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PCB007-Jan2021

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100 PCB007 MAGAZINE I JANUARY 2021 image-develop-etch); the same concepts apply, only with a slightly different twist where out- er layer technology is concerned. With a met- al etch resist and final etching as typically seen with outer layer technology, undercut contin- ues to be an issue. However, excessive under- cut and the overall etch profile (Figure 2) leads to the overhang of the plated etch resist. The concern here is the excessive undercut may allow a metal sliver to break off during pro- cessing and cause additional issues, such as a short. Regardless, the factors at play remain the same. One can review the requirements in the IPC- A-600 detailing issues with metal overhang. Again, process engineering needs to pay close attention to the etching process to min- imize the undercut both on inner layers and outer layers. The undercut is exacerbated when the thick- ness of the copper foil and plated copper is thick. Think of 1-ounce copper foil (typically 1.4 mils thick) along with a minimum 1.0 mil of electroplated copper. In practice, the elec- troplated copper is often considerably thicker than 1.0 mil, further increasing the undercut opportunity. Alkaline Etching What can be done then, from a process stand- point, to minimize undercut? And in a worst- case scenario, what can be done to reduce sig- nificant line width reduction or complete etch- out (open)? To answer these questions, one must consider all the factors that can influence undercut and overall etching speed. It has been established that undercut fac- tors are grouped into three categories: etch- ing chemistry, etching equipment, and other effects that are not due to either chemistry or equipment. These include surface preparation, exposure, development, etc. More on that later. The process control of the chemical and oth- er operating parameters (temperature, specif- ic gravity, pH, etc.) of alkaline etching is para- mount to ensuring consistent quality and high yields. Cupric chloride etching (acid-based) has several other critical chemical parameters that affect undercut and etch rate. There are two specific parameters concern- ing alkaline etching that have a significant (read: heavily weighted) effect on the undercut and etch rate. Unfortunately, the etch rate and undercut move in opposite directions. A slow- er copper etching rate, while a drag on produc- tivity, allows for the reduced undercut. Keep- ing the pH in a tight range of 8.0-8.2 is ideal when alkaline etchants are employed for print- and-etch applications. Some formulated alka- line etchants can operate at a pH of 7.9. While this is ideal for minimizing undercut, if the pH drops further, there is the risk of the etchant sludging out. Conversely, as copper concentration builds in the etchant chemistry, specific gravity in- creases. A combination of low pH and higher specific gravity provides a positive benefit in reducing undercut. However, the reduced un- dercut comes at the expense of slower copper etching speeds. Indeed, higher operating tem- peratures will increase etch rates with a corre- sponding increase in the undercut. Outer Layer Etching There are several additional concerns when employing etching as part of the strip-etch- strip process (in preparation for SMOBC—sol- der mask over bare copper). First, one cannot use cupric chloride as a final etchant on met- al etch resist processed circuit boards. Thus, one can default to alkaline ammoniated etch- ing technology. However, there are addition- al challenges related to etching pattern-plated outer layers. These include the need to etch through thicker copper (copper foil thickness plus the pattern-plated copper on top of the foil copper). As the reader recalls, the great- er the copper thickness that the etchant must Figure 2: Illustration of undercut related to outer layer. Note metal etch resist overhang.

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