In modern electronics manufacturing processes, component miniaturization has led to the continual decrease in quantities of medium to be dispensed and increasing demands for greater dispensing accuracy. The smallest variations of quantity are now more intensively noticeable than had been the case a few years ago. Even with the best high tech dispensing valves, the characteristics of the medium to dispense changes with environmental conditions. Therefore, differences in volume as a result of this have to be avoided or corrected by suitable measures. The reasons for these varying dispensing volumes are found in the parameters that influence the process. The most obvious variables include pressure in the dispensing cartridge and time of dispensing. Less obvious are values such as environmental temperature, level of the cartridge and the retention rate of the cartridge at the machine. All these factors have considerable influence on the characteristicsof the medium to be dispensed. Time/pressure variation
Various dispensing methods react differently to process variables and therefore produce different volumes of dispensing. The most common variation is "time/pressure”. The mode of operation of a dispensing system is very simple. At pressurization of the cartridge, the medium will be pressed out of the cartridge by the dispensing needle. The volume dispensed depends on how long this pressure remains. If one leaves this pressure pulse constant, one can notice a decrease in volume dispensed. The reason for this is the increase in empty space in the dispensing cartridge (figure 1). At constant entry pressure and period of time at which the hollow is filled with air, the ultimate pressure in the cartridge reduces and this decreases the dispensed volume as well. 
A further factor which causes an irregular result is the heating up of the dispensing medium. This results in the continuous compressing and removal of the contents in the cartridge. These changes of pressure are similar to the effect caused by pumping of a bicycle tyre by hand where heat is generated. This heating of the dispensing medium leads to a change in the medium’s viscosity, therefore producingvariable results. Two dispensing methods that can ensure greater volume consistency are introduced below: • Archimedean screw
At the screw valve, the medium for dispensing is moved with a worm conveyor in the form of an Archimedean screw. The medium will be pressed out of the cartridge into the worm conveyor by air pressure. But the pressure at the cartridge is so low that at standstill, no medium will be conveyed. Therefore the pressure can remain at the cartridge constantly and the conveyed volume only depends on the number of the turns of the screw and the pitch. However, variation depending on viscosity can still result as the screw in the feeding pipe cannot tighten 100 percent. Thus, the age of the medium to be conveyed, as well as changes of the environmental temperature at various viscosities, exertsgreater influence in differences of the conveyed volumes. • Jetting
Jetting is an extremely quick, exact and contactless dispensing method suitable for a wide range of applications. Utilizing a quick pulsating ceramic fibre plunger, the smallest drops are single "shot” out of the valve. Today this technique is the dispensing method with the best reproducibility in terms of dispensing volume. But here also, air pressure and viscosity have an influence on the result. Temperature control
As described, such variations of volume are undesirable in manufacturing as they affect the quality of the product. Therefore, methods and technologies have to be used to prevent or compensate for the variations. An elegant method to negate the influence of temperature is the use of a heating/cooling valve. Such valves include Peltier elements which feed or dissipate heat to the valve face as required in order to hold the temperature constant. By this, the conditions at dispensing are independent from the environmental temperature. Such valves are obtainable as time/pressure or screw dispensing systems and can work with an air or liquid heat exchanger. Automatic dot size adjustment
The above demonstrated method of temperature control, however, is still not efficient to achieve exact volume consistency for longer periods of time as it cannot remove other factors of influence. Better results can be only obtained with regular measurement and calibration of the real dispensed volume whereby the dispensing machineadjust several factors to ensure that the dispensed reference volume is in the range of tolerance (figure 2). In this way,the dispensing result stays stable despite the changing ofparameters. 
Such a procedure is called "Automatic Dot Size Adjustment” or ADSA. For the determination of the dispensed volume two procedures exist – optical measurement and weighing procedure. For optical measurement, the diameter of a dispensing point will be recorded with a camera and the space of the point determined with quick image processing routines. If the diameter of the drop is too small, the process of dispensing will be influenced in such a way that the next time a greater volume will be dispensed. Such test points are mostly dispensed on known reproducible surfaces as the dimension of the point not only depends on the volume but also the wetting angle of contact as well. The measuring method with a scale is independent from the wetting angle of contact but requires a very exact measuring system. This system works well with contactless jet dispensing as it can be integrated with a collecting vessel and the scale only has to determine the difference in mass. However, in needle dispensing, the tear off behavior of the drop is important which is why the connected dispensing space has to be easy to clean as the mass of one or several points in the dispensing parameter will be corrected in such a way that the result corresponds to exact demands. Summary
In the past, various tests had been undertaken to control the dispensing volume by empirical models which were based on the measurement of several parameters such as temperature, pressure and level. Such models are extravagant and they are missing something important, that is the checking of the actual volume. Only with an active control procedure like ADSA can one guarantee the reproducible quality demanded. ADSA is a longstanding tested procedure which compensates active variations in the dispensing process resulting from varied environmental conditions. With adequate adjustment, the system is able to detect empty cartridges or clogged dispensing needles. This method can be utilized in both needle dispensing and jetting systems as well as simple time/pressure dispensing systems, producing very constant dispensing results. About the author
Anton Knupfer, Product Manager Dispenser, EssemtecAG, can be reached at info@essemtec.com. | 
