A Discussion of the Essentials for Microdispensing Including Accuracy and Volume Size Deposits
The dispensing required for optoelectronic assembly is not too different from the dispensing for surface mount and solder paste applications. It just needs to be placed more accurately (volumetrically), and of course, with much smaller deposits than have ever been required before. No problem, if your dispensing system has the inherent capability for precision volumetric dispensing on the order on 1 nanoliter! Fortunately, technology exists today to achieve these volumetric and repeatability goals. There are a few basic requirements that narrow the field of competing technologies.
This is a key feature in any dispensing system that attempts to accurately control the deposit of very 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 devices) the variables that must be controlled are too numerous. Material can continue to flow after the desired shot size has been achieved. This drooling (or run-on or leaking) is caused by a group of factors: insufficient control of pressure on the material reservoir, compression of material in the reservoir, or a change in the 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 shut-off.
Positive shutoff eliminates the need to be concerned about these variable factors, as well as the need for frequent re-calibration of the fluid mass that is dispensed. Valves and pumps that incorporate positive shut-off include needle valves, pinch-tube valves, spool valves, diaphragm valves and the piston positive displacement pump.
Getting the right dispensing pump is only half the battle. Consistent diameter deposits are possible only when the motion platform can repeatably place the dispensing tip a consistent height above the target. This is called the gap — the distance between the dispensing tip and the substrate at the time of dispense. The dispensing system nust be capable of using a height probe (either touch or laser probe)to create in memory a topographical map of the substrate surface and then be able to place the dispense tip by refering 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.
The main challenge with needle dispensing is encouraging the material to leave the end of the needle and stay with the substrate. Most dispensing technologies allow the material to “Bloom” at the needle orifice, then “dab” the material onto the sobstrate (a process akin to pin transfer). Relying on this transfer method requires extreme care to ensure that the preparation of the substrate surface allows the material to stict to the substrate and willingly leave the needle tip. Proper preparation of the needle tip is also esential. Control of the gap is crucial to achieve wetting of the material to the substrate. Anyone who has used an auger pump has experienced the “Big-dot, Little-dot” pattern that occurs when a dabbing type tool is not set for precisely the right gap. The first dab fails to leave the entire bloom of material, so the next dab leaves a bloom and a half on the board.
Syringe pump time-pressure devices and the piston positive displacement pump drive material down the needle tube with sufficient velocity that the material exits the nozzle with enough energy to “wet positively” onto the substrate. This is not spitting the material, with all the satellite-splatter and heating problems associated with that technology, but is a method of 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 Positive Displacement
Positive displacement has been a misused term in the dispensing industry for many years, and there are numerous devices available that claim to be positive displacement. However, the only way to achieve a consistent volume of material, deposit to deposit, is to meter material out using true positive displacement. This requires a dispensing system that incorporates positive shut-off, gap control and positive wetting capabilities. Many optoelectronic component applications require the high level of precision and repeatability provided only by true positive displacement dispensing systems..