Air Over (Time / Pressure) Dispensing

Air Over with a syring plused with pressurized air
Air Over with a syring plused with pressurized air

What is Air over (Time / Pressure) dispensing?

This is the first method most people think of. It is about 70% of all the systems used in solar and electronic assembly. It is more common on lower end systems and hand held systems, less popular on high end systems.

It works with a puff of air applied over the fluid contained by the syringe and a cap (a receiver head) attached to the top of the syringe. EFD, Musashi, and Iwashita are the major suppliers, but there are dozens of other manufacturers and places to buy these systems. They are used for everything, but can be commonly seen dispensing surface mount adhesives, solder paste, and gasketing materials. Air over is used in both manual operations and automated equipment. This method uses air pressure applied to the top of a syringe to force material out a needle. The amount of time the air pressure is applied is related to the amount of adhesive dispensed. In manual operations you hold the syringe in hand and controls the time duration for the air pressure with a foot pedal. You apply air pressure as long as needed to get the amount of fluid wanted. Any variables causing inconsistencies can be adjusted by eye. When the dispense is automated these variables (temperature, syringe level, fluid type to name a few) need to be controlled if the operation is to be consistent.


Air Over Issues

Air Over Issues with the pressure must be applied for longer and longer durations
Air Over Issues with the pressure must be applied for longer and longer durations

The air over systems have developed over the years and really focused on fixing stability issues for automation. These issues are: 1) The Syringe Level effect, 2) The frictional heating effect, and 3) inspection methods.

1. The Syringe Level effect

As fluid is dispensed, the fluid level in the syringe drops.  This has two factors. A) The fluid offers less resistance as the amount of fluid drops, and B) the air compresses and acts as a shock absorber as the amount of air increases. This effect can be seen in a number of ways. Without any compensation the dot diameter will drop as the syringe level goes down. This is shown in the graph here. Obviously this is not a good situation, so we need some compensator. There are only 2 ways to fix this. One way is to increase the pressure on the fluid. This is NOT the normal method because air regulators are normally manual and not easy to change. Also, increased pressure can separate solder paste, because drool and blow by (force air around the stopper into the fluid). The other way is to increase the time the air is applied. Increase the dwell time. 

Increasing dwell time when a few dots are dispensed by hand is not noticeable, but when you need 40,000 dots an hour, it slows down the assembly process. In SMT lines, this often makes the dispenser the bottleneck. If a $1 million assembly line is idle waiting on a $100K dispenser – it comes a big deal pretty quickly. This is the biggest throughput issue with Air Over systems.

 

 

 

 

2. Frictional Heating Effect

How heat lowers the resistance of the adhesive to flow
How heat lowers the resistance of the adhesive to flow

When dispensing with a hand held dispenser – you dispense a few dots a minute, but automated systems often are running at 40,000 dispenses a minute. That means the air is pulsed 40,000 times an hour. That is a lot of pulses and the air in the syringe gets pounded over and over – It gets hot. Air is a gas made up of very small elastic particles which are ceaselessly moving with high velocities, colliding with each other and with the walls of the containing vessel. The pressure exerted by a gas is due to the cumulative impact forces of the moving molecules against the walls of the containing vessel. The magnitude of the pressure is a measure of the number of molecules and their energy. Pulsing the air pressure at high frequencies to repeatedly force material out of a syringe continuously excites the air molecules in the syringe creating heat. The heat is induced by the dynamic frictional effects of air compression and decompression. Heat is also induced by friction within the material being dispensed as the molecules are forced to slide against each other and through the syringe. A measure of the internal frictional properties of a fluid is called viscosity and defines the fluid’s resistance to flow. This Heat effects the Rheology of the material being dispensed. As the material temperature increases its viscosity decreases – to a point. That is – the fluid .flows easier when the dispenser is beating the fluid and heating it (to a point). When adhesives (glue) get heat is small amounts it flows easier, but over a long period of time it starts to cure and get harder (viscosity increases).  If the heating is really high, the curing can happen pretty quickly (in an hour or a number of hours). Different adhesives cure at different rates. Solder paste heats up the same way, but the pulsing can separate the flux and the metal causing all sorts of issues. Clogging and inconsistencies occur with both materials.

The normal “compensation” for this is to avoid reusing air in the pulsing. Let the old air out and pump in cool air to replace it.  This helps, but does not solve the problems (It slows down the problems).  It also complicates the system.

3. Inspection methods.

Because this really simple Air Over system gets very complex at high speeds over time, most companies offer some sort of inspection system.  This takes the place of an operator’s eye. The machine dispenses dots (normally in an area away from the board) and uses a camera to measure the dots. If the dots are too small the software adjusts the dwell time – it increases the time the air is on.  If it is too small it decreases the dwell time.  If the dots are inconsistent – it turns the production off and the operator is called. This wastes time, but prevents bad product due to bad dispensing.  Normally this is done on a tape, over a back lighted stage. Some machines can do this inspection of the board, but the board normally has a lot of leads, holes and writing making this more complex.


Other ways to make Air Over Systems work better

A needle heater is a common option. By heating the needle rather than the syringe, the heated fluid flows better and does not have a lot of time to cure.  The heated fluid is dispensed out and away from the syringe.  This is commonly used to reduce tailing (stringing), allow for lower pressures and improve dispensing.
Cooled cabinet – Controlling the needle heat up and the syringe temperature down can be done with a syringe cooler or a cabinet environmental system (cool the cabinet).  This can become a difficult control problem


Speed limitations

The complete cycle time for a dot of material to be dispensed includes the X, Y and Z travel times and the cycling time for the material to be forced through the needle commonly known as the shot time clocked in milliseconds (ms). As the equipment industry is reaching its maximum throughput capabilities in the equipment motion control they are forced to optimize cycle times by decreasing the shot size. In Time Pressure systems the only methods of reducing shot time is to either decrease the material viscosity or increase the air pressure. Vendors of Time Pressure dispensing equipment are currently recommending the highest possible operating temperatures that the material industry can support. Therefore, the only possible method of decreasing the shot time is to increase the air pressure. Higher air pressures and shorter shot times result in faster cycle times but they also have serious effects on dot volume variability. Current shot times used in Time Pressure systems range from 50 to 500 ms. Even a best case shot time of 50 ms leaves only 40 ms to complete X, Y, and Z movements in order to achieve the 40,000 dots per hour cycle time. Some flexibility in these times exists because the syringe can be pressurized as the Z travel begins. This is commonly called “pre-dispense”. The graph below shows the relationship of cycle time in dots per hour to tact time. Tact time is the length of time it takes for one complete cycle which includes all X, Y and Z movements, and shot times.


Summary

Changes in temperature effect the viscosity of surface mount epoxy materials. In Time Pressure systems a small increase in dispensing cabinet temperature during dispensing will result in a significant increase in the dot volume of material dispensed. Small changes in the level of material in the syringe level effect the response curve due to the additional air volume being compressed. Changes in the response curve must be compensated for by either increasing the air pressure or the time constant. Increasing the time constant or shot time will have a direct effect on the equipment cycle time reducing the equipment’s throughput.

Although Time Pressure systems may be easy to implement into automation equipment it is extremely difficult to control all of the necessary parameters needed to maintain consistent dispensing. There is an increasing demand from dispensing users to move away from Time Pressure technology due to the variability of depositions over time. The only current dispensing methods capable of minimizing this variability are the Auger valve and the positive displacement piston pumps.