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Fig. 4. Diaphragm Valve
With the diaphragm valve (Fig. 3b), the material dispensed is controlled by a plunger acting on a diaphragm, which in turn activates a poppet to open and close the flow of material to the tip. The longer the poppet is in the open position and the higher the pressure on the material, the greater the quantity dispensed. Pressure typically is applied through an electric solenoid or motorized device. When the poppet returns to the closed position, the diaphragm pulls back from the seal, causing a "suck back" action that provides a sharp cutoff of material. The constant pressure at the material inlet and "suck back" action give the diaphragm valve much higher accuracy than the basic syringe valve.
The diaphragm valve dispenses beads and fills in medium-precision volumes but has less accuracy when dispensing dots (Fig. 4). It has limited applications and typically works best with low-to medium-viscosity materials such as solder masks and conformal coatings. The diaphragm valve is most widely used to dispense lines of solder mask for wave solder applications.