-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Aspects of Microdispensing
December 31, 1969 |Estimated reading time: 5 minutes
Microdispensing has become the primary focus of liquid dispensing research and development (R&D) over the last few years, with the prime motivator the pursuit of small deposits.
By Gary Helmers
An additional benefit has been improved volumetric repeatability for any size deposit, achieved by new dispensing technology developed to meet the small volume challenge (Figure).
Figure. Optoelectronic applications drive the quest for small, highly repeatable fluid deposits.
Digital dispensing is defined as the positive displacement of material from a controlled volume metering cylinder. Digital dispensing is an accretion dispensing technology as opposed to an extrusion dispensing technology. Older dispensing technologies such as the rotary auger pump, air-over-time-pressure or any of the on/off valve technologies are examples of extrusion dispensing technology in the sense that a flow is initiated, allowed to continue until a sufficient volume has been extruded, and then the flow is shut off — like squeezing toothpaste from the tube. The inaccuracies inherent in each step (the initiation of the flow, varying flow rate and shutoff of flow) were of low significance for deposits 0.020" in diameter, at 35 nanoliters and above. However, these flaws are fatal for smaller volumes because they cause unacceptable volume inconsistencies.
Digital dispensing technologies create discrete, highly repeatable "fluid pixels." These pixels are applied in an accretive fashion to "build up" a deposit that is high in volume consistency. Pumps* have been developed that can displace material volumes of less than half of one nanoliter, meaning that microdispensing has now graduated to nanodispensing. The pump's technology uses a piston that moves within the dispensing chamber to deliver an exact amount of fluid through the tip. Material is held in a syringe or reservoir and transferred under low pressure to a pumping chamber. The material then is dispensed under high pressure through precision tips by the stroke of the piston. The pump is unaffected by viscosity changes and allows dispensing of materials from 1 to 1 million cps.
The pump meters out a defined increment of material and can repeat that identical metering action 90,000 times per hour to create volumetrically accurate dots, beads, fills and underfills.
Three characteristics of a capable digital dispensing nano-pump have been identified — positive shutoff, positive wetting and true metering cylinder positive displacement.
Positive Shutoff
This is one of the most important features in any dispensing system that attempts to accurately control the deposit of small volumes. It is important that there be no open path from the material reservoir (generally a syringe) to the substrate at the end of the dispensing cycle. If the path is left open, as in an auger pump, linear pump or other time-pressure device, the variables that must be controlled become too numerous. Material can continue to flow after desired shot size has been achieved. This drooling, also known as run-on or leaking, is caused by several factors, including insufficient control of pressure, changing volume of material and compression of the material in the reservoir, or a change in viscosity of the material, usually because of an atmospheric temperature change. Even the type of piston in the barrel of a syringe can have a complicating effect on a dispensing valve without positive shutoff. Positive shutoff eliminates concern about these variables, as well as the need for frequent recalibration of the fluid mass dispensed.
Positive Wetting
The most significant challenge with needle dispensing is encouraging the material to leave the end of the needle and stay with the substrate. Most dispensing technologies allow material to "bloom" at the needle orifice, and then "daub" the material on to the substrate, a process akin to pin transfer. Relying on this transfer method requires extreme care to ensure the preparation of the substrate surface allows material to stick to the substrate and willingly leave the needle tip. Proper preparation and construction of the needle tip also is essential.
Consistent diameter deposits only are possible when the motion platform can repeatedly place the dispensing tip at a consistent height above the target. This is called the gap — the distance between the dispensing tip and the substrate at time of dispense. The dispensing system must be capable of using a height probe, either touch or laser, to create a topographical map of the substrate surface in memory and then be able to place the dispense tip by referring to that map. A valve or pump may cause a consistent amount of material to be metered to the end of the needle, but cannot ensure that the metered amount of material will depart the end of the needle.
Control of the gap is crucial to wetting the material to the substrate. Anyone who has used an auger pump has experienced the "big dot, little dot" pattern that occurs when a daubing-type tool is not set for precisely the right gap. The first daub fails to leave the entire "bloom" of material, so the next daub leaves a bloom and a half on the board, and the process continues — big dot, small dot, big dot, small dot.
One type of pump drives material down the needle tube with a controlled sufficient velocity so that the material exits the nozzle with enough energy to "positively wet" onto the substrate. This is not spitting the material, with all the satellite-splatter and heating problems associated with that technology, but is rather a method of liquid transfer from the needle tip to the substrate that encourages material to stay with the substrate and leave the end of the needle.
True Metering Cylinder Positive Displacement
The only way to achieve a consistent volume of material, deposit to deposit, is to meter material out using true positive displacement. A metering cylinder is a controlled volume cavity filled with material to define a specific liquid volume. The material then is ejected from the cylinder. The cylinder is refilled and emptied as often as required to "build up" the precise liquid volume required. The metering cylinder also must be placed as close to the nozzle of the pump as possible to minimize the "dead space" between the metering cylinder and the substrate. "Dead space" can be a source of volume inaccuracy.
Conclusion
Digital dispensing was conceived to meet the challenge of smaller dots, but also provides the ability to create volumetrically consistent, repeatable liquid deposits. In addition to a capable pump, digital dispensing must be mounted on a motion system that can deliver the positional accuracy and gap control required for dispensing at this level of precision. The most capable motion systems provide positioning repeatability of ±0.00008" on all axes.
*Piston positive displacement pumps.
Gary Helmers, vice president, Dispensing Division, may be contacted at Creative Automation Co., (818) 767-6220.