Each step on wafer processing such as mounting the wafer on the probe card during wafer test, or holding it while it is being rotated during photo resist coating, involves wafer handling and transfer. To be able to perform efficiently and to produce high throughput with sophisticated quality, wafer machines use vacuum chucks, which precisely handle large and complicated wafers. Vacuum chuck is a method used to hold a work piece for machining, and operates using the theories of vacuum. Aside from wafer processing, it is also used in assembling of platens and film transporting during film fabrication, and handling of platen for assembly operation of camera.
Vacuum as a Suction-Producing Machine
But before digging any deeper into vacuum chucks, what is vacuum in the first place? Vacuum is a space or a region with air pressure lesser than the atmospheric pressure. Its quality is often signified by the amount of matter inside the space. A vacuum in an ideal perfect state does not have any particles in it. But such state does not exist in any kind of technology today and the only space that comes close to being a perfect vacuum is the outer space. Vacuum has been very useful in a variety of applications in both industrial and commercial. Some of these are incandescent lamps, household vacuum cleaner, car brakes, water pump, evaporation and sublimation in vacuum (outgassing), and vacuum-driven machines (or the suction-producing machines like the vacuum chucks).
Generally, vacuum-driven machines are known to produce suction. But in contrast to this common knowledge that vacuum sucks parts, it actually works by controlling the atmospheric pressure beneath and above the workpiece. Since the atmosphere has always a tendency to flow from high-to-low pressure to keep its balance, a decrease of pressure on the machine side will set the atmosphere on the other side to flow towards the machine. As for the vacuum chucks, the air pressure on the side of the workpiece facing the chuck is decreased. Then, the atmosphere on the other side of the workpiece, which is at high air pressure, will try to equalize the atmosphere by filling up the low pressure; but since the workpiece is located in between them, the workpiece is instead pushed to the chuck.
“Short and fat”
Unlike the usual workholding techniques used in manufacturing machines that exposes only three out of six sides of a part, vacuum chuck allows five sides for machining. This enables vacuum chucks to handle soft and/or thin materials, which can be bent or broken by other techniques. However, vacuum chucks are not advisable for any kind of parts. While vacuum chucks are best for soft or thin materials, it is not always the preferred method for workholding. Most of the time, vises and clamps are used to hold workpieces because of the high mechanical holding/clamping force, which allows more aggressive machining on the part. The rule of thumb for identifying the suitable application for vacuum chucks is simplified as “short and fat”.
Fat denotes larger surface area. With a larger surface area the vacuum chuck can hold at a greater force, and hence, can allow more aggressive material cutting or removal. I.e. for a 5”x5” part held by a high vacuum of 28” Hg (or 14 PSI), its holding force will be at 350 pounds (lbs).
Surface Area (As) = 5 in × 5 in =25 in2
Material Holding Force = Surface Area × Vacuum Holding Force
Material Holding Force = 25 in2 × 14lbs / in2
Material Holding Force = 350 lbs
The length requirement, on the other hand, also plays an important factor for material cutting. Since vacuum chucks utilize air pressure to squeeze the part down, it can only hold as much as 13 to 14 pounds per square inch. Other forces that can lift the part are avoided. This is why materials held by vacuum chucks are cut through sideway forces. However, such method of cutting can lift tall parts because of the leverage. This means vacuum chucks are only recommended on shorter parts because these parts do not experience leverage. The safest length will be 25th ratio of the part’s surface area. So the 5”x5” part should be 1” tall.
Surface Area (As) = 5 in × 5 in =25 in2
Part length= 25 in2 ÷ 25 = 1 in
Porous Vacuum Chucks
To provide utmost downward holding force, vacuum chucks are fabricated using porous metal. Porous metal creates uniform, flat, and smooth surface which allows stable work platform and eradicates deflections. There are numerous number of chucks available for different kinds of applications. Like the cylindrical porous chucks for printing applications. Metal, resin, and laser processing use either square or round chucks. Round porous chucks as well as the multi-type ones are both employed in semiconductor and LED fabrications. For a more in depth discussion on porous vacuum chuck and its type, see Porous Vacuum Chucks – types of chucks.
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