By Paul Watson/Allan Brown, Cemco-FSL


It is not the purpose of this article to demonstrate the effects that a tightly controlled etchant can have on the etched panel, it has long been established that tighter control limits are a major factor towards improving etch quality. However the purpose of this article is to present an alternative method of controlling Cupric Chloride compared to conventional chemical dosing systems and the benefits it provides the board fabricator.

For those who are unfamiliar with the use of Cupric it is most commonly used for etching copper from inner layer circuitry. While copper is initially required to activate the etchant, as the copper content increases cuprous ions are formed and the etchant becomes less active. If allowed to increase too high the etchant will no longer remove the copper. A simple illustration to describe what happens is the use of sugar in a hot beverage. The liquid is able to dissolve sugar but if you are a sweet tooth and too much sugar is added the sugar will not fully dissolve. You can either add more liquid or stop adding sugar; either way there is a saturation level that cannot be overcome.   

The Cupric has a relatively wide operating window and will produce good quality with undercut within acceptable limits.  Where the product is not complex or fine line and undercut control is less crucial the process is often run with manual testing and dosing and this may be perfectly acceptable.

Conventional Control

Most cupric etchers are controlled by either manual or auto dosing systems. Depending on which method is used there will be a fluctuation of the chemistry from the 'ideal'. In an ideal situation the chemistry would be maintained as near a straight line on a graph as possible.

A manual method will tend to produce a fluctuating graph if plotted because as product is etched its effectiveness will fall until checked and adjusted. The negative affect of manual adjustment is that the etchant will operate within a wider band of minimum and maximum parameters. It will also require the operator or lab technician to frequently analyse the chemistry and make additions of hydrochloric acid or the oxidiser as necessary.

It must also be remembered that adding too much oxidiser will generate chlorine gas, which is highly dangerous and therefore great care must be taken when deciding how much to add to avoid this happening.
The most common and effective adjustment method will require the dosing of hydrogen peroxide along with hydrochloric acid, both of which present a cost, which must be considered. 

The best auto dosing systems will achieve a controlled balance by monitoring the etchant and automatically dosing fresh replenisher solutions. This will result in additional chemical in the etch sump which will have to be removed and stored. This method requires some form of bulk storage systems and real estate for both fresh and spent etchant.

Both methods involve the constant rotation of trucking hazardous chemicals from the chemical supplier to the PCB plant and the waste to the chemical recovery treatment facilities, which is a cost to be factored into the process.    

Whichever method is used to replenish the system the state of the solution must be measured and ORP (Oxidation Reduction Potential) is a convenient and useful method.  Ideally the monitoring will aim to keep the ORP of the solution between 525 & 560 millivolts, which will give a good compromise between etch speed and controlled undercut.

Figure 1 - ETCH v ORP

So in summary either chemical regeneration or auto dosing systems, if controlled well are capable of maintaining the solution within predefined minimum/maximum limits but do have a major disadvantage by producing additional chemical and the associated storage and disposal costs.

Electrolytic Regeneration

By contrast passing an electrical current through an electrolyte, which in this case is the Cupric Chloride, carries out electrolytic regeneration. During the oxidation process the Cuprous ions are converted to Cupric ions. However, unlike chemical dosing, fresh chemistry is not required to dilute the etchant, instead the excess Cupric ions are converted to copper, which is plated out onto a Cathode plate.

It is fair to say that if you are making tight tolerance product with fine lines it would be preferable to keep the etching solution in an optimum condition. The benefits of an electrolytic regeneration cell and simultaneous copper recovery is that with the exception of minor hydrochloric acid additions and periodic top ups as a result of drag out and evaporation the system requires little intervention and maintains the cupric close to the ideal without the disadvantages previously mentioned.
 

Figure 2 - TYPICAL CELL OVERVIEW

A single cell has two Anolyte or "Copper laden Etching solution" chambers. These are mirror images of each other, located each side of the central cell. A Anode +ve plate is immersed within each of the chambers. The center chamber contains the Catholyte or "Plating solution" in which the Cathode -ve plate is immersed. Semi-permeable membranes separate the Anolyte and Catholyte.
The Copper metal which is plated on the Cathode plate is scraped off by an oscillating mechanism and falls into the Copper collection chamber. When fully primed a 2Kg cell for example can contain aprox 200ltrs of etchant

Figure 3 - TYPICAL 2Kg PROCESS OVERVIEW

The process comprises four fundamental stages 1) Anolyte is fed to the Anode plate and returned to the etching module. 2) Catholyte is fed to the Cathode plate and returned to the sump located within the regeneration system. 3) Catholyte is circulated and cooled within the cell. 4) Transfer of solution between the Anolyte and Catholyte chemistries.

