MicroDot Dispensing
02/01/2003
Equipment and Process Create a Solution
BY BRIAN PRESCOTT, THOMAS KARLINSKI, YINGFEI (FRANK) KE and JOSEPH BATTAGLIA
The commercial marketplace is driven by consumer demand. Today, that demand focuses on the delivery of feature-rich devices.
The continued miniaturization of electronic devices is creating challenges for manufacturers. Ultimately, this new generation of devices will require attachment to a suitable interface, which is the thermal, structural and electrical "bridge" joining the very small device to the outside world. Without a suitable methodology for the attachment of these tiny devices, the next wave of commercial products will not occur.
The Tiny Dot Challenge
A most daunting task for manufacturing these devices was developing an entirely new process for the generation of microdots (<250 µm) of solder, adhesives or thermal compounds. Traditional dispensing approaches required close scrutiny and, through exhaustive testing, experimentation and development, some very basic assumptions about dispensing have been cast aside. One of the foremost assumptions, the "1.5X needle internal diameter (ID) dot" premise, has been cast out and, as described in the next section, is no longer valid for small dots required in emerging electronics products.
 Figure 1. Deposits as small as 150 µm in diameter are possible with the latest dispensing process and equipment. |
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The Standard Process
Traditionally, adhesive dots are formed by bringing a needle to within a certain distance from a substrate, pumping out a quantity of material until it wets the surface to a diameter of 1.5 times the needle's ID, then pulling away. The contact area of the dot on the substrate is much greater than the contact area of the needle. When pulling away, the material breaks and leaves behind a deposit. Whether the system is time-pressure or positive displacement, the same mechanics apply. It is the viscous nature of the material, combined with the dispensing height, that determines dot shape and size.
The volume of material dispensed must be in correct proportion to the distance between the needle and the substrate. If too much material is dispensed, overfilling the gap, the adhesive viscosity forces it to envelop the needle. The excess material will stick to the needle regardless of how well the tip is prepared. If too little material is dispensed, dot production will be erratic. Repeatable dot formation requires accurate and precise height and volume control.
The greater the viscosity, the higher the dot. These are well known and accepted rules. However, when applied to micro dots, extensive testing demonstrates that this is not a robust, high-volume method.
 Figure 2. Dot diameter as a function of needle gauge and shot size. |
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The MicroDot Process
A new method of forming small dots has been developed. This method requires the system to have very accurate substrate height measurement, needle height control and pump displacement control.
In small dot applications, deposits typically are placed in tight places, so the expectation is that the positioning system must be more accurate and repeatable. A robust positioning system is required to accurately control the position of the pump (and ultimately, the dot) in the X-Y plane, and it must have the stiffness and rigidity to provide high accuracy in the Z-axis plane. Proper materials selection used in the construction of the pump components (depending on the adhesive, paste or epoxy being dispensed) and high resolution are required. Rotating auger positive displacement pumps works well, as long as their resolution is high.
This new method is capable of dispensing dots with a diameter significantly less than the needle's ID. The needle is brought to a predetermined height from the substrate. Pressure is applied to the material in the needle by the auger displacement and a meniscus is formed. Surface tension of the material being dispensed, in combination with an accurate displacement by the auger, ensures that the meniscus is spherical in shape.1 As the sphere grows, or is brought closer to the substrate, it meets the surface and begins to wet it. Once it wets out to the desired diameter, the needle is retracted, causing the material to stretch out and break, leaving behind a small deposit (Figure 1), in much the same manner as the larger dots previously described.
This method is being used successfully to create dots as small as 150 µm in diameter. It works equally well with solder pastes, silver epoxies, conductive epoxies and surface mount adhesives. It does not require high-precision needles with exceedingly small orifices, as traditional rules dictate — 27 and 28 gauge needles work well (Figure 2), which reduces needle clogging typically found when dispensing non-Newtonian fluids.
The Equipment
To support the process previously described, high-quality equipment is necessary. A pumping system is required to accurately and repeatably displace a very small volume of material. Key features include auger pumps that incorporate a wide selection of compatible materials, proper diameter and pitch of the auger screw, and high-resolution feedback to the motion controller. Additionally, gear reduction between the motor and auger simultaneously multiplies torque and resolution. This gear reduction reduces the rate of acceleration and deceleration proportionally, and eliminates another variable for the programmer.
The clearance between the needle and the substrate must be controlled accurately. There are several factors that contribute to errors:
Needle height calibration. This calibration teaches the needle height to a known reference surface. A precise detection method combined with a low friction slide can be used. The needle can push the slide down until the switch is activated. It is important that any needle or cartridge compliance is eliminated.
Board height detection. A precise measurement system must be used to teach the height of the substrate to the motion controller.
These two subsystems must be accurate and well integrated to supply the necessary height data to the system. A well-integrated system will automate these functions, making it simple and efficient for the system to calibrate itself. Calibration accuracy must be better than 25 µm at three standard deviations and even less for dot diameters that are 150 µm or smaller. To maintain consistent height and accommodate local changes in topography, the system must have the ability to calibrate at multiple sites.
These subsystems also must ride on a rigid, stiff and accurate gantry system. It is useless to have accurate calibration that is not valid across the entire range of the dispense area. Lightweight, well-designed, stiff aluminum structures are well suited for moving parts. They must ride on precise bearing ways that are well supported. New-age polymer concrete materials are ideal for this because they are rigid and well damped.
User-friendly and intuitive software rounds out the preferred system. It must contain tools that allow easy, repeatable calibration without operator intervention, as well as the necessary variables for programming the correct motion at the correct time. These variables include programmable dispense heights, dwells and heights at which the velocity of Z motion is changed.
Conclusion
At present, testing and process development have allowed dots of 150 µm to be dispensed at production speeds without issues. Further honing of the process will soon allow 100 and 125 µm dots to be dispensed at high-volume production-capable rates. The "MicroDot" solution will continue to extend the ability of manufacturers to deliver the small, feature-rich devices that will come to market in the near future.
REFERENCE
Laplace's Rule governs the formation of the meniscus as a function of the pressure being applied and the interfacial energy of the material relative to its surroundings. In this case, it is liquid-vapor surface energy that resists the pressure and forms the meniscus.
Brian Prescott, senior engineer of advanced development, Thomas Karlinski, applications engineer, Yingfei (Frank) Ke, applications engineer, and Joseph Battaglia, product manager, may be contacted at Cookson Electronics Equipment, 16 Forge Park, Franklin, MA 02038; (508) 520-6999; E-mail: [email protected]; [email protected]; [email protected]; and [email protected].
Illustration by Gregor Bernard