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Peroxy-Sulfuric Etching Module
Operation/Overview II

Pumping the etchant through a heat exchanger during etching to remove excess heat before it poses a threat can often increase throughput. In a limited sense, you can also increase throughput (as well as the etch rate) by agitating the bath with compressed air (a.k.a. sparging). The benefits of "air sparging" are three fold.
  1. The turbulence in the wake of the bubbles breaks up the depletion layer immediately adjacent to the board's surface, delivering fresh etchant to the unprotected copper. As a result, the etch rate can be significantly enhanced.
  2. This same turbulence has the added benefit of similarly "freshening" the etchant down in the nooks and crannies of the resist pattern, effectively increasing the etching resolution of the bath. Resist geometries with 0.008 in. (0.2mm) traces and 0.008 in. spaces can be routinely etched with such a "bubble etcher".
  3. Although the total air volume is fairly low, air bubbles tend to carry away some of the heat generated during etching. This cooling effect is further enhanced by the evaporative phase change that occurs at the bubble walls as they rise through the heated solution.
The primary downside of bubble etching is that it generates a significant quantity of corrosive aerosol. Effective fume collecting or containment must be implemented if a bubbler is used.

Please note that using an air agitation system with an aggressive etchant chemistry does not remove the danger of the bath going exothermic. It will however, significantly increase the size of the safe operating window and improve the overall performance of your etcher.

Peroxy-sulfuric etchants are most effective, and safe when used in spray etching equipment. Spraying:

  • increases the etch-rate by increasing the delivery of fresh etchant and removal of depleted etchant,
  • enhances the effective resolution by improving the delivery of fresh etchant into finer resist geometries,
  • cools the etchant before it impacts the copper, rendering a runaway exothermic reaction virtually impossible
  • can produce far greater uniformity of copper removal from large area panels
As with the bubbler above, spray etching with peroxy-sulfuric generates a significant quantity of corrosive aerosol. Effective fume collecting with active scrubbing must be implemented if spraying is used.

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