George A. Riley
Tutorial 76 reported the advantages of jet dispensing of underfills with a spring-driven metering valve to precisely control drop size. Since then, Asymtek has introduced piezoelectric dispensing with higher performance for a wider range of fluids.
Jet dispensing, as described in the earlier tutorial, has become the preferred method for dispensing flip chip underfills, replacing pump-and-needle dispensing. Jetting shoots drops of fluids from a stationary or moving nozzle above the board. It offers higher board density, smaller fillet size, better placement accuracy, higher dispense speed, and lower cost than needle dispensing of underfill.
In jet dispensing of underfill, the jet nozzle is positioned 0.5 to 3 mm above the substrate, with a single height sensing per board. The vertical distance from the substrate is not critical, as the droplets leave the nozzle before contacting the substrate. XY positioning centers the nozzle 100 microns from the edge of the die. No Z-axis motion is required during dispensing.
Piezoelectric deposition has been standard for years in ink-jet printers. However, it is limited to low viscosity liquids such as inks, and thus is not suitable for higher viscosity fluids such as underfills and solder pastes. The Asymtek approach is to use piezoelectric actuators to rapidly modulate the flow of higher-viscosity fluids such as adhesives, Bio reagents, and lubricants.
FIGURE 1. Diagram of closed and open DJ-100 jet. (Courtesy Asymtek)
Figure 1 is a cross-section graphic of the Asymtek DJ-100 Piezo Electric Jet in the closed and open positions. In the lower picture, the sealing ball blocks the flow. In the upper picture, the sealing ball has lifted to allow flow. Air pressure in the reservoir forces the fluid out, forming a drop. The valve is capable of opening and closing again in 5 milliseconds, to create drops under 200 micrometers diameter.
In operation, the nozzle “flies” above the surface, depositing precise volumes of fluid in dots, lines or patterns. Larger dots are created by depositing multiple shots in the same location,creating larger dots. Lines and complex shapes are made by merging adjacent dots.
Versatility Deposits a wide range of fluids including underfill adhesives, surface mount adhesives, encapsulants, conformal coatings, UV adhesives and conductive epoxies.
Speed Up to 200 dots per second, 400 mg of fluid per second. In comparison tests, about twice as fast as auger-driven dispensers.
Accuracy Highly repeatable in position and drop size. Deposits in spaces as small as 175 micrometers, creates fillets as narrow as 300 micrometers, dot diameters as small as 200 micrometers, shot volumes as small as 1.0 nanoliters, stream diameter as small as 50 micrometers.
Lower Cost Allows denser component spacing, greater throughput, contact-free dispensing, and higher yields than needle dispensing. The shorter track uses less material.
Dispensing is used in many common micro assembly applications. Beyond these, the improved performance has led to applications in such diverse areas as conductive collector lines for photovoltaic wafers, dispensing uniform phosphor layers across LEDs, forming encapsulating seals for new types of displays, creating 3D assemblies, and applying inks or other materials for new forms of fuel cell.
In summary, improved performance and lower cost of depositing a wide range of materials has greatly expanding the potential range of applications.
FOR MORE INFORMATION
Tutorial 76, Jet Dispensing of Underfills, September 2007.
(1) Alec J. Babiarz, “Jetting Fluids in a Wide Variety of New Electronic and Semiconductor Non-Traditional Packaging and Assembly Applications,” SMTA Pan Pacific Microelectronics Symposium, February 2009.