Breaking a wafer is achieved by applying tensile strain lateral to the scribe line. The Loomis "break wheel" method is a hybrid of two standard methods.
The first method bends the wafer over a curved surface ("Roller Breaking").
The second method uses three points to bend the wafer ("Three-point Bending").
By combining these two methods, Loomis takes advantage of the precision and limited contact of three-point beam bending and the controlled strain of a curved surface.
Two of the three beam-bending points come from a break wheel that has raised tracks formed around the circumference. The third point is provided by the wedge-shaped mandrel. The shape of the mandrel also limits the bending angle and therefore the strain that is applied to the wafer. A crack will propagate only if the optimum scribe line exists. If it doesn't exist, nothing happens and that section of the wafer is saved.
The profiles on the break wheel cause the wafer to conform to the mandrel. They are usually spaced symmetrically on each side of the scribe line at a distance dictated by the die dimensions. The two bending forces are exerted on the separation grids between the dice. This prevents damage to air bridges or other sensitive structures.
With the Loomis Machine, the results of your scribing and dicing are instantly observable. After a scribe line is broken, holding film under the wafer pulls the newly formed piece away from the body of the wafer and a gap forms. This is thanks to the tension of the wafer film. This tension causes the film to expand as the wafer is diced and separated. This serves two purposes; it keeps the newly formed pieces from abrading each other and feeds energy into the break as it occurs.