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CD & DVD Quality Assurance  Quality assurance ensures that discs fully meet the necessary specifications

Quality Assurance is an essential part of manufacturing CDs to ensure that they meet specifications and to monitor the processes involved.

• QA in Mastering ensures only the highest quality stampers are used
• Disc Inspection of all replicas to identify defects
• CD QA measurements of sample replicas to maintain high quality in moulding and downstream processes and involve 
• DVD QA measurements of sample replicas which involve different and more measurements compared with CDs
• Environmental testing to ensure discs stay within specification during their lifetime and under differing environmental conditions
• ISO 9001 Accreditation ensures that procedures and processes are designed for quality at every stage

QA in Mastering

The purpose of mastering is to produce perfect stampers with good pit geometry so that replicated discs meet industry standards. To ensure high quality levels:

• Stampers are played on a disc stamper player.
• The first disc to be pressed is verified against the source to ensure that it has been mastered without errors.
• The pit geometry on a metalised glass master can be inspected using a suitable high power microscope.
• Each maiden stamper production run has a PQ test carried out to ensure that the start and finish times of all tracks correspond to the values in the TOC.

Errors which can occur during mastering include variations in track pitch and linear velocity and pit geometry which can produce high jitter in the moulded discs.  In addition poor stamper finishing can result in discs with eccentricity and/or unbalance out of specification.

Disc Inspection

All discs replicated are subject to automatic inspection before and after printing including:

• Evaluation of any visual defect such as pin holes, cold marks, bubbles and dimples.
• Automatic checks during printing to ensure that all discs being printed carry the correct catalogue number.
• For DVDs tilt and bonding gap are also checked

DVD inspection is similar to CD inspection but includes tilt. Discs must be inspected after bonding as this stage can introduce tilt and other defects. DVD-10 and DVD-9 discs need inspection of both top and bottom of each disc. DVD-9 discs need inspection of the semi-reflective layer and the bonding gap.

CD QA Measurements

To ensure that discs meet the necessary standards, sample CDs are tested at the start of all production runs.  Measurements include the following signals measured while the disc is playing.

• Reflectivity, which relates to the depth of the pits and the ability to read the disc easily.
• Asymmetry checks, which are needed to find out how central the I3 signal is in relation to the rest of the signal.
• I3, which is the signal coming from the shortest pits.
• I11, which is the signal coming from the longest pits.
• Push Pull, which is called the radial tracking signal which allows the laser to seek to a random position on the disc quickly.
• Cross Talk, which measures the difference between the reflectivity from the pits being scanned and the unwanted signal from the adjacent rows of pits.
• BLER, which is the Block Error Rate before any error correction take place.
• E22, which measures uncorrected errors correctable by error concealment.
• E32, which is a measure of uncorrectable errors and should be zero.
• Birefringence within the polycarbonate, which is a measure of the optical properties of the disc substrate.

• Radial Noise to determine the lateral tracking drift from the centre of the pits being scanned.
• Eccentricity, which calculates the accuracy of the centre hole.
• Track Pitch which should be between 1.5 to 1.7 microns.
• Scan velocity which should be between 1.2 and 1.4 m/s.
• BLI (begin of lead-in) should be at a maximum radius of 23 mm.
• BPL (begin of program area), should be at radius 24.8mm to 25mm.
• BLO (begin of lead-out) must not exceed radius 58.

These measurements are made at the same time as the electrical signals are measured.

Special industry test equipment is used to make these measurements and results are printed out and used to assess the performance of the replication equipment.

DVD QA Measurements

DVD inspection and testing requires the use of some different techniques, new parameters to be tested and new readers. DVD discs must meet certain stringent quality parameters, the most important of which are:

• Discs must be flat to ensure they are playable so tilt is an important measurement to ensure that the mastering and replication processes are within specification.
• Jitter must be low, requiring accurate mastering and moulding.
• Signals must be within certain constraints, which implies care at the mastering, moulding and metallising stages

In addition, mastering and replication is often the only way to ensure that a DVD title has been premastered correctly. Therefore it is important to verify the replicated discs using DVD players to ensure correct functionality. This is particularly true for DVD-9 discs for which a pressed disc is the only solution. Now that 4.7GB DVD-R discs are available, it is possible to test DVD-5 and DVD-10 applications before committing to mastering and pressing. However, CSS and APS copy protection can be tested only with a pressed disc.

Electrical and Optical Measurements include:

• Reflectivity, which relates to the depth of the pits and the ability to read the disc easily.
• Asymmetry checks, which are needed to find out how central the I3 signal is in relation to the rest of the signal.
• I3, which is the signal coming from the shortest pits.
• I14, which is the signal coming from the longest pits.
• Push Pull, which is called the radial tracking signal which allows the laser to seek to a random position on the disc quickly.
• Cross Talk, which measures the difference between the reflectivity from the pits being scanned and the unwanted signal from the adjacent rows of pits.
• Jitter must be within 0.8 per cent
• Birefringence within the polycarbonate, which is a measure of the optical properties of the disc substrate.

• Radial Noise to determine the lateral tracking drift from the centre of the pits being scanned.
• Eccentricity, which calculates the accuracy of the centre hole.
• Track Pitch which should be between 0.74 microns.
• Scan velocity which should be between 3.49 m/s for a single layer disc and 3.84 m/s for a dual layer disc.
• BLI (begin of lead-in) should be at a maximum radius of 23 mm.
• BPL (begin of program area), should be at radius 24.8mm to 25mm.
• BLO (begin of lead-out) must not exceed radius 58.
• Radial and tangential tilt must be within quite tight tolerances to ensure discs play.

Environmental testing

The parameters measured can change with time and environmental conditions, particularly DVDs where tilt in particular can become worse.  It is therefore very important to carry out age or environmental testing, which involves placing sample discs in an oven at a specified temperature and humidity for 48 hours and then testing again after 96 hours.  

Since this is a time-consuming procedure it is normally used to ensure that the process can produce good discs that will stay good and to determine if a tighter tolerance needs to be used when measuring discs before age testing.