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Vision Engineering Ltd.

Metrology

The Quality Process Behind Your Favorite Disc

Careful metrology ensures the clear sounds and images of CDs and DVDs

Published: Tuesday, March 1, 2011 - 08:54

From its humble beginnings as a music carrier, the compact disc (CD)—and later the digital versatile disc (DVD)—is now entering its third generation for the high-definition market as HD DVDs or Blu-ray. However, one thing that must stay consistent throughout these evolutions is the high-quality production of these polycarbonate replicas.

CDs are a part of nearly everyone’s life, and they have become versatile enough for us to store data, copy data, and, on occasion, use as coasters. This is generally common knowledge, but how many of us are aware of the production method for CDs and how this can affect the disc’s overall quality and its output.

CDs or any optical disc format are made from polycarbonate that has been injection molded to reproduce the exact profile of the disc’s data. Before the molten granules of polymer are fed into an injection molding machine from the hopper, the injection molder is set with a nickel master called a stamper. This is a critical part of the process, because if the stamper is anything less than perfect, the whole batch of CDs (which could be thousands) could be wasted.

Creating the stamper

The stamper is made of a disc (the “ID”) that is punched from a nickel sheet 3.0 mm thick. This procedure is implemented with precision because of nickel’s relative hardness and thickness. To ensure the stamper has been accurately punched, it is measured using a noncontact method. This is because using a contact method such as a spring-loaded bore micrometer will create denting in the punched ID and an oversize reading due to the relative softness of the nickel. This may also cause permanent damage to the hole geometry.

Sibert is a company specializing in supplying equipment to punch the nickel stamper. After punching, Sibert measures the stampers in three-axes with a noncontact system from Vision Engineering. The discs are placed on a glass stage with X-Y linear encoders with a resolution of 1 µm and measured with specially modified graticules for repeatable results. The measuring accuracy for the nickel plates is ±1.5 µm. However, before measuring the disc, any protective coating must be removed and any dust or debris gently removed from the punched ID.

One of the clear advantages of using a manual noncontact method is that measurements can be taken by the human eye and thus account for selective placement of the graticule. Automated processes may not take this into account and subsequently provide an inaccurate reading. The images below show how different the mono image can look when dust and debris are present on the profile edge.


Figure 1: A clean and clear
edge without dust and debris


Figure 2: An edge that isn’t
well-defined due to dust and debris

Measuring for precision

A noncontact optical method also helps when the punched radius is measured. Some automated machines focus on the radius rather than the bore (or punched entry point), and this can sometimes give an oversized reading. This can happen when the soft nickel becomes slightly inverted during the punching process. The punch forms a curve in the nickel, so objective placement of the cross-hair graticule on the curve is necessary.

“A four-point measurement is optimum for accurate hole size, with the ‘unroundness’ reading also displayed,’” explains Steve Knight, applications manager at Sibert. “The cross hairs should be placed carefully to avoid erroneous defects, which will affect the reading. It is equally crucial to make sure the stamper is held flat onto a glass stage during measuring.”

The ID size can be calculated and displayed in three different ways: minimum circumscribed circle, maximum inscribed circle, and the average hole size (or gauss). The minimum circumscribed circle, taking any “unroundness” into account, is the largest circle that can be inscribed over the maximum deviation point on the ID hole plot. This has no real use for a stamper and the ID measurement will always be larger.

The maximum inscribed circle is the maximum size perfect circle or shaft that will fit within the ID hole profile. In theory this should represent the stamper holder but in practice will produce an undersized reading.

Average hole size (gauss) is the average size between the two methods and represents the optimum fit of the stamper onto the holder. Any small amount of irregularity in the bore will be adjusted as the stamper is fitted onto the holder due to the softness of the nickel. At this point ID measurements should also be verified on the other side of the stamper. The results should be within 2 µm of each other.

To achieve a good quality, clean-punched hole, the stamper hardness should be within 200 ± 10-percent HV0.3. The Vickers hardness (HV) value is taken from the Vickers hardness test, determined by the resistance of the material—in this case, the nickel—to deform when an indenter is forced on it. Stampers harder than this may have a ragged cut but stampers softer than this have an extensive punch entry radius.

Measuring the eccentricity

Measuring the eccentricity of the stamper and the stamper ID hole require high accuracy. This can be obtained by measuring them simultaneously so the ID size does not influence the eccentricity result.

The ID is measured first, followed by a media band. The distance between the center points of the two circles is calculated as eccentricity; the total indicator reading is twice the eccentricity. When measuring for optimum eccentricity, the geometry of the punched hole also plays a part in the end result.

Closer to the real thing

Whether CD, DVD, HD, or Blu-ray, the disc must be mass-produced to consistently high quality standards—and they are. As the tolerances get tighter and manufacturing technology improves, we can expect these discs to bring us ever closer to the “real” experience of being at our favorite artist’s concert or sitting on the sidelines of a film set.

Copyright © Vision Engineering. All rights reserved. Reprinted with permission from visioneng.com.

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Vision Engineering Ltd.

Vision Engineering is a global leading-edge manufacturer of patented ergonomic stereo microscopes, digital microscopes, and noncontact measuring systems.Twenty-five years ago, Vision Engineering introduced the world’s first “eyepiece-less” stereo microscope, the Mantis. It was an ergonomic revolution that went on to win numerous design and innovation awards. Company headquarters are based in Send, Woking, UK, with manufacturing facilities in the UK and U.S. Regional offices are located throughout North America, Europe, and Asia.