Under the Wire
When machining glass and ceramic components for high-tech applications, criteria such as productivity and cutting quality are paramount. For these reasons, a number of companies are turning to wire saws. With benefits such as high throughput, accuracy and repeatability, as well as low kerf loss, wire saws can optimize slicing processes and minimize waste.
Wire Saw OperationIn most wire saws, a single wire (typically plain steel) is fed from a supply spool through a tensioning system onto two, three or four wire guide rolls, depending on the size of the machine. These rolls are grooved at a constant pitch, and the wire is fed around the rolls several times, always proceeding one groove after the other toward the front of the machine to form a "web" of adjacent wires (see Figure 1). The rotation of the guide rolls moves the wire web, and a collection spool on the output side of the machine receives the used wire. An abrasive slurry (usually silicon carbide or diamond for ceramic and glass applications) is fed through a multitude of nozzles into the wire web and is dragged along with the rotation of the wire; no direct mechanical contact occurs between the abrasive particles and the wire. As the workpiece is pushed into the wire web, the slurry-clad wire cuts the material into slices at a thickness determined by the pitched grooves of the wire guide rolls.
A more recent innovation is a fixed abrasive wire saw, in which the diamond crystals are mechanically bound to the wire and no slurry is used (see Figure 2).
The Wire AdvantageCompared to other machining processes, wire sawing offers distinct advantages in production efficiency, part quality and kerf loss. While the feed speed of the workpiece is relatively slow compared to other commonly used slicing methods, such as outer diameter (O.D.) and inner diameter (I.D.) saws, the parallel processing capabilities of wire saws typically enable a higher throughput. For example, some of the largest wire saws used to cut solar cell wafers in the photovoltaic industry can slice more than 2000 mm of feedstock into 4500 wafers in a single run. O.D. and I.D. saws, on the other hand, can only slice one wafer at a time, making them impractical for such high production volumes. Smaller wire saws used in other applications produce equally impressive throughput results.
In any machining process, the quality of the cut wafers is determined by the geometrical variation within one wafer and by the variation from one wafer to the others. Workpieces with a cross section of 200 x 200 mm sliced with wire saws have exhibited a variation of around 10 microns for both values. Even more importantly, these tolerances are repeatable from one run to the next.
Wire saws also generate a low amount of heat during the slicing process. Even the largest workpieces are not heated above 50°C, making the process ideal for heat-sensitive materials. Additionally, wire saws can cut any material, regardless of electrical or chemical properties. The resulting surface finish is excellent with low roughness values, and because the wire sawing process is gentle, little subsurface damage is imparted to the piece.
Another benefit of wire sawing is the very low kerf loss that is achieved-typically 200 microns or less. This compares to a kerf loss of around 400 microns with I.D. saws, and as much as 1.5-3 mm with O.D. saws. A low kerf loss is especially important for companies that machine expensive raw materials or that use expensive processing steps, such as hot pressing, before slicing. For these applications, every additional part that can be generated by reduced kerf loss is more profit for the bottom line.
When fixed abrasive wire is used, the wire sawing process can be optimized even further by providing greater reductions in cutting time and subsurface damage in the material being machined, as well as reduced abrasive disposal costs.
Increasing ApplicationsBy far, the largest applications for wire saws are in slicing silicon for semiconductor wafers and solar cells, but other applications are also growing rapidly. For example, in the manufacture of substrates for blue laser diodes, wire saws are used to machine sapphire (single crystal alumina) and single crystal silicon carbide, both of which are among the hardest known materials. Wire saws are also used to cost-effectively slice parts made of polycrystalline functional ceramics used for X-ray imaging and piezoceramic stacks. Since these parts often contain expensive chemical elements or must pass through expensive processing steps, the low kerf loss provided by wire sawing is a key benefit.
Sintered structural ceramics, such as alumina, zirconia and all varieties of polycrystalline silicon carbide, as well as aluminum nitride, glass and quartz, are also being successfully machined using this technology. As companies in the ceramic and glass industry continue to look for ways to improve their machining processes and workpiece quality, wire sawing will undoubtedly play an increasing role.
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