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Matching Fluid Dispensers To Materials for Electronics Applications

Positive displacement dispensers

This type of dispenser is commonly referred to as piston positive displacement because the technology uses a piston to displace material from a chamber (Figs. 3f and 7).To be considered true positive displacement, a dispenser must be designed so material flow is absolutely sealed off during the actual dispensing process. Positive displacement is available in two basic technologies. One is motor-driven and the other is solenoid-driven. The actual models available differ in design by individual manufacturer, but the key to the applications they are used for stems from which method activates the dispenser.

Both types of positive displacement dispensers are typically used with medium-to high-viscosity materials, and offer high-volume repeatability regardless of temperature, viscosity, and pressure changes. The actual volume of material dispensed is controlled by the adjustable displacement of the piston entering the chamber. The solenoid-driven version also works with low-viscosity materials when con figured with a special adapter valve.

Motor-driven positive displacement technology is relatively new. It works by feeding material under low pressure from a holding container into a small dispensing chamber. The feed mechanism typically differs by manufacturer, but this does not affect the actual dispensing process, which uses the same piston technology. After material is fed into the small dispensing chamber, a motor-driven piston activates in this chamber, pushing the material through the dispensing tip at high pressure. When the piston starts moving to dispense, the material inlet to the chamber is closed.

Figure 2

The motor-driven pump typically is used to dispense medium-to high-viscosity materials such as adhesives and silicones. Although this technology is generally slower than solenoid-activated piston devices, some motor-driven pump configurations can accomplish an accurately controlled dispense rate that allows beads of material to be dispensed in patterns such as lines and circles.

Solenoid-driven positive displacement technology uses a high-speed pump where motion is driven by an electric solenoid and compressed air. The material is super charged under low pressure into a chamber where the piston moves up and down, opening and closing the pumping chamber in the upper portion of the dispensing needle. When the piston is pulled up, material enters the pumping chamber. When the piston moves down, it enters the chamber, shuts the material inlet, and displaces the desired volume of material. This technology is considerably faster than the motor-driven piston pumps, and can dispense in much smaller volumes. It is typically used to dispense dots of high-viscosity materials such as solder paste, adhesives, and thermally conductive materials.

The force of the positive displacement action keeps non-homogeneous materials like solder paste from separating. Although this type of technology can dispense large volumes of material, it excels in dispensing small material dots, micro-potting, micro-encapsulation and micro gasketing. It typically is used where precise dot volume repeatability is critical. Beads of material are accomplished by dispensing tight configurations of multiple dots that reflow into each other. This type of dispenser is used widely in various SMT applications, particularly in high-density board configurations where precision and volumetric repeatability is critical.

Solenoid-driven positive displacement dispensers also are available in low-viscosity versions where an internal check valve is incorporated into the design. The valve is fitted between the piston and the dispensing tip and acts as a seal by springing open and closed as pressure is applied, eliminating leakage of materials as thin as water.

Conclusion

The descriptions provided here are the technologies that form the foundation for all of the most popular dispensers currently used in electronics assembly. However, with a myriad of manufacturers building dispensing pumps and valves, many designs have had subtle additions and enhancements that have created virtual technology "hybrids" of dispensing pump technology. To add even more choices, there are many non-contact devices available such as jet dispensers, spray dispensers and pin-transfer technology. These too, have their place in various applications within the electronics industry.

Dispenser manufacturers are always working on better versions of the popular dispenser designs outlined in this paper. And some older contact dispensing technologies are still being used, but, for the most part, they have been replaced because of the capabilities offered by newer dispensing methods. However, some manufacturers may still be looking at ways to resurrect the older technologies in more efficient forms to suit specific applications. That is what is best about any technology. There is always room for innovation whenever there is an application that can benefit from it.

GJHelmers@creativedispensing.com

Gary helmers, vice president, may be contacted at Creative Automation Company, 11641 Pendleton St., Sun Valley, CA 91352; (818) 767-6220; Fax: (818) 767-1243;

Article originally published in Advanced Packaging Magazine, July, 2003.