PCB Fab: Etching Innerlayers (Conditions and Operating Parameters)

Etch conditions and Operating Parameters

Given the fact that the Copper foil on a PCB laminate is extremely rough and uneven on the laminate side, it is impractical and difficult to discuss or measure the actual thickness of the Copper. A convention has been established to get around this problem. The thickness of the Copper is discussed in terms of its weight per square foot. The most commonly used thickness of copper foil is "one ounce", which means that the copper averages one ounce by weight per square foot, which is equivalent to a thickness of 1.2 mils (0.0012 inches, or 30 microns) if the copper were completely flat.

The convention by PCB fabricators today is to have any conveyorized process essentially complete by the time that the PCB is half way through the spray chamber in which it is being processed. This is to insure that any spurious, slow-to-process details are completely taken care of by the time the panel emerges from the process. The point in the spray chamber that the process is essentially complete is called the "break point". Thus the break point in the etch process is the point that the laminate under the copper just becomes exposed. This implies that a spray etcher must actually be about twice as long as is actually required to complete the majority of the etching.

Typical operating conditions for the Cupric and Ammoniacal etches are shown in Table 11.2.

Table 11.2 Operating Conditions for Ammoniacal and Cupric Etchants


                      Ammoniacal                   Cupric
                      Etch                         Etch*
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Copper Content        19-20 ounces/gallon          24 ounces/gallon
                      (140-150 grams/liter)        (180 grams/liter)

Temperature:          120-130 F                   120-130 F

Effective speed       1 oz Copper/ft/min.         1/2 oz Copper/ft/min.

Chemical               
Operating
Parameters:

* The operating parameters of the Cupric Chloride etch actually vary over a huge range,and these parameters listed are typical of the colorimetrically controlled process that is most popular in the PCB industry.

The issue of etch speed is difficult to discuss, as it is very much a function of the details of the etch chemistry. The speeds cited in Table 11.2 are reasonably common to the industry. However, etch rates of both etchants can be more than doubled by small changes in the etchant chemistry.

In the case of the Cupric etch, the etch speed is a sensitive function of the free acid. The most commonly used Cupric Chloride etch control technology today uses a colorimetric type etch chemistry controller. This type of controller is inexpensive to purchase, and reasonably reliable. However, the price of this control convenience is that it controls the etchant chemistry such that it has essentially zero free acid. This low free acid causes a very slow etch rate, and consequently the Cupric Chloride etchant has the reputation, perhaps unfairly, that it is a very slow etchant.

The ammoniacal etchant is nominally controlled by density or to use the common terminology of the industry, a "Baume controller". This device continuously monitors the specific gravity (or Baume) of the etchant solution. The specific gravity of the etch chemistry increases with increasing dissolved copper. When the etchant reaches a specified specific gravity, which is usually near the saturation point, the controller turns on a pump which pumps in etch replenisher, which is a solution of Ammonia, Ammonium Chloride, and the other proprietary ingredients, into the sump, or into the first rinse chamber, which then overflows into the sump. The excess volume in the sump then overflows into a holding tank where it is transferred by pump to a barrel, or holding tank. It is then returned to the vendor of the etch replenisher for recycling.

The Cupric Chloride etchant is controlled and replenished in a number of different ways. The most common in the PCB industry is colorimetric technology, which uses a combination of Sodium Chlorate NaCl03 , Sodium Chloride (NaCl), and Hydrochloric acid (HCl) to replenish the etchant. This system controls the etchant by making minor additions of either Sodium Chlorate NaCl03 , or Hydrochloric acid (HCl) & Sodium Chloride (NaCl), to a side stream of the etchant, to see which of the two additions increase the transmission of light through the etch solution. Since the presence of Cuprous ion, or spent etchant, darkens the etch solution, and diminishes light transmission, one of these two chemical additions will increase the light transmission. The chemical addition which is effective in increasing the light transmission in the side stream, triggers a large aliquot of the proper additive to be pumped into the etch sump itself. The NaCl03 and the HCl are diluted in water sufficiently that the etcher is kept at an acceptable Cupric ion concentration (and thus density, or Baume) just by the addition of these replenishers. The excess etch solution generated is overflowed into a sump and pumped to barrels or a holding tank for shipping out.

When Hydrogen Peroxide and Hydrochloric Acid (HCl) are used to replenish the Cupric Chloride () etchant, the Hydrogen Peroxide additions are controlled by an Oxidation/Reduction Potential (ORP) meter, which functions very similarly to a pH meter. The Hydrochloric Acid additions are controlled by a very sensitive conductivity meter, which has only recently been commercially available.

In both etches, the speed and quality of the etching is very dependent on maintenance of the chemical operating parameters. The "quality" of etch, or etch factor, is discussed in Outerlayer Etching Area. The speed of etching is unusually sensitive to changes in the pH (in the Ammoniacal etch) and to the HCl content in the Cupric Etch. The essential reason is the same in both etches, the free Ammonia, which is controlled by the pH, and the free HCl, are the etch reaction rate-limiting-reagents in the dissolution of the Cuprous salts formed during etching, and thus are the real etch rate controlling reagents.

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