Keeping Track of Quality
From its humble beginnings as a music carrier the Compact Disc (CD) or Digital Versatile Disc (DVD) is now entering its 3rd generation for the high definition market (HD DVD or Blu-ray). The one thing that needs to stay consistent is the high quality production of these polycarbonate replicas.
CD’s are a part of nearly everyone’s life, and they have become versatile enough for us to store data, copy data and for some, to use as a coaster… This is generally common knowledge, but how many of us are aware of how they are produced and how this can affect the overall quality of the disc and its output.
How they are made
CDs or any optical disc format are made from polycarbonate which has been injection moulded to reproduce the exact profile of the discs data. Before the molten granules of polymer are fed into an injection moulding machine from the hopper, the injection moulder is set with a nickel master called a Stamper. The nickel Stamper is an intrinsic part of the process, because if the stamper is anything less than perfect, the whole batch of CD’s (which could be thousands) could potentially be wasted.
With all the attention on the stamper preparation (due to the implications of any imperfections), the production of such an important part of the CD manufacturing process must produce a bead of sweat on the brow of the most experienced mastering engineer.
The Stamper is made of a disc that is punched from a nickel sheet 3.0mm thick. This procedure is implemented with precision because of the hardness and the thickness of the nickel. To ensure the stamper has been accurately punched it is essential that as part of the quality procedure, the stamper is measured using a non-contact method. This is because using a contact method like a sprung loaded bore micrometre 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 specialising in supplying equipment to punch the nickel Stamper. After punching, Sibert measure them in 3-axes with a non-contact system from Vision Engineering. The discs are placed onto the glass stage with X-Y linear encoders with a resolution of 1μm and measured with specially modified graticules for repeatable results. The measuring accuracy for the nickel plates is ± 1.5μm. 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 non-contact method is how measurements can be taken by the human eye and therefore accounting 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.
Fig.1 (left) shows a clean and clear edge without dust and debris.
Fig.2 (right) shows an edge that is not very well defined due to the dust and debris.
In addition to this advantage using a non-contact method of measuring, other advantages using a non-contact optical method is when the punched radius is measured. Some automated machines focus on the radius rather than the bore (or punched entry point), hence this can sometimes give a oversize reading where the soft nickel has become 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.
Steve Knight, Applications Manager at Sibert explains the importance or measuring the stamper and hole accurately.
“A four point measurement is optimum for accurate hole size, with the ‘unroundness’ reading also displayed. The cross hair should be placed carefully to avoid erroneous defects, which will affect the reading. It is equally as 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 including minimum circumscribed circle, maximum inscribed circle and the average hole size (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 undersize reading.
Average hole size (Gauss). This is the average size between the two methods and they represent 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. ID measurements should also be verified on the other side of the stamper. The results should be within 2μm of each other.
To achieve a good quality, clean punched hole, the stamper hardness should be within 200 ± 10% HV0.3. The HV value is taken from the Vickers Pyramid, determined by the resistance of the material (in this case the nickel) to deform when an indenter is forced upon it. Stampers harder than this may have a ragged cut but stampers softer than this have an extensive punch entry radius.
Measuring the eccentricity ECC
Measuring the eccentricity (ECC) 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 ECC result.
The ID is measured first, followed by a media band. The distance between the centre points of the two circles is calculated as ECC (Total Indicator Reading TIR is twice ECC). When measuring for optimum ECC, the geometry of the punched hole also plays a part in the end result.
Quality and the future of the humble CD
Whether CD, DVD, HD or Blu-ray, the quality of the disc needs to be mass-produced to consistently high standards. This insight into the manufacture of optical discs can only tell us what we already know, the quality and performance you can expect to achieve from optical discs is extremely high. As the tolerance gets tighter and tighter and technology expands, we can only expect to find ourselves getting closer to the ‘real’ experience of either being at our favourite artists concert or sitting on the side lines of a film set.