Media Sciences has developed special mechanical test fixtures that often reveal why discs that pass in quality testers can fail in the field. Such discs often satisfy reflectance, push-pull, I3, I11 or I14, jitter, asymmetry, radial noise, E22 or PO, burst, BLER or PI, and other electrical requirements. However, CDs and DVDs that function in high quality test drives can fail in some user drives. Mechanical quality issues are frequently responsible for these mysterious field failures.
ISO/IEC 10149 and the Color Books contains many mechanical and dimensional requirements for CDs, while ISO/IEC 16448 and the DVD Forum Books provides similar conditions for DVD discs. Ongoing efforts by Media Sciences develop mechanical test fixtures that are then integrated with our other quality tests. Our custom equipments generate data that frequently answer the question Why Did the CD or DVD Disc Fail? Since mechanical tests involve only length and weight, results are readily traceable to NIST.
RADIAL RUNOUT is measured by our latest test fixture that went online in December 2003. Precise disc centering is assured by a highly accurate, custom spindle. This allows both track eccentricity and outer rim concentricity, or radial runout, to be measured to an accuracy of 0.01 mm using a specially mounted, laboratory grade microscope. Magnification enables any burrs on the outer rim to be quantified.
Eccentricity evaluates azimuthal variation of the track radius, and provides important assurance that changes in track location remain within the dynamic range of the track servomechanism. High frequency variations of radial track position are evaluated by radial noise tests, while sudden, low frequency variations can be measured by radial acceleration tests. High eccentricity can be generated during stamper punching, molding, and subsequent handling of the ejected substrate before it cools.
Concentricity measures radial runout of the outer rim relative to the geometric center of the disc as determined by the center hole, and contributes to disc unbalance. Excessive values are usually caused during molding or subsequent handling of the ejected substrate before it cools.
AXIAL RUNOUT, or deflection, is also measured by our latest test fixture that was put into use in December 2003. The reference plane is accurately established using a precision clamping mechanism with proper clamping and center hole forces. Because the reference plane is flat within 0.4 mrad, axial deflection of both the entrance surface and the reflective layer can be determined by a special microscope to an accuracy of 0.02 mm. Lacquer buildup on the rim can be detected under magnification.
Axial deflection of the entrance surface is specified only for CDs, and is a valuable indicator of molding quality. Axial deflection of the reflective layer for both CD and DVD discs must be controlled so that variations remain within the dynamic range of the focus servomechanism. Low frequency variations are measured by a vertical acceleration test. Axial runout results from disc warpage, thickness variations, and unparallelism of the substrate surfaces. These are produced during molding or subsequent handling of the ejected substrate before it cools.
THICKNESS of the substrate to an accuracy of 0.01 mm has been a standard test of Media Sciences for many years. A custom fixture properly positions the disc for accurate measurement. Multiple point tests across the entire disc are important. Thickness in the information area is important for proper focusing, while variations can cause vertical acceleration or unbalance problems. Thickness is influenced by the molding cavity, and also by any subsequent label.
OUTER RIM DIAMETER is routinely measured by Media Sciences to an accuracy of 0.01 mm. Variations can contribute to unbalance, and are usually molding-related.
CENTER HOLE DIAMETER is determined to an accuracy of 0.005 mm using a special tool that avoids deformation of the plastic substrate. Incorrect values can affect centering on a spindle, and are usually related to molding or subsequent handling of the ejected substrate before it cools.
CENTER HOLE BURRS are axial extensions that protrude beyond the substrate surface. They are not allowed on the CD entrance surface, or both surfaces of DVD discs, and standards require a prescribed chamfer or radius of the center hole edge. Center hole burrs can interfere with proper mounting of the disc onto the drive spindle, and are usually caused by improper molding or subsequent handling of the ejected substrate before it cools.
UNBALANCE happens when the center of gravity of a disc is offset from the geometric center of the disc as determined by the center hole. It is measured by Media Sciences using a custom static unbalance tester. Since results involve only length and weight, they are readily traceable to NIST. Other dynamic unbalance testers are indirectly traceable, and may be sensitive to higher mechanical moments related to disc warpage.
Unbalance limits have been present since the first Red Book was issued in 1980, but became critical with the advent of high speed write and read drives. Since radial forces caused by unbalance increase with the square of angular velocity, a given amount of unbalance generates a force 2,500 times greater at 50X than at 1X. This same force would be generated in a 16r DVD drive, since DVD 1r velocity is about 3 times that of CD 1X velocity. In addition, angular velocity is usually dependent on pickup head radius, and is highest at the inner diameter where a drive always accesses critical information when a disc is first mounted.
Forces caused by unbalance cause vibration of a CD or DVD drive chassis. Although drives contain vibration isolating components, these are not 100% effective, especially near mechanical resonances. Vibrations are transmitted to components of the optical pickup head, including the suspended objective lens and coils. Motion of this lens can result in radial or focus errors, or in distortion of the focused spot. Radial forces can cause momentary clamping failure, and the sudden radial motion of the disc results in high radial acceleration. Forces between the spindle and the center hole caused by very high unbalance may result in center hole cracking, or even shattering, especially when significant strains in the substrate are present near the center hole. A few causes of disc unbalance are variations in substrate thickness and diameter, poor concentricity, and asymmetric labels. Operational unbalance includes both disc unbalance as well as clamping inaccuracy that is affected by disc centering and spindle runout tolerances.
ACCURATE FIXTURES for other mechanical tests are under current development at Media Sciences. Tilt refers to angular deviation of the reflective layer from the reference plane, and is specified for both CD and DVD discs. Tangential tilt, or deviation along the track, is critically important to DVD interchange. Some DVD drives even compensate for tilt. Tilt measurement requires both precise clamping and sensitive optical measurements in order to accurately determine angular deviations as small as one minute of arc, or 0.3 mrad. Our new tilt fixture will be online in early 2004, together with a capability to measure track pitch, that being the radial displacement of adjacent tracks. High capacity can be obtained with low track pitch, while high quality is related to nominal track pitch. This conflict is often resolved to the detriment of quality. Accurate determination of both average and instantaneous track pitch is necessary to achieve confidence that the disc will work in all read drives, and in all write drives if the disc is recordable or rewritable.
MECHANICAL QUALITY MEASUREMENTS are often overlooked because they may not be conducted by automated, computerized testers. Although manual measurements may be tedious, and a little boring, they are important to the prediction of successful interchange. All CD and DVD discs can only work first time, every time, in every drive when they fully conform to all mechanical, optical, electrical, physical, and logical requirements of ISO, Color Book, and DVD Forum standards. Confidence in predictable interchange is achieved through comprehensive testing, even of just a few samples, not by limited tests performed on a large number of samples. Rely on Media Sciences for accurate, objective support of your quality program.