Tuesday, June 19, 2018
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Enhancing Fluid Dispense Platform Accuracy
Calibration corrections help overcome path errors and motion system inaccuracies in fluid dispensing systems.

Miniaturization trends in semiconductor applications continue to push the limits of fluid dispenser capabilities to accurately dispense fluids into smaller spaces between printed-circuit-board (PCB) components, camera modules, and die. Customers are increasingly more concerned with the consistency and straightness of dispensed lines, and not just dispense system motion from dot A to dot B, to miniaturize their applications. Microelectromechanical-systems (MEMS) sealant lines and solder paste applications further challenge dispenser capabilities in precision Z-gap control and line path accuracy to achieve the thin lines and small dot sizes required. New dispensing systems must meet the needs of these markets by driving mechanical and software improvements to achieve ultimate precision and accuracy in all three axes — X, Y, and Z — at high motion and flow rates and with minimal footprints, to maximize productivity per square meter.

For semiconductor and microelectronics assembly industries, the continuing miniaturization of packaging has pushed the limits of capability in the X-Y and Z positional accuracies of fluid dispensers required to dispense fluids into smaller and smaller spaces between components. In the past, fluid dispensers were evaluated simply by their capability to accurately place individual dots with point-to-point moves. However, with challenges in underfill and MEMS sealant applications to reduce keep-out zones and minimize circuit-board real estate, it becomes necessary to understand and evaluate a fluid dispenser's capability to dispense fluid along a desired path to form consistent, straight lines while maintaining high system throughput and controlling thinner line widths.

The mobile electronics market, particularly the Smartphone market, is driving a large part of today's semiconductor, MEMS, and surface-mount-technology (SMT) component consumption. At the forefront of this market are the continuous needs to improve battery life and add capabilities to end products. To achieve both, overall product size is decreasing to minimize losses due to electrical resistance and to allow for addition of new devices and capabilities, such as finger-print scanners, gyroscopes, and multiple cameras and microphones. With this trend in higher levels of integration and miniaturization comes the need to improve fluid dispensing positional accuracy for underfill and sealant dispensing to accommodate requirements for reduced space between board-level components as well as within semiconductor packages and MEMS structures.

Line Dispensing
For underfill applications, fluid is typically dispensed in one of two patterns: the "I" pattern or the "L" pattern. In both cases, the goal is to bring the dispensed fluid line as close to the die as possible without spreading fluid on top of the die. Bringing such dispensed lines closer to the die edges allows for reducing the keep-out-zone (KOZ) around the die, enabling reduced circuit-board real estate. Deviations from a straight-line path or inconsistency in the line width negatively affects how close to the die edge a fluid line can be dispensed without risking fluid landing on top of the die or on neighboring components, which can result in product failures. Line straightness and width consistency are also important for solder paste and sealant line dispensing applications. However, in such applications, there are two additional considerations that affect fluid dispenser design and performance: oscillations (ringing) after cornering and thin line width.

The Cost Factor
Cost is always an important factor, whether for underfill or sealant line dispensing, and fluid dispenser productivity plays a major role in determining the costs of these actions. To maximize productivity, fluid dispensing should be performed at high line speeds. However, transitioning from high-speed motion in one direction to a perpendicular direction causes ringing oscillations in the subsequent dispensed line due to the motion system inertia. When determining how close to the die to target a line path, these ringing effects must also be considered, to prevent fluid from landing on top of die. This will also affect the minimum KOZ.
By correcting for variations in the motion system, fluid dispensing systems capable of more accuracy paths can be created.

When striving for thin fluid line widths, particularly for solder paste dispensing, it is important to understand a system's capability to maintain a consistent Z-gap above the substrate surface throughout the course of the dispense path. Z-gap is defined as the vertical distance between the fluid ejection point and the substrate surface. Line width is most directly dictated, apart from fluid and surface interaction characteristics, by the fluid dispense opening diameter and the Z-gap distance from this opening to the target surface. To achieve consistent, narrow line widths, it is critical to not only reach a minimal Z-gap without crashing into the substrate surface, potentially damaging the dispenser or the substrate, but to also maintain this minimal Z-gap without variation over the course of the dispense path. Achieving a minimal Z-gap not only requires precise control in the Z-axis motion, but also precise detection of the surface height relative to the reference Z-home position. Maintaining this minimal Z-gap along the dispense path requires the capability to ensure minimal variations in both surface height and reference Z-home position throughout the target dispense area.

