Automatic dispensers reduce worker fatigue, enabling assemblers to work without discomfort. Ergonomic syringes allow operators to hold devices comfortably, similar to a pen.

Air-powered dispensers are ideal for repetitive work where joints are uniform in size and shape, and where air lines are available.

Dispensing Solder Paste with Pneumatic Dispensers

To get an idea of the technical challenges of dispensing solder paste, imagine a bucket filled with equal parts of water and sand. If you swing the bucket from side to side, like a pendulum, you’ll notice that the water moves in one direction, while the sand moves in the opposite direction. The sand, being heavier than water, simply can’t keep up. Solder paste behaves the same way. It consists of spherical metal particles suspended in a gel-like flux. When pressure is applied to solder paste inside a syringe, the flux moves first, pushing the particles along as it goes. No matter what dispensing method you choose...the key thing is to be sure that the flux vehicle continues to transport the alloy.

You don’t want the flux to move faster than the alloy, leaving the metal behind. If that happens, engineers can expect clogs, inconsistent solder deposits, and bad solder joints.

Assemblers have several options for dispensing solder paste. Pneumatic dispensers use controlled pulses of pressurized air to dispense paste from a syringe. Simple, inexpensive and fast, these systems can be handheld or mounted to a dispensing robot. They are typically used for large deposits—dots 0.03 inch in diameter or larger.

If you need fairly substantial solder deposits, the basic pneumatic shot meter with a 10cc/ml syringe is fine. However, time-pressure systems are not as accurate as other dispensing methods, and they can be more stressful to the paste. Because each pulse of air affects all the paste in the syringe, the size of the reservoir that can be effectively used with time-pressure systems is limited. Variability from deposit to deposit can be as high as 10 percent. Another problem is paste flow after pressure has been shut off. A material filled with a heavy metal is hard to stop once it starts moving.

Dispensing Solder Paste with an Auger Valve

For paste deposits smaller than 0.03 inch in diameter, an auger valve is most commonly used. An auger valve consists of an Archimedes screw powered by an electric motor. The performance of the valve can be adjusted by varying the speed of the screw, and the pitch and depth of its threads. Paste is fed to the screw from a syringe under a constant low pressure.

With a time-pressure dispenser, the air pressure is typically 30 PSI, but with an auger valve, it’s approximately 8 PSI—just enough to feed the auger but not enough to have any negative effect on the paste, so you can use large reservoirs with auger valves.

Auger valves are not as fast as time-pressure dispensers, but they are more accurate. They can make sub-milligram deposits with less than 5 percent variability from deposit to deposit. An auger valve is sometimes the best and most consistent method for dispensing solder paste, because it puts less shear load on the material.

Dispensing Solder Paste with Air Free Dispensers

A variation of the time-pressure system uses a motor-driven lead screw to push the piston, instead of air. Based on the volume of the syringe and the pitch of the screw, the system automatically calculates how far the piston must travel to dispense the desired amount of paste. The amount dispensed remains constant, regardless of the viscosity of the material or the volume remaining in the syringe. In addition, the piston can be programmed to pull back after each dot is dispensed, which keeps paste from oozing from the tip.

Dispense Tips

Tips for solder paste vary, depending on the dot size and the dispenser. Time-pressure systems should be paired with either Tapered Tips, allow sighting fluid level at all times and feature safe, semi-transparent, non-scratching, all-plastic construction and UV Blocking Agent for any material including most light sensitive adhesives. 1 1/4" in length, they are rigid.

A higher end solution, for robotics and in cases when the material needs to flow quickly with no clogging, are Thin Wall Disposable Precision Tips feature a lower cost replaceable drawn steel core, which snaps into a reusable stainless hub and snap out for quick changeover, cleaning or changing gauge sizes or for use in any of your other applications as needed. Use in dispensing on robots or in any situation where application, hand held or robotic, calls for close tolerance.

With valve dispensing, you want very good positional accuracy, and stainless steel tips provide that, but whether the tip is plastic or steel, assemblers should choose the shortest possible tip with the widest possible inside diameter. A tip that is too restrictive will produce excessive back-pressure on the paste and foster tip clogging. Assemblers should not expect to produce a deposit with a diameter smaller than 1.5 times the inside diameter of the tip.

Other Considerations

Besides the dispenser, assemblers also must consider the equipment to which the dispenser is mounted. The dispenser should be isolated from sources of heat and vibration, such as reflow ovens and feeder bowls. Heat can reduce the viscosity of the paste, causing inconsistent deposits. Vibration can cause the material to separate. If the dispenser is mounted to a Cartesian robot, the system should be programmed to make slow starts and stops.

Servo driven tables are best for solder paste dispensing.

If the paste is mounted on the Z axis, it could get shaken pretty hard, causing the flux to separate from the metal. You’re not going to be able to dispense 40,000 dots per hour with solder paste. The material won’t stay in suspension.

To lessen the risk of paste separation during automated dispensing, assemblers should choose syringe size carefully. Assemblers must weigh the frequency of replacing the cartridge against the material’s sensitivity to heat and vibration. A small cartridge might have to be replaced more often, but less will be wasted if the paste separates because of excessive vibration.