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Dispensing

Sunny Agarwal

 

A look at piezoelectric-based pump technology for jetting materials in low volumes.

Requirements for higher speed and precision in applying very small volumes of material in precise locations have been met with the development of better pump technologies and dispense methodologies for a much more controlled and automated process with tighter tolerance levels and higher throughput. One such example is jetting technology. Jetting has been widely used since the 1990s to expel controlled amounts of material at low volumes. Jetting dispenses droplets of fluid from the nozzle tip and propagates them toward the target in precise volumes via the striking motion of a piston over a seat. Jetting replaced needle dispensing, which requires Z-axis motion to dispense good quality dots.  Jetting, which does not require Z motion, is performed on the fly; i.e., dots are fired while the head is moving from one dispense location to another. Jetting technology is much faster than needle dispensing and produces more stable, controlled, and tinier deposits.

The newest pump technology to emerge for jetting is piezoelectric-based, capable of depositing materials in low volumes to much tighter tolerances. These pumps are able to control dot size and volume consistently; mass depositions down to 12 nanoliters with precise dot placement accuracy have been achieved. One such pump works on the principle of a closed loop control system along with two sensors to track the position of a piston and armature yoke which holds the piston during the upstroke, as well as the downstroke, motion. An adaptive feed forward control works to drive the upcoming stroke of the piston, this being dependent on the preceding strokes to maintain the required charge, measured by the output of an encoder with an incremental resolution of 25nm. The piezoelectric motion is amplified by a lever mechanism to provide the maximum charge of 0.5mm needed to dispense different dot sizes at desired frequencies. Piezo actuators also permit a rapid valve actuation, repeatable in nature, to control the amount of fluid jetted out of the orifice during one complete stroke. This offers the ability to control dot size, operation speed, and improve higher dot positional accuracy with better dot characteristics, e.g., good circularity.

Faster dispensing. The blue column in Figure 1 illustrates a 65% reduction in cycle time operating at 500Hz versus 50Hz. A piezoelectric pump can operate at a high actuation frequency of 500Hz, providing a UPH advantage of 186% compared to a low frequency of 50Hz. Indicated UPH and cycle time percentages have been calculated by running a programmed test that includes lines and dot commands in the process program as were run in a production environment.


Figure 1. Improvement in throughput.

A frequency of 500Hz implies that the pump can dispense 500 shots per second on the substrate. Pump hardware configuration and characteristics of the dispense fluid can have a profound effect on droplet size and dispensing performance. For that reason, piezoelectric pump operation (which is complex) must rely on closed-loop control and powerful software tools that auto-tune the “charge,” which affects droplet size.

Dot positional accuracy. Dot positional accuracy is measured as the difference between the commanded position and the actual dispensed position for each individual dot through dispensing an array of dots. In multiple tests, dot positional accuracy test results are within ±25µm for Loctite Eccobond 3838T epoxy adhesive for different dispense heights varying from 3.0 to 4.0mm. Such good accuracy even at higher dispense height provides an advantage in the ability to dispense within tighter pitches between components.


Figure 2. Dot positional accuracy at different dispense heights (DH).

Dot footprint and circularity. Dot footprint defines how the dot gets placed on the substrate with good dot diameter accuracy and no contamination to adjacent components. Visual inspection shows good dot quality with good positional accuracy, even with dots close to 300µm in diameter, with no satellite formation.


Figure 3. Dot footprints on copper pad.

Line quality. Line quality depends on the dot characteristics in terms of dot positional accuracy and circularity. Line width is defined by the series of dots dispensed at a correct spacing to align properly to form a straight and non-scalloped line. Once the dots touch each other, the boundary is scalloped but the dots are compressed further, and fluid takes the shortest path to the deepest scallop to form a perfectly straight line. The closer the dispense head to the substrate, the better the line quality.


Figure 4. Line quality from dispense height of 3.5mm at line width of 3 dots per mm.

In summary, jetting has become a standard dispense method in assembly production. Real-time adaptive feed forward controls enable a more repeatable actuation of the jet, enabling higher repeatability at faster cycle rates. Jetting can be used with a variety of fluids for a range of applications in the PCB assembly, advanced semiconductor packaging, LED assembly, flat-panel display assembly, and electromechanical devices and products that support alternative energy resources.

Sunny Agarwal is applications engineer II at Speedline Technologies (speedlinetech.com); sagarwal@speedlinetech.com.

An overview of today’s adhesive application technologies.

Automated dispensing of electronic materials in fluidic form is employed across the full range of electronics manufacturing, from board-level assembly to semiconductor applications. Materials dispensed can range from very low (water-like) to very high (toothpaste-like) viscosity and encompass many different functions. These include solder paste to electrically connect components, encapsulants to protect devices from atmospheric conditions, thermal interface materials (TIM) to help dissipate heat from parts, adhesives to attach parts to a substrate or assembly, and others.

Each material may be dispensed in a range of dot sizes or complex lines and patterns, depending on application requirements (Figure 1). Common applications include underfill, selective coating, fastening, dam and fill, potting and dielectric dispense. Shape and function are determined by the type of pump mounted in the dispenser. A dispenser may be fitted with more than one pump head type so that it can perform multiple dispense operations on a single substrate. For example, for an individual PCB or workpiece being processed, one pump head might be dispensing tiny adhesive dots 300µm in diameter to hold very small passive or chip components onto the assembly, while the other head is performing an encapsulation operation on a wire-bonded chip, or applying a selective coating.

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Dispensing is a complex process with many different controllable variables. But essentially all dispensing is divided into two main sets of parameters: material and machine. Material parameters include such variables as viscosity, temperature stability, flow behavior, absence of air, wetting behavior and homogeneity. Machine parameters are all those software parameters a given system uses to be able to execute the process of dispensing the material.

With automated dispensing, there are different types of pump technologies used to precisely meter the deposition of materials, ranging from traditional auger-screw constructs to piston and streaming designs, and even cutting-edge technologies that involve noncontact and radical new fluid management technologies. Each type has its pluses, from reliability to speed to precision, whether the application is dot dispensing or streaming lines of material. Pump designs incorporate special materials or features to accommodate the types of material that they are dispensing; for example, some types of adhesives are filled with highly abrasive material that can quickly wear out pumps that aren’t built with carbide and sapphire components.

As requirements for smaller dot sizes and higher throughputs increase, OEMs must work even harder to offer dispense systems with higher accuracy and higher speeds. We see this in the new pump technologies offering faster cycle times and higher degrees of process control through more sophisticated software and more robust X, Y gantries for stability. To obtain higher accuracy and speed, DC linear motors and linear encoders are used to move the dispense heads around quickly and with precision. Proper gantry design enables higher speeds and accelerations up to 3g without sacrificing accuracy. With today’s automated dispense systems, speed, accuracy, and dispense control are paramount. Machine vision systems ensure accuracy, and more user-friendly GUI and software tools speed teaching and setup.

In terms of pump technologies, in addition to greater dispense control and smaller dot sizes, ease of setup and simple maintenance without overly involved cleaning procedures are goals, driven by the growing number of high-mix product environments where downtime between different product runs is money out of pocket.

Dispense equipment is trending toward smaller, more compact footprints to maximize limited factory floor space without compromising throughput. This often means dual-lane processing capability. Dual-lane processing permits parallel loading of production parts onto two lanes for continuous dispensing, eliminating lost time in non-dispensing activities such as material flow-out and substrate loading/unloading.

Michael Martel is product marketing engineer at Speedline Technologies; mmartel@speedlinetech.com.

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