Optimizing Time/Pressure Dispensing of
Adhesives
by Chrys Shea
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Over the past decade, depositing adhesives for SMT components on the solder side of a circuit board has seen more dramatic changes than any other production process. It's gone from pin transfer, to screening, to time/pressure dispensing, to positive displacement pumps. These changes have not been in the vein of typical process evolutions where experience and experiments give way to refinements and improvements. This process evolution has encountered complete conceptual changes in the approach to get a repeatable, reliable glue dot.
Everyone in the industry will agree that time/pressure systems are not the best way to form adhesive deposits. Some organizations can justify the capital expense of positive displacement pump equipment, while others have returned to screening adhesives with a more refined process than in years past. But many assemblers have time/pressure systems on their shop floor, and no choice but to use them to the best of their capability. Understanding the mechanics of the process can help the effort to optimize this less-than-perfect process, however.
The perfect glue dot looks like a Hershey's Kissâ
, and has a base to height ratio of 1:1. For an 0805 chip, the diameter and height should be 0.8 mm, for a 1206 component, 1.0 mm, with a tolerance of ±
0.1mm for both. If the base/height ratio is greater than 1:1, the failure mode of the dot will either be poor contact with the component or spread onto the circuit pads. If the base/height ratio is less than 1:1, the dot will most likely collapse and appear to look more like a string than a dot. Even the most stubborn dispensers can achieve these goals if the process is administered properly and the following "rules of thumb" are employed.
The nozzle diameter should be half the desired glue dot diameter. If an 0.8 mm dot is desired, then the nozzle diameter should be approximately 0.4 mm. If a 1.0 mm dot is required, then the nozzle diameter should be approximately 0.5 mm. Fortunately, both these common sizes can be achieved with a 0.45 mm nozzle on most machines. Larger dots, for tantalum capacitors or SOICS, can be achieved using the 50% diameter rule of thumb.
The proper standoff height is critical to achieve the right aspect ratio. Most dispensers have a standoff, or "foot" to prevent the nozzle from crashing into the circuit board. As the adhesive is dispensed, it fills the void between the nozzle and the board, the adhesive sticks to the circuit board, and as the dispensing head retracts in the Z-axis the surface tension of the glue causes it to snap off and form that perfect "kiss" shape. If the standoff is too low, the adhesive will stick to the nozzle, creating a thick string that does not snap easily. If the standoff is too high, the base of the kiss won't be formed, and there will not be enough resistance to the retraction of the dispenser to create a good snap off. The viscosity of the adhesive is usually blamed for stringing problems, but in most cases, improper hardware setup is the culprit. Viscosity impacts dot size far more than it affects stringing.
Shot sizes must be changed as the piston travels down the cylinder. The medium applying pressure to the piston is air - a compressible gas. The shot size of air required to move the piston and extrude a set volume of adhesive is smaller in a full cartridge than it is in a half-empty or nearly empty cartridge. As the cartridge empties, shot sizes should be increased. Smaller cartridges (5 or 10 cc) are more forgiving than larger (30 or 50cc) cartridges.
Shot sizes must be changed to accommodate viscosity changes. Lower viscosity adhesives will form larger dots; higher viscosity adhesives will form smaller dots when both materials are dispensed under the same parameters. But adhesive viscosity varies within a single cartridge. How can this happen? The shear frictional forces that are applied to the adhesive as the piston moves down the cylinder will cause the material to heat up. Many dispensers offer heated boots around the nozzle to maintain a temperature above ambient and balance the viscosity as the piston travels. But adhesives are cured by heat - the cross-linking process is initiated in the nozzle.
Purge the nozzles regularly. As the adhesive in the nozzle cures, it reduces the size of the nozzle, eventually violating the 50% diameter rule. A quick purge, or continuous flow of adhesive through the nozzle, will remove most of the buildup inside the nozzles. It is recommended that nozzles be purged after as little as fifteen minutes of downtime or after two hours of production.
Keep the nozzles clean. A fresh nozzle every day helps to guarantee the proper diameter. Nozzles can be cleaned in simple benchtop ultrasonic cleaners with approved solvents. The best practice is to have two sets of nozzles, so one can be cleaned while the other is in production. This method limits downtime to a few minutes for nozzle changeover. Removal and cleaning at the start of the shift also provides an opportunity to inspect the nozzles. Any dents or dings in the nozzle change the diameter and affect the glue dots.
The time/pressure dispensing process is operator dependent. There is no way the process can run optimally without operator intervention. The intervention required to produce a consistent process is one of taking simple actions, however. It does not require excessive amounts of training or decision-making capability. Using an optical measuring device like a microscope reticule, the process engineer or technician can measure dot sizes and implement a program that calls for standardized hardware, shot size increases at certain fill levels of the syringe, and set intervals for cleaning.
Implementation of these steps will assure a better adhesive dispensing process.
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Chrys Shea
Siemens Information and Communication Networks
Cherry Hill, NJ
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