BY CLAUDE BERGERON, PRODUCT LINE MANAGER, NORDSON EFD
Micro-dispensing has evolved to support the needs for higher throughput assembly production by developing specialized dispensing technology for automated inline assembly and manufacturing systems, and for stand-alone production devices. Such dispensing technologies can deliver very discrete, highly precise microdots of fluid onto substrates within extremely short cycle times, at rates of up to 1,000 cycles per second, and with infallible repeatability. Evaluating technological options in micro-dispensing, understanding how they work and their strongest and weakest attributes, is essential in determining the benefits of implementing a new technology into the production environment.
Constant improvements and innovations in virtually every industry continue to increase the functionality and interoperability of automated devices and equipment. Along with this has been the continual quest to reduce their power requirements, with the solution being the miniaturization of these devices and their components. Everywhere we look in our world of increasing automation, miniaturization is a key driving factor that makes these devices and equipment possible. Smart phones, digital cameras, smart TVs, computers, pacemakers, MRI, robotics, vehicle fuel injection, GPS and thousands of other consumer, commercial and industrial devices and processes are made possible because of component miniaturization. Indeed, the convergence of the IoT (Internet of Things) with the Ethernet, and the increasing possibilities of data and power connectivity for enabling devices, is being made possible because of miniaturization of components and their diminished energy draw.
This continuing miniaturization of components presents challenges for manufacturing, and particularly for electronics assembly. Surface Mount Technology (SMT), for example, is the dominant electronics assembly process used for the production of these electronic devices in diverse industries – ranging from automotive to medical devices to aerospace. The need to manufacture smaller and more complex assemblies with dense multi-layer circuitry and mixed technology boards, as well as the increasing range and mix of surface-mount component sizes and types, poses challenges for SMT process engineers.
Critical to SMT assembly is the need to deposit very small and precise amounts of fluid – such as adhesives and silicones – to the tiny micro-electronics and other minuscule parts, to keep these surface-mounted devices in place before and during the soldering process. These deposited fluids can also provide the added benefits of mechanical strength, thermal conductivity, dielectric strength and chemical inertness throughout the life of the assembly.
The tiny amounts of adhesive and silicone must be dispensed reliably and accurately in dosage and placement. The precise positioning and quantity of these fluids deposited on the SMT substrate is critical to that product’s assembly, function, quality, appearance and viability.
This technique of depositing tiny volumes of liquid media dosages – called micro-dispensing – is not isolated to SMT assembly, but spans a multitude of assembly applications in a diverse range of industries that require the precision dispensing of oils, grease, lacquers and a multitude of other media. Micro-dispensing is necessary for sealing the laminated glass composites of smart TVs and touch-screen computer displays, dispensing silicone phosphor in LED assemblies, securing components of compact digital cameras, making tiny deposits of sensitive fluids for assembling biomedical assays and lab-on-a-chip materials, and providing lubrication for the assembly of vehicle bearings, transmissions and engine parts.
Micro-dispensing has evolved to support the needs for higher throughput assembly production by developing specialized dispensing technology for automated inline assembly and manufacturing systems, and for stand-alone production devices. Such dispensing technologies deliver these very discrete, highly precise microdots of fluid onto substrates within extremely short cycle times, at rates of up to 1,000 cycles per second, and with infallible repeatability.
More than a dot
A dispensed microdot should not be thought of as being a single dot, independent from the next. This is seldom the case. In most applications, a series of small micro-deposits are being put down dot-to-dot, that stitch together, blending seamlessly to make a continuous line or pattern. Any shape or solid can be created on a substrate by stitching dots with micro-dispensing. Multiple microdots can be stacked to form larger dots.
Dispensed microdots are 300 – 400 microns in diameter, which is roughly the thickness of five to six human hairs. Within these parameters, real-world applications typically require many different size dot diameters to accommodate the different size components that populate a substrate. Larger components, such as those on SMT assemblies, may require multiple dots to be dispensed at a single placement site.
Given the requirements, more advanced micro-dispensing technologies can deliver extremely tight deposit tolerances that are within 1 percent of project specification for size, height and shape of the microdots.
