The size of a typical adhesive dot ranges from 0.7 to 1.5 millimeters in diameter. A dot for a very small component, such as an 0402, might be only 0.3 millimeter in diameter. Most components require only one dot, but a large part, such as an SO16, might require two or three.

"The big challenge today is to make smaller dots," says Gary Helmers, vice president of Creative Automation Co. (Sun Valley, CA). "The problem is, a lot of materials, especially silver-filled epoxies used in photonics applications, are extremely compressible. If you compress more material than you want to dispense, you have to find a way to relieve that pressure."

Both the volume of the dot and its location must be carefully controlled. If the process is off by only 0.1 millimeter or 0.1 microliter, the adhesive might flow onto a solder pad when the component is placed, causing a bad connection.

And then there’s the speed issue. The fastest chip-shooters can put down well more than 50,000 cph. Most automatic dispensers can’t match that speed, but they must come close to avoid creating a bottleneck.

"Adhesives are a necessary evil of printed circuit board assembly," says Scott Wischoffer, applications engineering manager at Fuji America Corp. (Vernon Hills, IL). "You would rather not glue components and run the boards through a solder wave, but someArial you have to."

Dispensing Options

The fastest is to print the adhesive with a stencil in the same way that solder paste is applied. This method is simple and repeatable. It’s also inexpensive, because the same machine used to print solder paste can also print adhesives.

However, it’s not very flexible. If the size or location of a dot changes, or if a dot needs to be added, a new stencil is required. In addition, the adhesive supply is exposed to the air, which can lead to problems related to water absorption and solvent loss.

"In a high-mix, low-volume environment, you should go with an automatic dispenser," advises Wischoffer.

Automatic dispensers have changed little in the past 5 years. At its most basic, an automatic dispenser consists of a valve mounted on a high-speed gantry. Assemblers have four options for the valve: time-pressure, augur, piston and jetting.

The oldest of the four is the time-pressure valve. In this system, the adhesive is packaged in a disposable syringe. An electronic controller applies a measured amount of air pressure, for a measured amount of time, to the back of the syringe, forcing the adhesive out the tip.

"The beauty of time-pressure systems is their simplicity," says Wischoffer. "You attach the needle directly to the syringe that the material comes in." When the syringe is empty, it’s thrown away. No cleaning is required.

One problem with time-pressure systems is that their performance can decline as the syringe empties. Less time and pressure are needed to dispense a given volume from a syringe with 25 cc of adhesive and 5 cc of air than from a syringe with 5 cc of adhesive and 25 cc of air.

Equipment suppliers have developed novel ways to overcome this problem. For example, a machine vision system can monitor dot size and send a signal to increase air pressure when it detects volume fluctuations.

Nevertheless, as dispensing speeds have increased and dot sizes have decreased, time-pressure valves have become less popular. Time-pressure valves can cause filled materials to separate, and some have difficulty producing consistent dots for very small components, such as 0603s and 0402s.

"With a time-pressure system, you might need more than 80 psi of air pressure to dispense a material with a viscosity of 50,000 centipoise through a small needle," says Can La, applications engineering manager at Techcon Systems (Garden Grove, CA). "It will push it out, but it won’t be fast enough or consistent enough."

The augur valve is the most common valve used in automatic dispensers for electronics assembly, says Christian Vega, business development manager for GPD Global (Grand Junction, CO). An electric motor drives an augur inside the dispensing chamber. As the augur turns, it forces adhesive out of the chamber. How much material is pushed out depends on how long the motor runs, the viscosity of the adhesive, and the diameter, pitch and depth of the augur.

Augur valves are faster than time-pressure systems, but slower than piston and jetting valves. They can dispense a dot for an 0806 component in 50 to 80 milliseconds. Augur valves dispense a wide range of volumes, and they can handle filled materials.

"Augur valves are very good for moving solder paste and materials with particles in them," says Steve Adamson, product manager for semiconductor packaging and assembly at Asymtek (Carlsbad, CA). "Augur valves can also produce pretty small dots. We can dispense a dot of silver-filled material as small as 8 mils wide."

One downside of augur valves is that they can introduce air into the material. They also must be cleaned regularly. "If you don’t clean the valve, material builds up on the inside walls and creates problems," says Wischoffer.

Of the four valve technologies, only the piston valve is a true positive-displacement technology. That is, each actuation produces a set volumetric output. The valve consists of a matching piston and cylinder. Adhesive fills the open cylinder from the top. The piston seals the cylinder and forces the adhesive out the bottom.

The piston can be very fast and very small. For example, Creative Automation offers a cylinder and piston arrangement that is 10 mils in diameter and actuates up to 50 Arial per second.

"The advantage of piston valves is they compress only the amount of material that you want to dispense," explains Helmers. "The rest of the material is not stressed at all. It’s at 2 or 3 psi, which is just enough pressure to keep it flowing."

"Piston valves are the least affected by viscosity changes," adds Adamson.

Like the augur valve, the piston valve requires regular cleaning. But unlike the augur valve, the piston valve may have difficulty with filled materials, and dot size is not programmable.

