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Data Storage - Lapping Plates

Data Storage Magnetic Head Lapping Plates


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Magnetic head lapping is accomplished with either tin alloy or zinc lapping plates. The lapping plates are typically planed using a carbide tool bit, trued with alumina lapping films and then textured with a water soluble abrasive. The lapping plates are then charged with polycrystalline diamond ranging from 0.10 micron to 0.50 micron. Lapping lubricants are used during the lapping process to maintain lapping plate life and control PTR, alumina recession, smearing and surface finish.

Tin Alloy Lapping Plates

Lapping Alloy

Properties

Tin-Bismuth

Structure has a fine grain structure and machines as a powder instead of a chip. These features result in minimizing metal smearing during lapping, thus producing improved electrical performance.

Tin-antimony

Structure has tin-rich dendritic grains in a eutectic matrix. This provides localized areas for the diamond particles to become embedded.

Tin TAC-3000

Also know as tin babbitt, this structure contains fine cored dendrites with the harder grain boundaries produced by small Cu6Sn2

Zinc A10

Zinc alloy lapping plate, fine grained lapping plate with a recrystallization temperature close to room temperature. Results in lower and more consistent plate life.

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

Magnetic head lapping plates

Tin-Bismuth

Most common lapping plate for reducing smearing for improved electrical performance for GMR and AMR recording heads.

Tin bismuth magnetic head lapping plates

TAC-3000

Also know as tin babbitt, this structure contains fine cored dendrites with the harder grain boundaries produced by small Cu6Sn2

Magnetic head tin alloy lapping plate

Tin-Bismuth Lapping Plates

Properties Eutectic Bismuth-tin alloy
Melting Temperature (F) 281
Tensile Strength Lbs/In2 8000
%Elongation in slow Loading 200
Brinell Hardness No. 22
*Specific Heat - Liquid 0.045
*Specific Heat - Solid 0.045

*Latent Heat - Fusion Btu/Lb.

20

Conductivity (Electrical)
Compared with Pure Copper

4.5%

*Maximum Load -
30 Seconds Lbs/In2

15000

*Maximum Load -
 5 Minutes Lbs/In2 

9000

*Safe Load Sustained - 
Lbs/In2

500
* Approximate values

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.

Material

Recrystallization Temperature

Lead

-25C

Tin

-25C

ZINC-A10

Room Temperature

Copper

200C

Aluminum

194C

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-A10s 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).

Recommended Application

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
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