The principle of time-pressure dispensing is shown in Figure 1. In this simple syringe for a hand application, as might be used for solder paste dispensing during rework, an air-driven plunger drives the resin out of the nozzle.
Dispensing equipment must be able both to control flow rate and to start and stop the dispense cycle as required. This may be easy to state, but is not always easy to achieve. Many adhesives are compressible, thixotropic and even tend to solidify under load. The principal factors likely to prevent correct flow rate being achieved are the viscosity of the material, the effects of temperature change on the viscosity, and hysteresis in the equipment.
For these reasons, more sophisticated mechanical methods are increasingly favoured, especially for the fast dispensing of glues and underfill. Figure 2 shows one solution, which uses an auger screw to drive predetermined quantities of material to the needle.
Practical dispensers have a considerable degree of automation in order to achieve high throughput rates. The needle distance is the critical factor in determining the dot profile and to reduce stringing, and the correct distance varies with needle size. In particular when dispensing glue, it is necessary both to maintain a constant syringe temperature, and to sample check dot size and profile, especially after a break in production.
Single-component materials can often be applied direct from the package, whether syringe, cartridge or drum. However, the need to proportion and mix two-component polymers before applying them results in equipment being quite complex. Some materials are supplied in ‘side by side’ and ‘coaxial’ cartridge dispensing units, where the two components are contained in a single package but only meet when dispensed through a mixing nozzle; more generally the two components are transferred from separate packing or reservoirs through metering units which feed an in-line mixer. The mixing action can be carried out by having a rotor in the mixing chamber, or by pumping the materials through a series of baffles. Once mixed the polymer has a finite working life, and mixer, down-stream pipe-work and nozzle all need to be cleaned from mixed resin.
Syringe-based dispensers apply pressure to the rear of a plunger to displace adhesive from the syringe. Until it is dispensed, the resin is not under pressure, so there is no surge effect. The dispense pressure is supplied from a controller unit which is normally fitted with a venturi valve. This produces a vacuum behind the plunger when dispense pressure is switched off, pulling the material back to assist cut-off.
There is generally a minimum pressure required to ensure adhesive flow through any nozzle, which depends on both the viscosity of the polymer and the diameter and length of the nozzle used. A relatively high dispense pressure is normally used to ensure that the compound is adequately dispensed. Note that it is also easier to regulate from a higher supply pressure, thus reducing dispensing variation due to pressure variations.
Most automatic dispensing machines use syringes pre-loaded by the paste manufacturer. Syringes are usually made of rigid polyethylene or polypropylene which will not bow or bulge, so that, at normal working pressures, the piston is both air-tight and paste-tight. Great care must be taken to exclude any trapped air which would otherwise cause ‘misses’ in dispensing and continued dispensing of paste after pressure has been released.