Data Storage - Lapping Plates

Tin Alloy Lapping Plates

Tin alloy lapping plates have has been specifically designed for lapping thin film magnetic heads. The advantages of these tin alloys, which makes them superior for lapping over traditional lapping plates, includes the following:

• Rechargeable Fixed Abrasive Lapping Process
• Decreased metal smearing and therefore higher electrical yields
• Less Residual Lapping Plate Metal Contamination on the Sliders
• Lower Diamond Consumption

Tin and its alloys are a class of materials, which have recrystallization temperatures below room temperature. This means that they do not work harden at room temperature. This is very important for embedding diamond into a lapping plate because the lapping material will not work harden during the charging process, thus allowing the diamond to embed into the lapping plate. The problem with most of these low recrystallization metals and alloys is that they do not effectively hold the diamond. The tin TAC-3000 and tin-antimony lapping plates get around this problem by having a very tightly controlled grain structure. As a result the embedded diamond gets pinned at the grain boundaries, which prevents the diamond from smearing and rolling. Thus the tin TAC-3000 and tin-antimony lapping plates produce a fixed diamond lapping process.

Fixed diamond lapping processes have lower cutting profiles, are more efficient cutters (energy is applied to cutting vs. rolling), produce less wear on the lapping plates, and cut vs. smear soft magnetic materials. The result is more characteristic of a polished surface.

The benefits of fixed abrasive lapping processes for thin film magnetic heads include:

• Lower Pole-Tip Recession (PTR)
• Lower Alumina Recession (ALR)
• Less Alumina Roll-off
• Less Smearing across the MR/GMR Gaps
• Reduced Embedded Diamond in the Slider
• Increased Plate Life
• More Consistent Lapping Plate Profiles
• Polished Surfaces

Tin Alloy Properties   Tin Traditional lapping plate for magnetic tape and hard drive heads Tin-Bismuth Most common lapping plate for reducing smearing for improved electrical performance for GMR and AMR recording heads. TAC-3000 Also know as tin babbitt, this structure contains fine cored dendrites with the harder grain boundaries produced by small Cu6Sn2

Tin-Bismuth Lapping Plates

Zinc Lapping Plates

ZINC-A10 is an engineered lapping plate designed to easily take a diamond charge during the embedding or charging process. Once embedded the diamond is rigidly fixed. In order for the diamond to be effectively embedded into the lapping surface the lapping surface must not work harden during charging. Thus the lapping plate must have a recrystallization temperature below room temperature. The following Table lists the recrystallization temperatures for various metals used for magnetic head lapping.

ZINC-A10 is the only lapping plate with a recrystallization temperature close to room temperature. The diamond embedding process for the ZINC-A10 lapping plate is as follows:

• As a force is applied to the diamond, the pressure at the ZINC-A10 interface increases the interface temperature
• Once this temperature increases above the ZINC-A10’s recrystallization temperature, the diamond embeds into the plate
• As the diamond embeds the surface area contact of the ZINC-A10 plate increases around the diamond particle. As this area increases the force on the embedding diamond particle gets distributed over a larger area thus reducing the overall pressure on the plate
• As the pressure decreases so does the point contact temperature at the diamond/ ZINC-A10 interface. As this temperature decreases below the recrystallization temperature the area around the diamond particle is work hardened, thereby rigidly fixing the diamond into the lapping plate

The result is a fixed diamond lapping/ polishing abrasive.

Note that tin and lead have traditionally been the most common lapping materials. This was because of the ease at which an abrasive could embed into its surface. Subsequently, because of their low recrystallization temperatures they do not hold diamond as well as zinc. Thus the diamond is only semi-fixed, and will eventually smear across the lapping surface, resulting in wear of the lapping plate (plate profile and plate life).

The following is a list of recommendations to maximize the performance of tin-alloy lapping platens.

• Optimize lapping surface roughness to match diamond size and type
• Use a lapping plate texturizing abrasive which will not remain embedded in the lapping plate.
• Charge diamond at higher pressure than the lapping process
• Use a lapping lubricant designed to remove lapping swarf from the plate
• Pre-charge and apply diamond abrasives to lapping plate for coarse lapping and use only a pre-charged diamond for the polishing operations