Lapping is an abrasive machining process that helps improve material surface characteristics such as roundness, roughness, waviness and flatness. This process can be applied to fragile parts and brittle materials such as alumina, sapphire, silicon nitride, zirconia, quartz and other ultrahard materials.
In Insaco's lapping process, loose abrasive grains with sharp edges and uneven shapes and sizes are suspended in an aqueous or oil medium. The fine abrasive particles are then applied between the lapping plate and the material surface to form a thin film of abrasive. The sharp edges of the abrasive grains create indentations or grooves when forced on to the workpiece material. The depth of the grooves largely determines the surface roughness of the finished component. Harder and larger grains give rise to increased surface roughness. Insaco typically uses diamond abrasives for the ultrahard materials they machine
Factors Influencing the Lapping Process
Major factors influencing the lapping process are;
- The size and type of the abrasive particles
- The kind of lapping plate used
- The type of lapping fluid used in the process
- The speed of lapping
- The pressure applied during lapping
The Function of the Lapping Plate
The kind of lapping plate Insaco uses plays a vital role in the lapping process. It significantly affects the output of this abrasive machining technique. A plate should not be too hard as it damages and contaminates the workpiece surface. At the same time, a hard plate does not allow abrasive grains to be embedded in it, resulting in a rolling motion and the removal of material through stress induced microfracture.
A soft lapping plate allows partial insertion of abrasive particles into the plate surface, allowing more sliding movement and the removal of workpiece material by ploughing. The result is a surface with a finer but less planar finish.
A hard plate will give rise to a more planar and better surface finish. Hence, hard plates along with extremely fine abrasive grains are widely used in Insaco's lapping processes.
The Role of the Lapping Fluid
In the lapping process, abrasive particles are suspended in an aqueous or oil medium, known as the lapping fluid, and then applied to the lapping plate. The liquid carrier acts as a lubricant between the work material and the abrasive grains. The fluid also helps in dispersing the abrasive particles evenly across the lapping plate and removing debris from the lapping zone.
The lapping fluid should be delivered to different positions on the lap to avoid a high slurry concentration in one area giving rise to uneven abrasive wear rates during the machining process.
Double-Sided and Single-Sided Lapping Techniques
Single-sided lapping is used at Insaco for creating very flat surfaces. Lapping machines used in this method have a rotating wheel, which is the lapping plate and is annular shaped. The slurry of abrasive is evenly applied on to this flat wheel during lapping. The workpieces are kept in conditioning rings and pressure is applied, thus pressing the parts against the abrasive film.
Three significant issues to be controlled during single-sided lapping process are the following:
- Applying a consistent pressure
- Maintaining a uniform flow of the abrasive slurry, and
- Maintaining a flat lap.
Figure 1. Single Sided Lapping Set-Up
Insaco also uses double-sided lapping, where both sides of the work material are machined at the same time. The lapping machine used for this method has two lapping plates having the same diameter and rotating in opposite directions. This method gives high accuracy in size uniformity and parallelism. One additional advantage of this process is that the chance of foreign particle entrapment between the load and the workpiece is reduced.
Single sided lapping is done at higher speeds and with a finer abrasive size compared to double-sided lapping. Therefore the single sided process produces a much better surface finish.
Accuracies Achieved Using Lapping on Engineering Ceramics
By using proper techniques, Insaco's lapping process can achieve high levels of accuracy on flat surfaces. Total thickness variation (TTV) using this lapping process can achieve .000040" (1um) TTV on a 4" (100mm) diameter wafer for example with thickness tolerance to +/- .000060" (1.5 um). Surface roughness will be less than one micro inch Ra (0.025 um).
All of these processes require highly skilled operators and high precision equipment to achieve the highest levels of precision, surface finish and low levels of surface and sub-surface damage.
This information has been sourced, reviewed and adapted from materials provided by INSACO Inc.
For more information on this source, please visit INSACO Inc.