CD (optical disc) longevity -- How durable and reliable are they?

Do CDs last forever, or will they gradually go bad with age? The aluminized data surface will corrode if exposed to air, so it is over-coated with transparent lacquer over which the label is printed. But while the basic technology of the CD is standardized, manufacturing details vary from one pressing plant to another. There are now [ca. late 1992] about 200 pressing plants in the world, producing more than a billion [ca. late 1992] discs a year. If you examine the CDs on your shelf you will find some with vertical outer edges produced by a biscuit cutter, and others with rounded, polished edges; some whose edges are bare plastic, and others whose aluminum coating covers the edge of the disc; some with a brilliant mirror-like surface and others whose reflective coating is semitransparent; some whose label is printed directly on the transparent lacquer, and others that are painted a solid color, and then overprinted with information. Each CD plant makes these design choices and determines its own quality-control procedures, within the overall error-rate limits set by Philips. So the longevity of your CDs may depend on where they were made.

A few years ago a batch of CDs that had been manufactured in a Nimbus pressing plant went bad when acid-based printing inks ate through the lacquer and into the aluminum. This episode threw a scare into other manufacturers. Meanwhile corporations and government agencies, including the IRS, would love to substitute small boxes of CD ROM discs for their air-conditioned vaults fill of bulky computer tape—but they need a guarantee that after a hundred million Social Security files have been archived on CD ROM discs, they won’t be found to be corrupted by microscopic pinholes.

Since 1988, quality-control specialists at many European CD plants have been exchanging information about their testing procedures and about factors that may affect CD longevity. They discovered, for example, that if the aluminum layer extends all the way out to the edge of a disc that has a vertical biscuit-cut edge, there is a much greater risk of air getting at the aluminum and causing it to oxidize. They learned that ultraviolet light is better for hardening the lacquer than hot air, and sorted out which printing inks to avoid and which are safe.

If many of your CDs were manufactured before this research was done, a few of them could be self-destructing right now on your shelves. But today’s CDs are likely to last a thousand years under average conditions, according to Bert Gall (general manager of optical disc technology at Philips). Gall was quoted in the British magazine New Scientist as saying that, even under the worst environmental conditions, new CDs should be good for at least 20 to 50 years.

Such estimates are based on accelerated life tests that have been adopted as a standard procedure among European CD makers. Discs are placed in a chamber that is raised from room temperature to 120°F within 30 minutes, kept hot with 95% humidity for 12 hours, and then cooled for the next 12 hours, cycling up and down every day for a month to produce stresses that may warp the disc or separate the layers of the thin plastic/aluminum/lacquer sandwich. Finally, to ensure objectivity, different factories inspect each others’ discs for flatness, data errors, or any evidence that air has reached the aluminum layer. This procedure may become part of the Philips Red Book standard for all CD manufacturing.

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ALSO SEE: audioholics.com article, NPR article, the x-lab report, CLIR report

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