Assessing parameters for micro-dispensing
Evaluating technological options in micro-dispensing, understanding how they work and their strongest and weakest attributes, is essential in determining the benefits of implementing a new technology into the production environment. The key is selecting a micro-dispensing method that lends itself most closely to the requirements of a specific production process.
Initially, several critical areas need to be examined before micro-dispensing methods can realistically be assessed. These are: a) substrates; b) fluid properties; and c) requirements for micro-dispensing.
The physical characteristics of the substrate – the material onto which the microdot fluids will be applied – strongly influences the selection of micro-dispensing technology. Cycle times for dot deposition and throughput rates are regulated to some extent by the substrate surface topography.
Dispensing of fluids onto hard-to-access areas, uneven or irregularly shaped surfaces, or delicate substrates are key factors that need to be carefully assessed as they can considerably impact assembly production. These directly affect the Z-axis movement of the micro-dispensing system, influencing its ability to move over uneven surfaces and dispense the correct volume of fluids in the right locations.
As the trend toward miniaturization progresses, substrates are becoming more crowded and uneven in nature. Such is the case with substrates used in electronic wafers and thin film electronics, and Printed Circuit Board (PCB) assembly.
The variety of fluids and fluid viscosities that can be micro-dispensed spans a considerable range – encompassing epoxies, adhesives, silicones, greases, oils, flux, lacquers, solder paste, weak acids, ester and weak alkalis. There are many formulations of fluids from varied suppliers, each specially formulated for different application techniques.
First, and foremost, the fluid to be dispensed must readily flow through the dispensing heads. Viscosity is the resistance of a fluid to flow, and is one of the primary rheologic properties used to determine if a fluid is dispensable.
Then, once the fluid reaches the substrate, it must have the ability to restructure and recover to keep it from spreading and contaminating the other components on the substrate. The property of the fluid that allows the material to return to its original viscosity is part of the thixotropy of the fluid – the condition of the fluid becoming less viscous when subjected to an applied stress. Understanding the thixotropy of fluids to be dispensed is a critical component in successful micro-dispensing.
Other properties of the fluid rheology that must be considered for micro-dispensing include its density and weight, whether or not it has abrasive fillers, and whether it is safe to dispense or combustible. The fluid properties can also be modified by the dispensing process being used. Heating of the air within the dispensing system can change the viscosity of the fluid, as can the fluid pressure and line speed. Also influencing the rheology is how the fluid is stored. Is it kept in cold storage? How is it warmed? The properties of a fluid change as temperatures change, and as it expires.
Characterizing different fluids and determining the best dispensing parameters for a specific application are important factors for implementing a successful micro-dispensing process.
Requirements for micro-dispensing
Requirements for micro-dispensing are as varied as the differences in production environments. Many factors need to be assessed, such as: Is the micro-dispensing to be used for a manufacturing/assembly process? If so, is it to be utilized within an industrial environment, or possibly within a cleanroom? Or, is the system to be used within a production process, possibly producing prototypes? Are these processes semi-automated or full-automated? How skilled are the operators?
Then, more specific process questions need to be assessed. These include determining: a) expected parts per hour to be produced; b) production cycle times; c) speed that the parts can be fed into placement for dispensing; and d) if the micro-dispensing system will be mounted on a robot, or stationary with the parts moved beneath.
Identifying these requirements will facilitate the selection of the most optimum micro-dispensing system for the application.
Jetting versus contact micro-dispensing
There are two basic types of fluid micro-dispensing techniques: classic needle-based contact dispensing and non-contact jet dispensing. Although traditional contact fluid dispensing is still the predominant technology in use, it is hampered by definite disadvantages of speed, particularly when dispensing on irregular substrates. While the best method is ultimately determined by the material and application at hand, for many automated fluid dispensing processes, high-speed jetting technology is a popular and innovative alternative to traditional, needle-based contact dispensing.
In contact dispensing, the fluid drop forms at the exit of a nozzle, and is deposited on the substrate by contact while the drop is still on the nozzle.
