MEMS | ARTICLE << Figure 8: Motion profile. >>
<< Figure 9: Mechanical jet applicator ejecting small droplets. The jet uses a pneumatic piston with a ball tip to raise the piston by air pressure and to push fluid through a narrow orifice at the jet nozzle. As the ball tip on the end of the piston engages in a seat at the nozzle, the fluid is energised and shoots a droplet from the end of the jet [3]. >>
<< Figure 10: a) Arrays of dots right after landing (FAR LEFT); b) Dots connect to form a continuous line after laminating with top glass (LEFT). >>
Figure 4 shows an example of conductive adhesive jetting on a wafer to create uniform thin lines of 0.3 mm width.
2) VOLUMETRIC ACCURACY There are many variables to control the flow of the material to dispense tight sealant lines. Fluid pressure, needle diameter, length of needle and fluid viscosity determine volumetric flowrate shown in figure 5.
<< Figure 5: Volumetric flowrate - Poiseuille’s law. >> Q=πd4
∆P 128µL
d = inside diameter of needle or nozzle μ = fluid viscosity L = length of needle or nozzle ∆P= pressure drop Q = volumetric flow rate
Many varieties of materials have a pot life, and viscosity will change over time. This changes the flow rate for a given set-up. Along with flow rate change, dispensing volume changes if dispensing parameters are fixed. Thus dispensing parameters need to be adjusted to keep dispensing volume consistent, which calls for the use of a flow rate compensation tool. Dispensing uniform lines can be a challenge due to material characteristics if a flow rate compensation tool is not applied. Figure 6 shows an
example of two-part silicone flow rate changes over a 4-hour period (after fluid mixed) while dispensing parameters were fixed.
Once flow rate over time is characterised, the weight calibration tool can be applied at a given frequency for routine maintenance. Examples of variables for calibration are dispensing on-time, fluid pressure, or the number of shots (when jetting), depending on available tools and chosen applicators.
Dispensing by auger or time-pressure valves Certain materials are suitable for needle-type dispensing, like solder paste or dam epoxy material. There have been many efforts to create uniform thin lines and to dispense consistent volume at the knitting point for sealing. Sealant tends to come out slowly from the needle at the beginning and more excessively at the end of the sealing line. These inherent effects make it challenging to dispense a uniform knitting point. When the needle dispenses the sealant on the surface, controls at the starting and ending points are critical to make volumetrically consistent dispensing. Figure 7 shows the typical dispensing point when dispensing parameters were not optimised on the left (a) and improved case on the right (b).
Impacts on the sealing line volume control RESPONSE TIME Fluid will start to flow once fluid pressure is on. The duration between the valve turning on and sealant coming down to the surface depends on viscosity of fluid, fluid pressure, valve motor speed and signal delay from the system. Somewhat uncontrollable
42 | commercial micro manufacturing international Vol 6 No.5
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