In dispensing of adhesives and solder paste, reliability and quality control problems, such as variations in viscosity and dot size, dripping and waste of the material, and even changes in homogeneity, are conditions that challenge the accuracy and consistent performance of fluid dispensing systems. In most cases, problems can be attributed to the use of pneumatics to "push" the material out of the barrel (syringe) reservoir.
There have been attempts by some manufacturers to develop alternative techniques of driving the dispensing mechanism — such as the use of hydraulics and a flexible metal cable — less-than-ideal solutions as well. Hydraulics can leak and cables can break, disrupting production until the system can be replaced.
Here, we will examine a proprietary and proven type of dispensing system that relies neither on air, nor on hydraulics or mechanical cables, and as a result, offers consistent and reliable performance and absolute process control, dot after dot after dot (or bead after bead after bead).
First, consider the downside of pneumatics, since, even today, such dispensers remain popular in electronics assembly.
What's Wrong with Air?
The process of dispensing adhesives and solder paste began with pneumatics. Whether a handheld gun, a benchtop system, or an automated workstation, whether a diaphragm, spool, needle, or piston, such dispensers depend on a column of air under pressure to force material through a dispensing tip and onto a substrate. Heat, moisture, fluctuations in air pressure, contamination, turbulence in the fluid due to pulsing of compressor air, changes in viscosity — all of these can undermine the ability of air-driven dispensers to deliver prescribed amounts of material consistently, especially in the small amounts required by high-density electronic circuitry. Pneumatic dispensers still serve a purpose, but where precision and reliability are paramount, dispensers based on other methods of delivering the adhesive or solder paste are gaining wider use. One in particular, The Fishman Corporation AirFree LDS9000, uses a unique drive system that offers a superior level of performance without the limitations of other types of dispensers on the market.
Linear Drive System
With the LDS (linear drive system) 9000, a notable difference over other dispensers, including valves, is a casehardened drive screw in the dispensing gun. The screw is machined with threads designed not to carry paste, but rather to rotate through a fixed block, or nut, mounted in the gun housing. As the screw threads through the block, it changes the linear position of the piston at the end of the screw, applying pressure on the material in the barrel or syringe. Movement of the piston is precisely controlled by a stepper motor driven by a linear actuator in the gun. The actuator directly engages the screw, eliminating lags and backlash in the drive system. Rotation of the actuator is programmed and controlled by software, the motor stepping in 3.5° or 7.5° increments, with the lead screw having either a 0.012-in. or 0.024-in. thread. The increment of rotation combined with the selected screw thread for the dispensing gun enables volumes ranging from 0.00023cc to 30cc and dot diameters as small as 30 mils for mounting of semiconductor components. A programmable end-of-cycle "draw-back" prevents dripping of the adhesive or solder paste.
The linear drive system is a true positive displacement pump, in that a specific volume of material is displaced each and every time, even during ramp up and ramp down. Also an advantage, the smooth motion of the linear actuator and the resulting movement of the piston enable the syringes to be filled with more material than is possible with other types of dispensers. What this means is less syringes are required to dispense the same volume of solder paste in a production run. Since the cost of adding adhesive and solder paste to syringes is minimal compared with the price of additional syringes, significant cost savings can result.
The linear drive system is just that: a system. The dispenser itself is but one component. Complementing the gun is an "intelligent dispensing" control unit connected to the gun with an electrical cable. As such, key data are programmed and stored, namely: the exact dimensions of the syringe (inner diameter and length), dispense volume, dispense rate, drawback, and drawback delay. With these parameters entered, the software calculates the "steps" forward and backward to produce the desired dispense amount, each step being either 3.5° or 7.5° in rotation, depending on the selected dispensing gun. In other words, the software determines the exact movement of the piston in the barrel to dispense the required amount. Neither the viscosity of the material nor the amount left in the syringe has any impact on dot volume; and simply activating the control, either manually or automatically, ensures high repeatability and absolute process control in delivering the desired amount.
For more information, contact: Fishman Corporation, 192 South Street, Hopkinton, MA 01748 800-433-2115 fax: 508-435-2119 E-mail: firstname.lastname@example.org Web: http://www.fishmancorp.com