Figure 4 - STAGE 1 Anolyte

The Anolyte or Etching solution from the Etch module is pumped to the regeneration cells. The Anolyte enters the bottom of the cell and rises up inside, passing between the Anode plate and the membrane. The solution weirs over at the top of the Anode plate, returning to the bottom of the cell, which in turn can be returned to the Etching module either by gravity feed or pumped return. The ORP is constantly being monitored as the Anolyte exits the cell and returns to the Etch module. When the ORP falls below a pre-set level (approximately 524mV) power is applied to the cell via a Rectifier and electrolytic regeneration of the Anolyte takes place whereby cuprous ions are converted to cupric ions.

Figure 5 - PROCESS STAGE 2 Catholyte Lift

The Catholyte or Plating solution is pumped from a holding sump to the cell. The Catholyte enters at the lower section of the cell and passes over a Cathode plate as it rises and weirs at the top of the cell, returning to the holding sump. The Catholyte and Anolyte are separated by means of two membranes. 

Figure 6 - PROCESS STAGE 3 Catholyte Cooling

During the plate-out process heat is generated as current passes through the membranes requiring the Catholyte to be cooled. This is achieved by recirculating the solution through a cooling bottle, which lowers the solution temperature to that of the incoming Anolyte. The cooling process is to ensure that when the solution is transferred to the Anolyte loop the Etching process is kept to within preset limits. Maintaining the correct Etchant operating temperature also reduces the 'heat on' time cycle at the Etcher, normally caused by conventional non pre-heated chemical dosing systems and normal heat losses.

Figure 7- PROCESS STAGE 4 Anolyte/Catholyte transfer

The transfer of chemistry between the two chambers is required in order to move surplus Cupric Ions that are created by the oxidation of the Cuprous Ions within the Anolyte chamber and to maintain the Copper concentration in the Catholyte to feed the plate out process. The total amount of plated copper is equal to the Copper ions introduced to the Catholyte.

To maintain the Catholyte volume, an identical quantity of Catholyte is transferred to the Anolyte circuit, which is accomplished by a pair of matched and calibrated diaphragm pumps. The Copper concentration within the Catholyte chamber is maintained to within 25 - 45 grams/litre. Hydrochloric acid concentration 4 - 6 N. The Anolyte (Etching) Solution Copper concentration is typically 130-140 g/ltr. Hydrochloric acid concentration 2.0 - 3.5 N, however this can be controlled to operator pre set limits. 

Process summary: 

Etchant is pumped from the etching machine into an anode compartment of an electrolytic cell at approx 200 ltr/min per 2kg cell. The rapid turnover of chemistry provides quicker control reactions in response to small changes in the chemistry, both in composition and temperature.

The ORP (oxidation/reduction potential) of the etchant is monitored. When the ORP falls below a pre-set level (approximately 524mV) power is applied to the cell via a rectifier and regeneration takes place.

The cell is divided into an anode and cathode compartment by means of a membrane. In the anode compartment, electrolytic oxidation occurs and cuprous ions are converted back to cupric ions and the solution then returns to the etching machine. Simultaneously, a proportion of the etching solution is transferred into the cathode compartment, where electrolytic reduction occurs and copper is removed from the solution in metallic form.

Copper depleted Catholyte solution is returned to the etchant solution to balance the Anolyte and Catholyte volumes. The rate at which etchant is introduced is regulated by the rate at which copper is plated out, thereby maintaining a constant copper concentration in the Catholyte (plating) solution.

The copper is plated in dendrytic form and is removed from the cathode by an oscillating scraper mechanism and falls to the bottom of the cathode compartment. The collected copper is periodically removed manually.

Process benefits:

* Consistent etch results due to tight process control
* No Chemical oxidising agent required (excludes natural losses)
* No storage of fresh and spent etchant
* Low Copper level in Etchant promotes a sludge free process sump
* High Etchant Flow Rate provides quick response to changes in etchant solution
* No waste, the only by-product is copper.
* No running costs due to profit from sale of the recovered copper.
* Anticipated short-term payback on investment