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Matching Fluid Dispensers To Materials for Electronics Applications

Understanding the technology behind popular non - contact dispensers for electronics assembly.

Dispensing Technology

Figure 3

This type of dispenser is very temperature sensitive, both to ambient temperature changes and to internal heat generated as a side-effect of the dispensing process. This causes changes in material viscosity that affect volume accuracy over time. This dispenser has virtually no competition in dispensing short pot life materials, because an expensive dispenser is not lost if the material is not cleaned from the unit in a timely manner. It also does an adequate job dispensing epoxy and adhesives in some surface mount applications where precision and repeatability are not an issue.

The spool valve (Fig. 3c) is a variation of time-pressure technology that offers higher precision and speed than the basic syringe or diaphragm valves, and can be used with higher pressure. The spool valve meters material flow with a spool that is seated in a circular, fluid outlet seal. The spool is attached to a piston and when it is cycled on, the spool pulls out of the valve seal and the pressurized material flows to the dispensing tip. When shut off, the spool moves back into the seal, and the motion of the spool pulling back through the material causes a "suck back" action. The spool controls the flow of material to the tip, and the longer the spool is in the open position and the higher the pressure on the material, the greater the quantity dispensed.

The spool valve is best used for dispensing lines of material and fills. It is not accurate in dispensing material dots. It excels in dispensing homogeneous materials of medium to high viscosity One area where it is quite effective is dispensing room temperature vulcanizing fluids. It is also used for gasketing and sealing applications, or wherever bulk dispensing is required.

Needle valve technology adds a valve that has a sealing element consisting of a long, tapered needle and cone-shaped seat (Fig. 3d). In most cases, the needle sits inside the tip. Material is dispensed by lifting the needle out of the tip, or valve seat. The farther the needle pulls back from the tip, the longer the needle is held out of the tip, and the higher the pressure applied to the material, the greater the quantity of material dispensed.

The needle valve offers more volumetric accuracy and speed than the other variations of time-pressure technology because the action of the needle provides better metering of the fluid flow, particularly when used to dispense low-viscosity materials. It handles the dispensing of lubricants and oils far better than the previously mentioned technologies, and often is used for medium-precision patterns and fills in applications such as gasketing and sealing.

Rotary screw valve dispensers

Also known as an Archimedes or auger screw dispenser, this technology is extremely popular because of its high versatility and satisfactory volumetric accuracy for all but the most precise applications (Fig. 6).These valves can be found dispensing virtually any material depending on the internal tolerance configuration of the individual valve. It works best however, with medium - to high-viscosity materials.

Rotary Screw Dispensers

All rotary valves or pumps are based on the same technology (Fig. 3e). An auger screw controls the material flow through the dispensing tip. Air pressurizes a syringe that feeds material to a motorized auger screw which rotates and forces the material from the dispensing tip. The amount of material dispensed is directly proportional to the rotation of the screw; i.e., the longer the screw rotates, the greater the quantity of material dispensed; to a lesser degree, the volume dispensed also is dependent on the material pressure and viscosity.

The rotary screw valve offers high flexibility in dispensing dots; however, volume and repeatability are influenced by the syringe level, air pressure, material viscosity, temperature, and the motor or clutch timing of the mechanism. However, the accuracy of any specific valve model is dependent on the precision of its construction.

This technology is best used for dispensing beads and lines of material, but the rotary screw valve can dispense dots with satisfactory results for a wide range of applications. It is a general workhorse in electronics assembly and PCB production, excelling in applications such as glob topping, underfill for flip chip applications and gasketing. The rotary screw valve does a far more accurate job than any of the previous valves, and works well with both adhesives and solder paste.

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.