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," explains Don Cornell, market development manager for solder products at EFD Inc. (East Providence, RI). "You don't want the flux to move faster than the alloy, leaving the metal behind." If that happens, warns Cornell, engineers can expect clogs, inconsistent solder deposits, and bad solder joints.
Printing vs. Dispensing
The most common method of applying solder paste to a circuit board is with a stencil printer. In some cases, however, it's better to dispense it. For example, the paste might need to be applied in a recessed area. Or, a component, such as a flip chip, might already be mounted to the board, making it impossible for a stencil to lie flat against the substrate. After reflow, a dispenser can be used to put down solder paste for attaching interference shields and other components.
"In terms of cycle time, there's no comparison between printing and dispensing. Printing is much faster," says Brian Toleno, application engineer at Henkel Loctite Corp. (Industry, CA). "But, with dispensing, you can deposit larger volumes than you can with printing."
Dispensing is sometimes used in conjunction with printing. For example, if a family of boards shares the same basic layout, a stencil can be created for the group, and a dispenser can deposit paste in those areas that are unique to each board. Or, after a board has been printed, a dispenser can add extra material to some locations for pin-in-paste assembly.
"Pin-in-paste is becoming more popular, especially with the move to lead-free solder," says Toleno. "Wave soldering with lead-free solder is difficult."
When dispensing is the best method of applying solder paste, assemblers should choose a paste specifically formulated for that technique. "You can't put paste that was formulated for printing in a syringe and attempt to dispense it," says Cornell. "It's not suitable for dispensing."
Paste for dispensing differs significantly from paste for printing, says Toleno. For example, the viscosity of printable paste ranges from 800,000 to 1 million centipoise. In comparison, the viscosity of dispensable paste is 200,000 to 400,000 centipoise. The metal content of printable paste is 88 percent to 91 percent, while the metal content of dispensable paste is 83 percent to 87 percent. Whereas tackiness, slump and work life are the chief characteristics of printable paste, lubricity, flow and resistance to separation are the key traits of dispensable paste.
As with printable paste, the size of the particles in dispensable paste should match the application requirements. But, for good dispensing, the quality of the particles is critical, says Cornell. The particles should be uniformly sized, smooth and spherical. Ovoid particles or particles that are just 5 percent too large can clog narrow dispensing tips.
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 10-cc syringe is fine," says Jere Donohue, president of Integrated Dispensing Solutions (Woodland Hills, CA).
However, time-pressure systems are not as accurate as other dispensing methods, and they can be more stressful to the paste, says James Dornan, vice president of I&J Fisnar Inc. (Fair Lawn, NJ). 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. 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," he says.
A variation of the time-pressure system uses a motor-driven leadscrew 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, explains Scott Beebe, president of Fishman Corp. (Hopkinton, MA). 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 needed. 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," says Cornell. "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 submilligram deposits with less than 5 percent variability from deposit to deposit.
"An auger valve is the best and most consistent method for dispensing solder paste," says Toleno. "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, says Cornell. 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.
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.
"If the paste is mounted on the Z axis, it could get shaken pretty hard, causing the flux to separate from the metal," says Toleno. "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, says Cornell. 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.
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," says Cornell.
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.