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VOLUME -23 NUMBER 7
Publication Date: 07/1/2008
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July 2008 Issue
Special Feature: Assembly and Packaging
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Scalability Adds Flexibility for Dispensing Systems
Three configurations for a scalable dispensing system: dispense and heat, dispense and preheat, and preheat, dispense and post-heat.
By Steven J. Adamson, Market Manager, Asymtek, Carlsbad, CA
Scalable dispensing is a new concept in dispensing system design. It answers the question, "Why purchase a large machine when the boards/part carriers are often no larger than 300 x 200mm?" Many of today's automotive boards have to fit in a space no larger than 125mm square, and many portable devices are pocket-size.
Further, no one wants to pay for features that they won't use. Customers want the ability to upgrade in the future, to include more heat stations and higher levels of feedback, and to have control of the dispensing process. From these concepts the idea of a scalable dispensing system emerged.
To some, a scalable system may seem to be one of add-on features, which often have the appearance of afterthoughts. Often, a new option is just bolted to the dispense head. This can present problems, such as the weight of the new option being cantilevered from the original robot motion, leading to vibrations that affect accuracy and repeatability. It's possible to add new features to any system, but if they are not anticipated during the initial system design, these new features may not give optimum speed and accuracy. Customers are asking for systems that offer precision, accuracy, high throughput, and low cost of ownership. Scalable systems address these customer requests.
The new scalable automated fluid dispensing systems are much narrower than most in-line dispensing systems. Factory space costs money, particularly if the system is operating in a cleanroom environment. Many companies cannot add more space to cleanrooms and are already running at full capacity. Management does not want to build more cleanrooms, so they are pushing engineering to use the space they have more effectively. Therefore, equipment size is important.
Sleek equipment design and purchasing only options that are needed can contribute to a more compact size. In dispensing systems, it's accomplished by having a central dispense area with an optional single heater. If a customer is dispensing silver epoxy or UV cured material that does not require heat, they do not need to buy a heater module. A heater module can be added at a later date if the product being manufactured changes. A no-heat or single-heater dispensing system has a working dispense area of 300 x 415mm, and the entire platform is only 600mm wide. Because the laptop computer used to control the system is typically 300mm wide, the entire dispensing system is only as wide as two laptop computers. Factories in Asia typically allocate a 750mm wide work space per employee, so an automated dispensing system could fit in that same amount of space.
Heaters are used in two of the most popular dispensing applications, encapsulation and underfill. The standard dispensing system configuration for these applications includes preheat and dispense heat stations. A preheat module can be mounted to the system with conveyor extension rails. If a post-heat station is required for flow-out after dispensing, it can also be added. Each additional heater adds to the width of the system, but a two-heater configuration is still only 850mm wide compared to the typical three-station systems on the market today that measure 1100 to 1300mm wide.
Process control features have become an essential part of automated equipment and contribute to ensuring an accurate process, yield improvement, and in turn, a reduction in total cost of ownership. There are a number of process control features that can be found on some of the new, scalable dispensing systems.
New heat control features have become important to more efficiently use energy. Through system sensors, a line stoppage can be detected and if that stoppage is greater than the programmed time in the dispense recipe, the heaters can be programmed to revert to a standby state or be turned off completely. The goal is to get the heater settings under recipe control, rather than rely on manual adjustment by the operators.
Many large global companies buy equipment to be used at multiple sites and want to have the same processes at all plants to produce consistent products with the same high yield. If operators are dialing in settings for fluid pressure, air pressure used with jets or valves, impingement air flow rates for heaters, or heater temperatures, mistakes can creep into the manufacturing process and yield will vary between plants.
One way to avoid random setting changes and maintain process control is to have integrated sensors and regulators control fluid and air pressure and a mass flow controller coupled with impingement heaters to ensure a consistent flow and air volume. Adding calibrated process jetting (CpJ
) makes it a complete recipe-controlled system. With CpJ, the system is constantly monitoring itself, and with sufficient air pressure, volumetric repeatability can be maintained during long production runs. Dispense patterns are programmed with a specified weight and the CpJ periodically samples the dispense weight per shot and then re-computes the number of shots for each pattern.
Any time you have a lot of height sensing to perform, because you are needle dispensing, this can become a significant proportion of the total manufacturing time. To increase speed, an alternative height sensing process called laser height sensing range finding can be used. The height sensor moves in an X-Y plane without Z motion to capture all substrate height variations at one time.
This is significantly faster than using a mechanical height sensor that moves into position, then moves vertically to trip a light sensor, and records the position of the head. Although jet dispensing has reduced the number of required height senses to usually one per board or part, range finding can significantly reduce throughput time in needle dispensing, especially if multiple parts are located in a carrier.
As parts get smaller, the accuracy of underfill or adhesive dot placement becomes more demanding. In the past, 75µ accuracy was an acceptable motion tolerance. However, higher accuracy machines are now typically requiring 50µ accuracy. Newer dispensing systems can achieve 50µ positional accuracy in all three axes, X, Y, and Z, and not have to sacrifice speed.
Many factors contribute to higher accuracy motion control, such as the use of linear encoders for all axes, including the Z-head. A built-in mechanical height sensor or laser height sensor can be mounted directly to the base of the Z-head, avoiding the cantilevering effect caused by a bolt-on height sensor.
Conveyor motion transports boards into the dispensing system from the upstream conveyor or releases them to the downstream conveyor. This can significantly impact throughput. Throughput is increased if boards or parts move simultaneously, rather than in series. If a system has a multimove conveyor, no time is wasted waiting for one board to register before allowing another board to enter the system. Since boards come in a variety of sizes, weights, and with different surface finishes, a scalable solution includes a range of available conveyor belts like 3, 4, and 6mm flat belts and a chain conveyor option.
Although a dispense head can move quickly, analog vision systems may not keep pace. New surfaces on circuit boards and smaller fiducial marks viewed at high speed can cause problems when trying to locate a board's registration marks. Using a digital camera is faster and removes the need to have an analog-to-digital conversion. A digital vision system is more reliable when compared to today's analog cameras. In addition, the camera works at a frame rate of 60Hz compared to analog cameras which capture images at 30 frames per second. This improves throughput by speeding up the vision portion of the process.
Manufacturing needs to be flexible because products and models change at a rapid pace. Scalable dispensing systems allow companies to meet their present capitol requirements with the ability to adapt to future needs. Buy what you need for today's production requirements with the knowledge that you can scale up for future demands at a later date.
Contact: Asymtek, 2762 Loker Ave. W., Carlsbad, CA 92008
800-279-6835 or 760-431-1919 fax: 760-431-2678 E-mail: firstname.lastname@example.org Web:
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