Saturday, December 10, 2016
VOLUME -22 NUMBER 7
Publication Date: 07/1/2007
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Archive >  July 2007 Issue >  Special Feature:
Components and Distribution
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Specifying Fluid Dispensing Equipment
Configurable dual-lane dispensing system can be retrofitted?in the field.

Fluid dispensing and coating processes are required for many assembly and packaging operations. These processes provide combinations of electrical, mechanical, and thermal properties to components and assemblies for functionality or enhanced reliability. Common processes are surface mount component bonding for assembly, encapsulation and underfill for reliability, thermal interface dispensing for heat dissipation, and conformal coating for moisture and contaminant protection. Many processes serve more than one function.

New products put demands on dispensing equipment to accommodate smaller parts, higher temperatures, and provide increased precision, accuracy, and throughput. Flexibility in equipment to accommodate these changes saves time and money, essential to being competitive. Equipment flexibility depends on features, options, the ability to upgrade, and the ability to scale and transfer processes.

Common Fluids
Dispensing processes start with a fluid that provides the physical properties required for the part. It's the dispensing equipment which needs to accommodate these various fluid and process requirements.

Solder flux is an oxygen reducing agent that prevents oxidation during solder reflow. It's often mixed with solvents to reduce viscosity or improve fluid wetting. After dispensing, the solvent evaporates, leaving active flux on the surface. It's important to balance the flux needed to prevent oxidation, with limited solvents, and edge definition. For flip chip or BGA fluxing, "tackiness", or the ability to hold a component in its wet state, is important. Flux is usually held in a reservoir at ambient temperature. Precise temperature control of the dispensed fluid helps meet process tolerances.

Phase change fluids are usually waxes or polymers. They are solid at ambient temperatures, liquid at temperatures from 70°C to 150°C, and can create a temporary bond that is removed by heating. Many epoxies, such as surface mount adhesives, ball grid array corner bonding materials, and die edge coating materials have the hardening agent encapsulated in a phase change coating. When the material is heated, the hardening agents are released so the epoxy solidifies and bonds. The materials can be deposited in small dots or larger continuous lines.

Epoxies containing silver fillers are used for electrical connections, thermal conductive paths, or both. Other epoxies, such as encapsulants and underfill materials, are pre-mixed and frozen for storage. When thawed, they have a limited working life because their viscosity changes dramatically. Abrasives, such as silica or alumina oxide fillers, are often added to reduce the coefficient of thermal expansion or increase thermal conductivity. Dispensing equipment must accommodate these materials and their properties.

Conformal coatings. Many circuit board assemblies require protective conformal coating. Conformal coating fluids are available in chemistries including: solvent-based acrylic materials that are "dried", moisture cure materials, UV cure materials, heat cure materials, epoxies or combinations that provide specific moisture, physical, or chemical protection.

Gasketing and sealing materials are used in electro-mechanical, electro-optical, and electro-chemical devices. Requirements fall within ranges mentioned for other devices; however, the critical dimensions are determined by how they flow when compressed between surfaces.

Dispensing Methods
Jetting technology has emerged as a standard for many applications. Jetting is characterized as ejecting discrete amounts of fluid from a nozzle with sufficient velocity for the momentum of the fluid to break it from the nozzle. The primary advantages of jetting are speed and the ability to place fluids in places that are difficult or impossible for needles to reach, such as along the edges of stacked die, through RF shields, or between closely spaced die. Fluid can be deposited on a surface without waiting for wetting to occur. Faster cycle times are possible since z-axis motion is reduced. Many applications benefit from higher line quality. Sharper corners, smaller wetted areas, and more consistent knit points are possible.

Needle dispensing. In needle dispensing, fluid is extruded from a needle to contact the substrate. The needle is retracted and the surface tension on the substrate breaks the material string. The mechanism for extruding the fluid can be a piston pump, an auger screw, air pressure, or some other fluid pump. While typically much slower than jetting, it is possible to create smaller dots and finer lines.

