What is fluid dispensing in electronics?
Read the following to find out:
What is timed-pulse dispensing?
Timed pulse dispensing is a simple and efficient way of applying precise, consistent amounts of solder paste, cyanoacrylate, conductive adhesive and other assembly fluids used in electronic production. It suits both bench top and automated assembly operations.
In bench top applications, fluid is contained in a disposable syringe that is either held by the operator or mounted on an XYZ table. The syringe is attached by tubing to a dispensing unit, which is connected to a compressed air source and electricity. When the dispenser is actuated, a timed air pulse pushes a pre-determined amount of fluid through the syringe and dispense tip and onto the part.
In automated processes where fluids are consumed in larger quantities, fluid is contained in a pressurised reservoir and fed under constant pressure to a dispense valve fixed on an assembly tine or XYZ table. In valve applications, the timed air pulse opens the valve and determines how long the fluid is permitted to flow.
In either instance, deposit size is adjusted through a combination of time, pressure and tip size.
Of these three factors, time adjustment is the most convenient and effective way to regulate deposit size. A good timed pulse system will feature a microprocessor based controller that permits dispense time to be adjusted in small increments, typically 0.001s. This provides exceptional accuracy and process control.
Although the principles are simple, results will be influenced by the quality of the equipment and disposable components: tips, barrels and pistons.
What fluids can be dispensed?
Bench top systems, where fluid is contained in syringes, will handle virtually any fluid used in electronics assembly, from thin cyanoacrylates to thick solder pastes.
For high volume automated dispensing, there are several different styles of valves available, each designed for different fluids and applications. For thin cyanoacrylates and other reactive fluids, diaphragm valves with inert wetted components are used. For solder masks and thicker materials, a piston valve with an adjustable snuff back will provide a clean cut off.
For solder paste, conductive epoxies and other particle filled materials that tend to compact or `coin`, an auger valve system should be considered. With this set up, paste can be supplied to the valve under either constant or pulsed pressure, with time used to regulate the amount of auger travel.
What factors affect accuracy?
Trapped air, changes in fluid viscosity and disposable component quality all affect accuracy. If air bubbles are encountered in prepackaged fluids (this would be indicated by dripping and oozing from the tip) the issue should be resolved with the packager.
Users who load barrels or transfer material from one barrel to another themselves can minimise problems by using a pneumatic barrel loader to fill barrels from the bottom up.
What about changes in fluid viscosity?
With certain materials, such as multi component epoxies, viscosity is going to change, regardless of the dispensing method. The question is how to deal with it most effectively, with the least impact on quality and production rates.
Timed pulse systems feature either a microprocessor based controller (valve systems) or timer (bench top systems) that simplifies adjustment and allows deposit size to be fine tuned online, without stopping production. Another advantage is that settings can be easily recorded, then duplicated the next time that job is run.
Adhesive can also be purchased in srnaller quantities (5cc instead of l0cc) so that the syringe is ernptied before the fluid viscosity changes. Any increase in packaging costs should be more than offset by eliminating adhesive waste.
Does disposable component quality
really matter ?
Component design and quality are absolutely critical to accuracy and system performance. Best results will be obtained when tips, barrels and pistons are all engineered to work together as a system.
Barrels and pistons require a clos tolerance fit to assure consistent, accurate and complete emptying. Pistons should move smoothly and barrels should have perfectly straight, untapered interior walls. This will ensure the last deposit is exactly the same as the first. Metal and moulded tips need to be free of burrs, flashing and other obstructions that could obstruct fluid flow and cause variations in deposit size.
Matching the cornponents to the fluid and application is also important. Using the shortest tip possible when dispensing solder paste, for example, will allow shorter pulse times and reduce the risk of clogging. Flat wall pistons versus Neoprene pistons will stop oozing at the end of the dispense cycle. Special tips and pistons allow cyano -acrylates to be dispensed safely and accurately.
Components are available in a wide variety of styles and sizes to accommodate many different fluids and applications. Positive locking of tips to barrels via a Luer Lock, eliminates the danger of tips detaching during use. There are even ESD safe components with an integral grounding wire. With such a broad selection available, there`s no reason to settle for less than optimal results.
How high should barrels be filled?
For general applications where material is supplied from barrel reservoirs, barrels should be loaded no more than two thirds full. For micro deposit applications, filling barrels one third full will maintain a large air-to-fluid volume ratio, and assure that any variations in shot size will be virtually unmeasurable.
Why should the user consider a timed pulse system?
Reasons why many users select tirned pulse systems include accuracy and repeatability, ability to handle a wide range of fluids, simplicity of operation and maintenance, and low cost compared to other dispensing technologies.
Any advice on selecting a dispense system ? Talk to a reputable manufacturer with the technical expertise and practical experience to provide knowledgeable application assistance. Evaluate the equipment design, is it simple or complicated ? Ask questions, can maintenance be performed on site by the user or does the equipment have to be returned to the factory ? Also, don`t forget to check on the availability and lead time of components and spare parts, an important but often over looked consideration.
How cost effective are timed pulse systems ?
Timed pulse systems can provide significant cost savings on the production floor and also eliminate the danger of CTS. When lower fluid usage, more parts per hour, fewer rejects, reduced maintenance and downtime are taken into account, many manufacturers find that a timed pulse systems pays for itself in months, not years
Timed pulse dispensing is a proven technology that is well suited to electronics manufacturing. It provides fast, consistent deposits, is versatile enough to dispense virtually any material used in electronics assembly, minimises.maintenance and downtime costs on assembly lines and quickly allows new operators to achieve the same skill level as an experienced one. For these reasons, timed pulse equipment has gained widespread acceptance in the electronics manufacturing industry.