Frequency (PRP)
During the electrodeposition cycle, using a PRP rectifier, frequency is the number of occurrences of a repeating event per unit of time. A period, on the other hand, is a duration of one cycle in a repeating event, so a period is the reciprocal of the frequency.

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June 14, 2004

Description:

Thickness variation is the difference between the maximum thickness and the minimum thickness of a stamper. Thickness is generally measured in microns at many points within a defined radius from the center of the stamper. Thickness variation can be expressed as absolute variation or as a deviation of +/- from the mean thickness.

Causes and Solutions:

• Cell Alignment
  
  When the Digital Matrix electroforming cells are originally setup, they are aligned properly. This alignment consists of moving the Head Assembly left, right, up, and down, until the shaft is aligned to the center of the baffle. If the Head Assembly is not properly aligned, excessive thickness variation may result. It is recommended that the alignment be checked on a regular, monthly basis. A special alignment tool is used for this.
  
  Periodic adjustments need to be made to either the anode to cathode distance, or to the size of the baffle. As the bath ages, either one or both of these needs to be adjusted. Once this adjustment is made, the operator needs to be aware, that adjustment screws, holding wheels, or other fasteners need to be checked, to see that they are tight, before production can resume.
  
• Alignment of tthe Backplate(Workholder)
  
  The backplates were designed to remain on the shaft at all times. Periodically they do need to be removed for maintenance purposes. When the backplate is screwed back on, the technician off times does not screw the backplate on all the way. This changes the anode/cathode distance and causes wobble of the workholder with respect to the cell. This in turn causes thickness variation issues.
  
  The workholder can also become loose if the operator manually turns the workholder counter-clockwise. Always turn the workholder clockwise unless removing the workholder.
  
• Workholder Damage
  
  Contact RIngs: The contact rings have a life. The number of times a contact ring can be used depends on the handling and care from the operator. It is necessary to keep these rings as straight and bend free as possible. This ensures that the current is equally distributed to the part.
  
  Nickel Buildup: Periodically the backplte needs to be examined. If there is nickel buildup on the stainless steel insert, maintenence needs to be done. To remove nickel buildup from the stainless steel insert, the backplate need to be removed, and the stainless steel insert removed. Soak the stainless stell parts only in nitric acid 20% overnight. NEVER PUT THE PLASTIC PARTS OF THE WORKHOLDER IN NITRIC ACID.
  
  O-Rings: Check the o-ring in the Holddown Insert (HDI) and in the plastic base on the workholder. These o-rings need to be replaced every 6-months or so. If these o-rings are damaged or worn excessively, they will allow leakage of the nickel solution into the contact area of the workholder causing nickel buildup.
  
  Tension Adjustment: It is important to adjust the backplate knobs, such that a little bit of effort is required to close them. Over tight adjustment knobs can cause leakage and subsequent nickel buildup.
  
• Chemistry
  
  An imbalance in the chemistry of the bath can cause thickness variation issues. Follow recommended maintenance practices, clean anodes properly, clean mesh properly, and clean any debris in the bottom of the cells.
  
  Low Flow can cause excessive thickness variation. Chack and maintain filters periodically. Do not exceed the filter life.
  
  Nickel and Boric concentrations should be regularly analyzed. These parameters, especially Nickel Baume, along with the pH, flow, temperature, and wetter level, affect the efficiency of the solution, which affects the rate at which the solution plates.
  
  Anodes: Packing every part; addition of new anodes, which have been activated in Sulfamic acid, every shift, help to push the anodes in the center of the basket, into a tighter formation; periodical maintenance to remove the excess anode sludge, as the sludge will cause a barrier between the anode and cathode, which increases the resistance, and slows down the plating rate.
  
  Packing the anodes is necessary, to ensure that the basket contents remain tight. Too much packing, or packing the anodes too hard, or with a rod that is too big, will results in an expansion of the anode basket's front face. When the anode basket expands, the anode to cathode distance will change, which affects the overall tapir of the stamper.
  
  Periodic checking that the connections between the anode basket and the buss-bar are tight.
  
  When the basket and anodes are cleaned, making sure the basket is placed back and securely tightened, so that the anode to cathode distance hasn't changed.

Molding - Molding relies on the thickness quite heavily. If the TTV is above specifications, the mold will have difficulty venting. As a result, the bumps on the stamper will not be formed properly, as residual air will prevent the liquid plastic from forming around these bumps.

Criteria - With the introduction of the CD, thickness variation (TTV) was 295 +/-5. When DVD was released, the same criteria was used. Soon it was found that a much tighter spec was required. Today DVD stampers are expected to be maintained at 295 +/-3. In fact, the industry standard is even tighter, because a new term has been introduced (TTTV). TTTV is the difference between the highest measurement anywhere on the stamper, and the lowest measurement anywhere on the stamper. The new industry standard is a TTTV of 4, or +/-2.