblies. Non-contact dispensing also has been the biomedical industry’s method of choice. For creating assays and lab-on-a-chip materials, jetting offers a contamination-free method for making tiny deposits of rare, sensitive, or expensive fluids with little waste.