Dispensing needles

Using a dip needle dispensing the correct amount of fluid... 

Needle dispensing is described as extruding material under pressure to deposit dots or lines of materials and can be used for a wide variety of materials. In all cases, the tip of the needle or nozzle is positioned close to the dispensing surface and a combination of gravity and surface tension pull the material away from the dispensing tip. 

Discrete deposits or dots can be created by holding the dispense tip stationary relative to the dispensing surface. Lines or beads of material can be created by moving the tip relative to the surface while extruding the material. The difference between needle dispensing technologies lies in the mechanism used to create the extrusion pressure. 

Auger pumps and piston pumps are also commonly used in needle dispensing. Examples are shown in Figures 10-2 and 10-3. 

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. 

The jetting mechanism propels small dots at rates up to 200 Hz in four software-controlled modes distance based, time based, fixed number of dots per line, and continuous line with breaks, enabling the construction of many different sizes and shapes of dots and lines. Because the momentum of the fluid comes from the jetting action, proximity to the substrate (dispense gap) is much less critical than it is in needle dispensing. 

The inherent simplicity of the jetting mechanism enables it to adapt to a wider variety of adhesive fluids, dispense patterns, and circuit board geographies than is possible with a needle dispenser. This simplicity facilitates multiple processes using a single jetting dispenser. 

For most users, the most attractive feature of jetting is the increased speed compared with needle dispensing. The primary reason for jetting to be faster than needle dispensing is the reduced amount of mechanical travel required by the jet mechanism. 

Stencil printing is a fast efficient process for long production runs and is widely used in surface mounting of components. In surface mounting, the solder paste is generally dispensed first by stenciling, followed by needle dispensing the adhesive however, these processes may be reversed or both the solder and adhesive can be selectively dispensed by needle. 

Jet dispensing, also known as jetting, is a direct-dispensing process whereby the adhesive paste is forced through a fine nozzle and programmed to flow onto selected areas controlled from a computer database. It is an automated method that differs from syringe/needle dispensing in that contact with the surface is not required to produce the dots, lines, or patterns. 

---