Time-pressure dispensing

One of the simpler methods is time-pressure dispensing, shown in Figure 10-1. 

In time-pressure dispensing, the fluid is pressurized pneumatically. The amount of fluid dispensed is controlled by cycling pressure during the dispense cycle. 

The primary disadvantage is that time-pressure dispensing is less precise than some other dispensing methods. 

Time-pressure dispensing is often used in thermal interface applications, but lacks accuracy compared with other available technologies, with typical tolerances of 20%... 

Time-pressure dispensing can be used effectively on many simple, low value-added applications. 

Automated dispensing Time/pressure dispensing Data-driven process, rates to 50,000 dph, flexible, simple operation, and easy cleanup Higher speeds result in less consistent dots... 

Although time-pressure, auger, and piston systems are the most commonly used methods of needle dispensing, some other pump designs exist. For the purposes of fuel cell stack assembly, however, the considerations listed above apply to most technologies. 

A wide variety and numerous suppliers of automated dispensing equipment are available as listed in Table 4.22. An important consideration in selecting an automated, contact-type dispensing system is the pump used in the system, of which there are three types time-pressure, auger, and linear piston or positive displacement. " ... 

The third type of pump used in dispensers, linear piston, is a true positive-displacement pump. This pump uses a piston to change the volume of a reservoir that is fed from the main syringe. The displacement of the piston produces an equivalent positive displacement of fluid through the pump. Changes in viscosity have litfle or no effect on the repeatability of dispensed mass. Of the three methods, the repeatability of the linear-piston type is the best ( 1%, except for very small dispense volumes of less than 20 pl). " Repeatability of the time-pressure method is 10% and for the auger method is 4% (short term less than 30 min). 

Jet dispensing (low-viscosity adhesive), True volume positive dispensing wide range of solder paste and adhesives, TmFlow auger valve for medium viscosity adhesives, and time-pressure for low-to-medium viscosity adhesives. NA - Not Available. ... 

Time-pressure pump dispensing is a method to deposit adhesive or solder paste by applying a pressure pulse for a specific time duration on a reservoir of material (see Rg. 40.17). A precisely controlled quantity of adhesive or solder paste emerges from the selected orifice size, which is deposited on the circuit board. Often, the material comes prepackaged in a syringe that is inserted into the machine. 

FIGURE 40.17 Time-pressure pump dispensing technique (a) The dispenser is positioned at the circuit board site (b) the dispenser is lowered to near the surface at which time, a pulse of air pushes the piston a distance Ax to dispense a set quantity of adhesive (c) the dispenser is raised from the site, leaving the adhesive deposit. 

Archimedes screw pump dispensing utilizes an Archimedes screw to push the adhesive or solder paste out of a nozzle (see Fig. 40.18.) The speed and duration of a turn, as well as the size of the opening, determine the quantity of adhesive or paste deposited on the site. As in the case of the time-pressure technique, different deposit quantities can be realized by multiple spindle (heads) or a computer program that alters the screw speed or turn duration to change the amount of material dispensed from the same orifice. All other considerations with respect to the roles of adhesive or solder paste viscosity and shelf-life also apply to this technique. 

For syringe applications, programmable time/pressure controllers are used to produce beads and dots. An adjustable vacuum also can be used to control drips between dispense cycles. Pressure options relate to material viscosity with zero to one bar specified for low-viscosity applications, for example. Shot duration can typically be controlled between 0.01 and 99.99 s. 

Recently, we have developed a new version of this system, and this is shown in Figure 36. In this system, instead of the capillary being used as a blow-out pipette, the capillaries are used as wash-out pipettes. The capillaries are dispensed in a block, and the solution flows through the capillaries sequentially as they are presented to 2 press plates which apply pressure to the side of the block and maintain contact at all time. This done with 0-rings and has an effect similar to a suction cup sliding along the block. In Figure 36, it shows how 3 capillaries can be simultaneously emptied into three different channels of an autoanalyzer so that three determinations can be done simultaneously. 

A room is used for dispensing flammable liquids. The liquids are expected to have fundamental burning velocities less than 1.3 times that of propane. The room is 9 m long by 6 m wide by 6 m in height. Three of the walls are shared with an adjoining structure. The fourth and larger wall of the room is on the outer surface of the structure. The three inside walls are capable of withstanding a pressure of 0.05 bar. Estimate the vent area required for this operation. 

---