Fluid Dispenser Design
Calibrating out motion system inaccuracies, known as mapping, is a common solution to reduce the inaccuracy between a desired path and a fluid dispensing machine's actual motion. This approach does not require changes to system hardware and therefore is often used to correct consistent or smaller deviations from the desired path. However, mapping does have some drawbacks for long-term accuracy stability. Calibration maps tend to lose some of their accuracy as operating and environmental conditions change. Thermal expansion effects from changes in the operating environment will impact the accuracy of the calibration map. Likewise, a change in the platform location can affect the system's balance and the rigidity of its anchoring, in turn affecting the consistency of the gantry motion. Further, changes in the mechanical system inertia, such as changing applicator configuration or fluid density and quantity, can have effects on the accuracy of a calibration map. As such, calibration maps should be periodically updated to ensure optimal results. With larger dispense areas, mapping time can be lengthy, reducing equipment availability for production. In addition, calibration maps should be copied and stored in case of storage medium failure. Because of the many details required for successful mapping, eliminating or reducing the sources of dispenser motion error may be a more desirable long-term solution.

By redesigning the aspects of the motion system that are sources of inaccuracy, a more robust platform can be created. Although improvements might be gained by applying a calibration map to the system, these gains are often minor in comparison to the time required to create the map or improve the overall system design. With a well-designed system, the amount of deviation from the desired path can be brought within specification limits, eliminating the need for a calibration map.

Understanding a fluid dispensing system's capability to hold to a desired dispense path requires a new measurement technique. Previously, the fluid dispensing industry measured accuracy by examining only a system's capability to dispense dots in target locations. While still an acceptable measurement approach, it does not adequately detect the path that the system travels from one point to another. Dispensing a line along the desired path, and then comparing the center of the dispensed line at regular intervals along the path against the desired path, provides more complete information on the system's capability to stay true to a programmed path. Reducing deviations from the desired path, reducing ringing effects at higher line speeds, and maintaining consistent dispensed line width all make it possible to reduce the KOZ because the fluid can be deposited closer to the die edge or neighboring components.

New dispensing systems available on the market offer improved capabilities to automatically calibrate and maintain desired dispense paths, although the metrics for evaluating these capabilities are only now being published. Customer requirements for placement accuracy and line width control, for current and future applications, must be carefully considered when evaluating and selecting new fluid dispensing equipment. To achieve performance to meet and exceed these requirements, it is likely necessary to redesign the motion system to achieve accuracy and precision in all three axes, further improving path accuracy and narrow line width control. Some of the newest fluid dispensing systems on the market have demonstrated the capabilities to increase units produced per hour productivity by over 25 percent, while also reducing KOZs and achieving line widths and dot diameters below 200µm.

As packages requiring fluid dispensing continue to evolve and require smaller fluid volumes dispensed into tighter spaces, fluid dispenser capabilities to achieve these targets must continue to improve and the abilities to evaluate these capabilities must also evolve. New test methods are being used to evaluate fluid path accuracy and Z-gap control to closely reflect actual customer use. The newest dispensing systems have been designed to take these factors into account and compensate for them. These methods for evaluating dispensing performance will likely become the new standards for dispensing system specifications, replacing older dot placement and point-to-point motion specifications. New applications and requirements will continue to force equipment manufacturers to push the limits of capabilities and technology to provide customers with products and solutions that meet their current production needs as well as their future requirements.


Contact: Nordson ASYMTEK, 2747 Loker Avenue West, Carlsbad, CA 92010-6603 800-279-6835 or 760-431-1919 fax: 760-431-2678 E-mail: info@nordsonasymtek.com Web: www.nordson.com/en-us/divisions/asymtek

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