Frequently Asked Questions About CD-R and CD-RW Discs

Please contact Media Sciences if your questions are not answered on these pages.

Do gold CD-R discs have better longevity than green discs?

Claims have been made of CD-R lifetimes as high as 100 years. These claims are based upon results from accelerated aging under environmental extremes that are extrapolated to normal temperatures and humidities using various mathematical calculations and assumptions. For example, it may be assumed that longevity is affected only by chemical reactions in the dye layer, dominated by a single processes having a unique activation energy. Such an assumption may not be applicable. Also, since accelerated stability tests are normally conducted under stable conditions, these tests ignore stresses caused by fluctuations in temperature and humidity that are common in ordinary use.

Are predicted archival stability values correct? We will know in 100 years! Longevity figures provide superb ammunition for marketing departments, but are not very helpful to the user. Longevity was an issue when the first CD-R discs were manufactured by Taiyo Yuden using a cyanine dye. Unexpected degradation in sunlight or at elevated temperatures was corrected by the addition of stabilizers. This stabilized Taiyo Yuden cyanine dye was licensed to other manufacturers, and the "TY green" disc became the de facto standard.

Mitsui Toatsu subsequently developed the "MT gold" disc using a stable phthalocyanine dye as an alternative to cyanine, and licensed it to other manufacturers. The NCC subsidiary of Mitsubishi recently developed a "blue" CD-R disc using metallized azo dye. All three dyes, "TY green," "MT gold," and "NCC blue" have acceptable longevity based upon data furnished by their manufacturers. Although sales literature may suggest that one is superior to the other, other factors are more important.

Longevity is no longer limited by the stability of the dye, but instead depends on two other issues. The first issue is the original quality of the recorded disc. Each dye and recording speed requires a different optimum recording power and write strategy. CD-R writers vary in their capability to provide optimum recording conditions. Inconsistent recorded quality is the result, even though initial quality must be high if the disc is to degrade gracefully over its lifetime.

The second issue is loss of performance with time. Studies have shown that high quality discs with a BLER of only 2 per second can degrade to 700 per second after one year of office use, and to a BLER of 4,000 per second when the readout surface is heavily scratched. ISO Standard 10149 requires BLER to be less than 220 for successful interchange, indicating that improper handling can cause a disc to fail in months or even weeks.

Although damage to the readout surface can cause a disc to fail, the label side is even more delicate. A CD-R dye layer is covered by a thin metallic coating and then a fragile protective layer. This very thin acrylic or lacquer layer is only 5 to 10 micrometers thick (200 to 400 microinches). Physical damage to this surface will destroy the underlying recorded data, or will admit atmospheric contaminants that corrode the metallic coating. Silver is particularly vulnerable to attack by sulphur, a common air pollutant. Even aluminum layers used for CD-ROM discs oxidize when exposed to clean air, resulting in loss of performance.

Application of a label or other marks can also cause degradation. Special nonreactive adhesives or inks must be used to avoid attack of the metallization layer by migration of chemicals through the protective coating. Hard overcoat protection is added to the label side by some manufacturers. Although the resulting surface is less susceptible to damage, protection is not perfect.

The disc itself is made of polycarbonate, normally thought of as an indestructible material, that can degrade over time. Internal stresses created during storage or by mishandling can produce birefringence, a form of optical distortion. Molecular breakdown can result in yellowing or crazing (a network of fine, microscopic cracks internal to the disc). Contaminants in the polycarbonate can degrade the plastic itself or can attack the metallization.

Liquid water must always be avoided since polycarbonate will absorb water molecules. Oil from fingerprints or organic vapors in the environment can also attack either surface, resulting in long term degradation. Delamination can occur when the critical metallized layer separates from the CD-ROM polycarbonate substrate or from the CD-R dye layer.

Longevity is an major issue for CD discs, and will become even more important when the smaller data and track spacings of DVD discs enhance sensitivity to defects. Although extremes of temperature, humidity, and exposure to sunlight must always be avoided, degradation often occurs gradually as a result of "normal" handling and storage. Certainly select a supplier of high quality CD-R discs, but also ensure that recorded quality is high and that care is taken in the handling and storage of your discs. Excellent longevity will be achieved while the misleading issue of green versus gold dye will be avoided (see Measures of CD-R Longevity.)