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.
 

Air-powered dispensers that use a timed air pulse to apply controlled, repeatable amounts of fluid are a highly productive alternative to hand applicators such as squeeze bottles, tubes, swabs and toothpicks. Controlled air pressure, instead of hand or finger pressure, reduces repetitive motion and the risk of carpal tunnel syndrome.

Dispensing Solder Paste

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. Time-pressure systems 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 Cartesian 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.

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.

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.

A fourth option is a piston valve. These pneumatic devices provide the accuracy of auger valves, but the large-shot capability of time-pressure systems. Like the auger valve, paste is fed to the dispense chamber from a syringe under a constant low pressure, but instead of a screw, the chamber contains a piston. Paste is drawn into the chamber when the piston rises. When the piston comes down, the paste is expelled.

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.

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.

Dispense Tips

Tips for solder paste vary, depending on the dot size and the dispenser. Time-pressure systems should be paired with cone-shaped, tapered plastic tips. However, a chamfered stainless steel tip, 0.25 inch long, should be used with auger and piston valves, since resistance to flow is less of an issue with valves than with time-pressure systems. Steel tips are available with standoffs, if maintaining a consistent dispensing height is important.

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.

Auger Valve

For paste deposits smaller than 0.03 inch in diameter, an auger valve is typically the dispensing method of choice. An auger valve consists of a screw,  powered by a controller. 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. With an auger valve, it’s more like about 8 PSI—just enough to feed the auger, 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.

Time-pressure systems 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 Cartesian 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. They come in a standard analog and digital model.

Accuracy and repeatability of dispensing paste can be very different according to method. Remember, there are some drawbacks to this inexpensive method of dispensing paste:

• Many time-pressure systems are not as accurate as some other dispensing methods.
• Because each pulse of air affects all the paste in the syringe, the size of the reservoir that can be used with time-pressure systems is limited.
• Variability from deposit to deposit can be as high as 10 percent.
• At times, paste continues to flow even after pressure has been shut off.
• A material filled with a heavy metal is hard to stop once it starts moving.

Stepper Dispensing

A variation of the time-pressure system, the IntelliSpense Air Free stepper dispenser uses a motor-driven lead screw to push the piston. Accurate and repeatable, it is, in some cases, an excellent choice. Please consult a sales person.

The piston can be programmed to pull back after each dot is dispensed, which keeps paste from oozing from the tip. The smooth dispense cuts down on the "tunneling" effect of pneumatic syringe pistons. To increase this benefit, use a Tapered Dispense Tip. This is an excellent, almost unbeatable method of dispensing paste from a syringe, as long as speed is not crucial to the dispense process.

Besides the dispenser, you 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.

Points to remember:

• 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.
• If the syringe full of paste rapidly shaken it could cause the flux to separate from the metal.
• The material won’t stay in suspension very well with syringe dispensers.
• To lessen the risk of paste separation during automated dispensing, you should choose syringe size carefully.
• Weigh the importance of the frequency of replacing the cartridge against the material’s sensitivity to heat and vibration.
• A smaller cartridge or syringe might have to be replaced more often, but less will be wasted if the paste separates because of excessive vibration.

Dispensing Needles and Tips

Dispense Needles and Dispense Plastic or Specialty Tips for solder paste vary, depending on the dot size and the dispenser:

• Time-pressure systems should be paired with cone-shaped, tapered plastic tips.
• Steel tips are available with standoffs, if maintaining a consistent dispensing height is important.
• With valve dispensing, you want very good positional accuracy, and stainless steel precision tips provide that.
• Whether the tip is plastic or steel, you 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.
• You should not expect to produce a deposit with a diameter smaller than 1.5 times the inside diameter of the tip.