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Peroxy-Sulfuric Etchant
Stabilizer Replenishment

Stabilizers used in peroxy-sulfuric etchants fall into two broad categories, those that are replenished in a fixed proportion with the hydrogen peroxide (sacrificial stabilizers), and those that only require replenishment as a result of bath drag-out (permanent stabilizers).

Sacrificial Stabilizers

Generally speaking, the members of the sacrificial class are less effective in stabilizing the peroxide, resulting in more rapid H2O2 consumption and higher etch rates, often a much as 10 times the etch rate possible with the so called permanent stabilizers. As a result, etchants that employ these additives are preferred for use in immersion etchers where solution agitation is at a minimum and simplicity of construction is emphasized over etch rate and control. However, the multi-step etching process of peroxy-sulfuric etchants is very exothermic. The agressive nature of these etchants can allow significant heat to build-up in immersion tanks. Continuous heating of the bath, and the exothermic heating at the copper surface during etching contribute to the accerlerated deterioration of the stabilizers in the bath. As the stabilizers become less effective, the excessive heat, and the always present dissolved copper can cause all of the H2O2 (and much of the remaining stabilizer) to be spontaneously consumed, releasing even more heat. So much heat can be released in fact, that polyethylene and polypropylene tanks have been known to melt down into slag, spilling their entire contents onto the shop floor. In the industry, this catastrophic process is referred to as the bath "going exothermic". Exotherming can usually be avoided by diligently maintaining the chemistry and operating temperature of the bath at their optimum levels AND limiting the amount of copper you try to etch at any given time (i.e. limiting throughput). Constant attention must be paid to the bath temperature and all etching must be stopped if the temperature should exceed 48 degrees C.

If the bath does start to exotherm, and the etchant sump is not fitted with cooling coils, the runaway reaction can often be stopped by adding very cold (i.e. just above freezing) deionized (DI) water to the tank. The amount needed depends on the severity of the exotherm but, generally speaking, a volume of cold DI water equal to 10% of the total bath volume is usually enough to lower the temperature into a safe range. If cold DI is not available, cold tap water can be used. However, you may have to dispose of the bath afterwards due to the presence of chlorine and other contaminants.

Maintenance

Direct analysis of any peroxide stabilizer is difficult at best, and beyond the means of most small shops. Primary maintenance of sacrificail stabilizers consists of adding stabilizer every time hydrogen peroxide is added to the bath. Although it varies between vendors, a common practice is to add one part of stabilizer with every 3 parts of 50% H2O2 (or 4 parts of 35% H2O2). Unfortunatley, in most cases, this simple regimen does not totally compensate for stabilizer consumption.

Since the purpose of a stabilizer is to stabilize the H2O2 in the presence of heat and dissolved copper, very often, the bath itself will tell you when it is depleted by the appearance of vigorous effervescence. In the case of severe stabilizer depletion, the bath will actually roil as the peroxide is consumed. To calm the bath, add stabilizer in increments equal to 5% of the initial full charge until the effervescing is reduced to an occasional bubble. Once the bath is still, return to the regular schedule of adding stabilizer with each peroxide addition.

NOTE: An excess of dissolved copper can also cause similar effects, so the copper content must be determined and corrective action taken (if needed) before proceeding with catalyst level adjustment.

Permanent Stabilizers

Stabilizers that require replenishing only as a result of bath drag-out tend to produce somewhat less agressive etchants. In spite of reduced etch rates, their near total immunity to uncontrolled exothermic reactions, reduced H2O2 consumption and higher allowable operating temperatures (55 degrees C) makes members of this class the preferred stabilizers for peroxy-sulfuric etchants used in high-throughput production environments. Production operations usually allow for the use of more expensive spray etching equipment that, in many cases, allow these less vigorous etchants to meet or exceed the performance of their more agressive cousins. Even low-cost, low-pressure randomized spray etching with these etchants can acheive etch rates acceptable for most PCB prototyping houses. Unfortunately, corrosive aerosol generation is still a problem and must be dealt with to insure your safety and the health of the environment.

Analysis

Permanent stabilizers are so effective in stabilizing the peroxide against the effects of heat and dissolved copper, it is very unlikely that solution effervescence will be observed, even in severely depleted baths. A simple, but reliable technique devised by Bill Good of the M.F. Good Co. (Westminster, CA) follows.

Equipment needed:

  • 2 ea. Pyrex brand test tubes
  • metal test tube stand
  • 4 quart sauce pan

Reagents needed:

  • ferrosulfate indicator solution (F)
  • 50% stabilizer solution (S)

Procedure:

  1. fill the sauce pan with water and pre-heat to 82 degrees C (130 degrees F)
  2. pipet a 50 mL sample of the etchant into the two test tubes
  3. identify the samples as 'A' and 'B'
  4. add 1 mL of F to each sample tube (this will increase the instability of the peroxide)
  5. add 1 mL of S to sample tube 'A'
  6. set the test tube rack (with the sample tubes) into the hot water for 30 minutes

    The test samples will reach equilibrium with the hot water in 3 - 4 minutes. At this temperature, some outgassing (mild effervescence) will be evident even when the stabilizer content is optimum.
  7. remove the rack from the hot water (be careful, it is very hot) and quickly cool the sample tubes by dipping them in cold water
  8. carefully analyze both samples for peroxide

Small differences in peroxide concentration, in the tested samples, is normal. However, if the peroxide content of 'A' is higher than that of 'B' by 2% of the volume of the sample (e.g. H2O2 content of 'A' is 8% vs. 6% H2O2 for 'B'), additional stabilizer is needed in the bath. Add 1% by volume of stabilizer and repeat this test in 2 hours.




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