SAVINGS WITH GOOD ETCHANT CONTROL:

Graph
This graph illustrates how Optrol's precision Control/Monitor systems reduce process costs.  For any process there is some dominant process parameter such as speed or yield and process variables which can be adjusted changing the dominate parameter performance and process cost.  Curve C shows how the dominant variable such as speed would typically vary with process cost.  There is a range of acceptable process points on curve C which are above the minimum allowable line, B.  One would like to operate at the perfect lowest cost acceptable point, P.  A controller will hold the process within some control band and alarm if the process departs significantly from the control band.  The real control band must be set somewhat above point P to allow room for the control and alarm bands.  A good precision control lets the real control and alarm bands to be set near point P as shown in BLUE in the figure.  A low precision control shown in RED must have its control band set farther from point P to keep process variations out of the unsatisfactory region to the left of point P.  The process cost difference between the RED and BLUE control bands is wasted money.

Good etchant control for ammoniacal etchants will hold etchant density well within +/- 0.1 degrees Baume of a desired set point. Compared to more ordinary control precision of +/- 0.5 or even +/- 1.0 degrees Baume there are significant economies for the user. He will save on replenisher costs and/or increase etcher conveyor speed and productivity while at the same time reducing scrap.

REPLENISHER COST REDUCTION

The figure below shows the density and copper content of the solution in the sump for what used to be called adequate control (+/- 0.5 degrees Baume) and the +/- 0.1 degrees Baume which is now available.
 

Copper in solution versus time for good and poor control

In both cases the control turns on the replenisher feed pump when the solution density rises to the turn-on value and the pump runs until the density drops to the lower turn-off value. In the figure it has been assumed that the conveyor speed is the same in both cases which means that the maximum densities are the same for both controls. During the control cycle the sump density fluctuates between its maximum and minimum values. With the 0.5 degree control the average sump copper content is 21.5 oz. per gallon in our example. With the 0.1 degree control the average sump copper content is 21.9 oz per gallon. Since sump solution is what comes back as spent, the 0.1 degree control has increased the copper content of the spent 0.4 oz per gallon by eliminating the lower density excursions permitted by the 0.5 degree control. This 1.9% increase in copper content in the spent means a 1.9% saving in replenisher consumption per ounce of copper removed. If the actual performance of the control being replaced is worse than +/-0.5 degrees Baume then the replenisher savings would be proportionately larger.

PRODUCTIVITY

The above saving in replenisher cost is the easiest to calculate accurately but the savings from increased conveyor speed and scrap reduction are quite likely to be very much larger. Conveyor speed must be slow enough to etch panels completely when the process variables fluctuate to their lowest etch rate. The tight control of solution chemistry makes it possible to increase the conveyor speed because the etch rate does not fluctuate to such low values. Increased conveyor speed directly reduces labor cost per panel, increases productivity, and increases ultimate machine panels per hour capacity. The tight control means that the density can be set to lower values if desired to get even more conveyor speed without risking over etching during low density fluctuations in the control cycle because the fluctuations have been nearly eliminated. Setting to a lower density will of course give up replenisher savings in order to get larger savings in labor, machine charges, and overhead per ounce of copper removed.

SCRAP REDUCTION

Scrap reduction is probably the biggest saving and the hardest to prove without actually trying out the control in each case.
All etching on the conveyor beyond the bare point at which unwanted copper is gone is over-etching. Moving the bare point nearer the end of the etch chamber reduces over-etching. Over-etching can destroy lines which are correctly imaged and masked; it also puts extra demands and stress on masking weak points to create imaging defects which would not occur with a bare point nearer the end of the etch chamber.

Since it is not unusual for the panels inside an etcher to have a value exceeding $2000, it is important to detect process problems before a bad panel comes out of final rinse to announce that the whole load may be destroyed! Alarms can detect process faults before they generate scrap. The 0.1 degree controller now available alarms on high density, low density, and excessive replenisher feed duration. These alarms occur before the etchant is out of control and causing scrap. They will alert the operator to:

  1. Replenisher interruption which can be due to empty tanks, blocked lines, suction tube displacement, or feed pump electrical or mechanical problems;
  2. Excessive replenisher in the sump which can be due to valve failure, syphoning, relay sticking, inadequate sump mixing, or excessive manual replenisher operation;
  3. Water intrusion into etchant which can come from rinse or cooling water or from poor machine cleaning procedures.
The controller alarms should eliminate the scrap caused by any of the things on the above list. The value of the scrap saved must be assessed for each individual situation. One etcher had a 2% scrap rate before and a 0% scrap rate after installing a 0.1 degree controller with the alarm functions. In many situations eliminating a 2% scrap rate will save enough to pay for the 0.1 degree controller in less than one 8 hour shift!

For alarms to be effective they must be real. Repeated false alarms will desensitize the operator and reduce the effectiveness of real alarms. The new Model 136 Process Monitor has five internal alarm inhibit parameters which the user can customize to his process to eliminate most false alarming.

For quick ways to evaluate the effectiveness of existing controls see Etch Control Evaluation.
 

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