In the common way that printed circuit boards (PCB) are made today, a key difference in the process of making an innerlayer, versus an outerlayer, is the fact that the photoresist is used in two distinctly different ways. When making an outerlayer the photoresist functions as a “plating resist”, and is used to define the pattern of where the electroplating of the metal that will subsequently become the etch resist, occurs. When making an innerlayer the photoresist is the “etch resist” itself, and directly defines where etching will occur.
When etching innerlayers, the etch resist is an organic compound (photoresist), rather than a metallic layer, and as a consequence, the choice of etchant can be thrown open, and etchants other than the classic ammoniacal etchant become practical. Whatever etching chemistry is used, the chemistry is best applied in a conveyorized, high pressure spray chamber that exposes every PCB to a controlled, reproducible, constantly refreshed spray of etchant. This is critical to insure a uniform etch, with straight side walls.
At this writing, there are two common approaches to etching of innerlayers, Ammoniacal and Cupric Chloride. These are not the only two possible, these are merely what are in common usage by the vast majority of PCB fabricators today.
The two approaches are most clearly differentiated by the fact that the Ammoniacal etch uses proprietary chemicals, whereas the Cupric Chloride etchant uses basic chemicals. The net effect of this is that when using the Ammoniacal etch, the technology and engineering of the process are supplied by the vendor. When using the Cupric Chloride etch, the technology usually must be supplied by the end user, although this can be assisted by the supplier of certain Cupric Chloride etch chemistry controllers.
It is important to note that at this time, there is available technology for turning both of these etching chemistries into closed loop systems. In other words, to have either system return only metallic copper, and not spent etchant, to the user.
While these closed loop systems change the economics of these etchants dramatically, it is likely that these systems will be economically attractive only to very large users, because of the large capitol investment required. Both closed loop systems electroplate the Copper from the spent etchant, either directly, or indirectly, and return the regenerated etchant back to the process.