Atomizing and Non-Atomizing Sprays. These are coating applications. Atomization may be airless, air assisted, ultrasonic, or a combination of these methods. It produces thin coatings, but lacks selectivity. Non-atomizing applicators are usually film coaters or swirl coaters. They can be applied quickly with good edge definition, but the coating isn't as thin and viscosity is limited.

Flexible dispensing systems require a number of options for fluid handling to meet the needs of stable production. These include reservoirs, temperature controls, tools for position accuracy, part handling, and calibration, and software.

Fluid reservoirs. For fluids with limited working life, the fluid reservoir should be sized to match the production rate. If the reservoir is too small, production will be interrupted for frequent fluid replenishment. Too large, and the material will exceed its working life. Care must be taken to prevent contamination or loss of volatile components in these larger systems. Pressurized bags for moisture sensitive materials are common.

Temperature control. Fluid temperature may be controlled independently in three zones: in the reservoir, at the point of dispense, and at the substrate. A heated reservoir is required to melt phase change fluids. Heated reservoirs can lower the viscosity to facilitate flow for other fluids. The reservoir may be cooled below ambient to extend the working life of fluids. To maintain consistency and quality in production, the fluid may be heated or cooled at the point of dispense. The same is true for the substrate. The amount of wetting and the fluid profile are often affected by the temperature of the substrate. This is particularly true for underfill and encapsulation processes.

Position accuracy. Typical dispensing robots have position accuracies from ±10µm to ±125µm. Besides the position accuracy of the motion control system, it's important to specify the position where the fluid is dispensed. This is a composite of the part alignment, the repeatability of the applicator, and the motion control robot. Consider any speed vs. accuracy trade-offs. Systems offering selectable accuracy may provide the best flexibility for multiple applications. Automated vision systems can reduce tooling cost, but may reduce production speed.
Jetting can reduce the wetted areas and increase speed for underfill processes.


Z-axis position accuracy can be important. For precise needle dispensing, the distance between the needle and the surface affect how the material wets. Most automated dispensing equipment includes options for detecting the surface either mechanically or optically. For jetting systems and spraying systems, z-axis accuracy is less important.

Parts handling. Flexibility in dispensing requires flexibility in parts handling. Systems can be configured to handle printed circuit boards, package carriers, bare wafers, or film frames, or for batch loading or conveyorized automation. Electrostatic dissipation and part heating are additional considerations. Pre-heating stations are available. Tools that heat the parts from both top and bottom can reduce heat-related stress and reduce heating time, thereby increasing production rates. Dual-lane conveyorized systems can improve productivity. Flexibility in dual-lane systems includes the ability to run multiple lanes asynchronously or in pass-though modes.

Calibration. Productivity is enhanced by self-calibration options. The ability to calibrate the amount of fluid dispensed, the dispensing position, fan-width (for film coating), and dispensing pattern optimization can improve productivity and quality to reduce overall cost of ownership.

Software. Software ties the system features together and enables many of the options. It must be easy to use. For it to be flexible, it must also be transportable. A process that starts out in a lab or on a batch system may be transferred to a different piece of equipment with different hardware options. The ability to choose data logging options, CAD import standards, and SECS GEM interface ensures the equipment can be used for a variety of applications.

Other Considerations
Cleaning and maintenance, safety, and technical support should be factored in when selecting automated dispensing equipment. The system should be easily cleaned or contain low cost disposable components. Fluids that harden in dispensing equipment can be costly.

Safety requirements depend on the country in which the equipment is used and the application. Choose equipment that meets global safety requirements. Attention must be paid to ventilation in coating processes and processes which use flammable materials.

In today's production environment, global support is key. Beyond the ability to keep the equipment running, the ability to provide process knowledge and support enables fast production ramps and fast changes in process. A strong global support team is essential to make this happen. Good working relationships with both the equipment manufacturer and the fluid supplier ensure quick time to production and fast recovery from process problems.


For more information, contact: Asymtek, 2762 Loker Ave. W., Carlsbad, CA 92008 800-279-6835 or 760-431-1919 fax: 760-431-2678 E-mail: info@asymtek.com Web:
http://www.asymtek.com

 
 
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