"In general, piston valves are a little faster than augur valves," says John O’Neill, applications technician with Universal Instruments Corp. (Binghampton, NY). "However, piston valves are more labor-intensive for setup and maintenance, because the volume is adjusted mechanically. With augur valves, the volume is controlled by software. The software tells the motor how much to turn the augur."

The fastest of the four valve technologies for surface-mount adhesive is the jetting valve. This noncontact technology can deposit 50,000 dots per hour, or 12 to 15 milliseconds per dot.

Here’s how it works: Adhesive is fed into a heated chamber. At the bottom of the chamber is a ball and seat. When the piston retracts, the ball lifts from its seat and adhesive fills the void. When the piston moves forward, the ball returns with enough force to break the stream of adhesive, which jets through the nozzle. The broken stream of adhesive strikes the substrate and forms a dot.

"The benefit of jetting is that it takes the Z-axis movement out of the operation," says Adamson. "The valve only needs to move in the X and Y axes, which saves time."

Like piston valves, but unlike either time-pressure or augur valves, jetting valves offer positive shut-off. "With time-pressure and augur valves, there’s an open path from the material supply to the surface," says Helmers. "So if the pressure isn’t exactly right, the material can flow out when you don’t want it to."

One limitation of jetting technology is that it can only produce one dot size at a time. If a larger dot is needed, multiple dots must be deposited in the same location. However, one supplier may have found a solution to the problem. GPD Global is working on a jetting valve that uses piezoelectric technology to drive the piston, rather than pneumatics.

"The piston is driven by a rocker arm that moves up or down based on the amount of current that runs through it," Vega explains. "This enables the valve to vary shot size on the fly. It’s also quieter than pneumatic versions."

Another limitation of jetting is the inability to jet materials such as solder paste, says Adamson. Solder balls in the paste will "coin" when they are struck by the ball.

Fuji’s Wischoffer warns that jetting technology is not for every application. Depending on the material and the volume dispensed, jetting can produce microscopic particles of adhesive, called satellites, around the dot. "That little bit of contamination on nearby pads can make them less likely to solder," he says.

In the end, which valve technology to use depends on the material and the application. To ensure trouble-free operation, equipment suppliers recommend extensive testing. A 10,000-dot test is better than a 3,000-dot test, Adamson advises.

The Right Height

Maintaining that gap throughout the process prevents stringing and ensures that the dots will be the right volume and shape. "If the needle comes down too far, you squash the dot, and it comes out a doughnut," Wischoffer says. "If the needle does not come down far enough, the adhesive won’t release completely and you get an insufficient dot. Then, at the next dispensing site, you get a dot that’s too big."

To make matters worse, circuit boards are not perfectly flat. Solder mask might be slightly thicker in one area of the board than another, or the board itself might be warped.

Equipment suppliers have invented some innovative ways to deal with the problem. Clamping the board rigidly on both sides reduces warpage, and a footed, adjustable standoff alongside the needle ensures that the tip is always at the same height.

For a lighter touch, a spring-loaded mechanical sensor can be used instead of a standoff. If touching the board with each dot is impractical, the machine can be programmed to measure board height only at certain Arial and locations.

In some cases, however, touching the board is impossible. There may not be enough room for both a stand-off and the needle, or the board might have sensitive materials, such as gold contacts, that could be damaged by a standoff.

In that case, an optical or laser sensor can be used to measure the dispensing height, says Helmers. Another alternative is to use machine vision to create a topographical map of the board, a process called contour mapping. An additional advantage of machine vision is that it can also measure stretch in flexible circuits.

Jetting avoids the height issue altogether, because the dispenser does not contact the board, Adamson points out. The valve dispenses dots from a height of 1 to 3.5 millimeters above the board. Because it’s a noncontact method, jetting does not require support for the bottom of the board.

Niceties and Necessities

To solve the problem, some automatic dispensers are equipped with heaters to keep the adhesive at a constant temperature. Such heaters are mandatory for jetting valves, but optional for others.

"We like to keep the fluid at 40 C, so that it doesn’t matter if you’re in Finland in winter or Kuala Lumpur in summer, the temperature of the operation stays the same," says Asymtek’s Adamson. "We’re not trying to cure the fluid, or even thin it, we’re just trying to keep it at the same temperature worldwide."

Here are some other equipment options to consider:

• Multiple heads. For high-mix, low-volume applications, mounting two or three dispensing heads on the gantry can be worthwhile. For example, an augur valve can be used to dispense solder paste, while a jetting valve dispenses adhesive.
• Buffers. Few automatic dispensers can keep up with today’s high-speed chip-shooters. Installing a first-in, first-out buffer after the dispenser but before the chip-shooter can help balance the line.
• Needles. When dispensing surface-mount adhesive, the right needle can make a big difference. GPD’s Vega recommends using a stainless steel needle with a conical tip and a low ratio of outside diameter to inside diameter. "When the adhesive comes out the end of the needle, it will be attracted to the surface area of the needle tip and the surface area of the board," he explains. A needle with a low ratio of outside diameter to inside diameter will ensure that the board wins that tug-of-war.

  When dispensing filled materials, the inside diameter of the needle should be eight Arial larger than the diameter of the largest particle in the material, adds Universal’s O’Neill. "With anything smaller, the particles will jam under pressure," he says.