Contact dispensing frequently relies on a time-pressure method to dispense assembly fluids. The amount of fluid dispensed depends greatly on the time the valve is open and amount of air pressure applied to the fluid reservoir. The amount of fluid dispensed may vary from deposit to deposit, for example, if the shop air pressure fluctuates.
Contact dispensing can also rely on a volumetric method to dispense assembly fluids. Here, the volume of fluid dispensed remains constant. However, there can be repeatability issues when fluid gets sucked back into the dispense tip and dispensed with the next deposit.
The benefits of contact dispensing are: a) versatility to dispense virtually any type of assembly fluid; b) easier and shorter time to set-up and program; and c) reduced risk of deposit satellites splashing off the substrate.
Contact dispensing has disadvantages, such as: a) slow dispensing rates due to Z-axis movement, typically up to about 300 deposits per minute; b) the substrate part has to be touched to dispense the fluid; c) the substrate part can potentially be damaged; d) the dispensed fluid is not always precisely in the expected place; e) fluid deposit amounts are difficult to reproduce; and f) residual material can adhere to the tip, which can affect repeatability.
High-speed jetting is non-contact – the jet valve never contacts the product or substrate surface. Because of this it offers a higher degree of flexibility, and can be used in a wider variety of applications that otherwise would require a Z-axis system with height-sensing and positioning functionality. These distinguishing features provide advantages in dispensing speed and agility, dot capability and quality, maintenance, throughput, and cost of ownership.
High-speed jetting is made possible by piezoelectric technology, which enables this style of jet valves to dispense fluids at speeds at up to 1,000 Hz. Modular piezoelectric jet valves that can be configured for multiple uses combine the benefits of high-speed jetting, piezoelectric technology, and incredible application flexibility. This makes them a powerful alternative to traditional dispense valves, which are often limited to a single application capability.
High-speed jet valves that incorporate piezoelectric technology can achieve extremely fast open-and-close cycles that eject, or “jet”, fluids onto a substrate. When pressure is applied to certain materials – such as layered structures of specialized lead zirconate titanate (PZT) ceramic stacks interleaved with electrodes – it creates voltage. Conversely, when voltage is applied to PZT, it changes its shape. In the case of piezoelectric jet valves, voltage is applied to a piezoelectric actuator inside the valve. The applied voltage causes a bulk change in the ceramic’s length, allowing the actuators to be used for fast, high-force, real-time position control at the nanoscale level.
The benefits of jetting include: a) highly precise, repeatable deposits independent of part topography or tolerance; b) dispenses at a continuous speed of up to 1,000 deposits per second; c) jets from any direction, including horizontal and upside down; d) meets exact deposit tolerances as small as +/–1 percent; e)eliminates substrate damage since there is no contact with the surface while dispensing; f) laser-based light barriers can count every deposit jetted – adding a level of process verification and quality control not possible with contact dispensing; g) eliminates dispense tip damage since there is no contact with the substrate surface; and h) jet valves can create patterns not possible with needle dispensing.
The disadvantages of jet dispensing include: a) larger deposits when compared to needle contact dispensing – dots as small as 0.3 mm in diameter vs. 0.05 mm in diameter with contact dispensing; b) some fluids, such as certain particle-filled and highly-abrasive fluids, cannot be jetted; c) possibility for satellites or splashing can be greater than with contact dispensing, requiring time to adjust dispensing parameters; and d) additional training of operators may be necessary, due to more sophisticated programming requirements.
The ability to deposit very small and precise amounts of fluid is an ever-increasing necessity for manufacturers of tiny micro-electronics and other minuscule parts. Jet micro-dispensing meets these requirements with the capability to deposit extremely accurate and very small amounts of fluid, with higher repeatability and greater consistency than contact dispensing.
Because so many factors can impact a micro-dispensing process, it is important to consult an experienced fluid application specialist who knows the specifics and priorities influencing micro-dispensing for different applications.
Consulting with an application specialist early in a project will ensure the right micro-dispensing equipment is utilized, and the most optimum process has been put into place. This will facilitate manufacturing to achieve the desired production throughput, and improve process control, while reducing rework, rejects and fluid waste.