PCB007 Magazine

PCB-Mar2018

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70 PCB007 MAGAZINE I MARCH 2018 Pits and Mouse Bites, Part 3 Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY In the first two columns in this series, the author presented two critical areas of the PCB fabrication process thought to contribute to the mouse bite and pitting defects seen in produc- tion at a fabrication facility. In those first two parts, photoresist lamination and exposure pa- rameters were investigated as to the possible root cause of the defects. Since the original assessment determined correctly that the mouse bites and pits had different origins, the troubleshooting effort focused on the potential for air or gas bub - bles in the acid copper plating tanks (Part 2). While corrections (with positive results) were made in the acid copper plating tank filtra- tion, the issue of pitting down to the lami- nate or electroless copper was not complete- ly eliminated. As stated previously, the devel- oping operation was scrutinized as a possible contributing factor. Developing of Negative-Acting Resist Unexposed resist is selective- ly removed through the chem- ical and mechanical actions of a sprayed carbonate solution (a.k.a., the developing solu- tion). The addition of a foam control agent may be required. "Time to clean" and "break point" (i.e., the percentage of useful development chamber length traveled by the board to achieve the visible removal of unexposed resist) are charac- teristic data that is generated in the development process. A water rinse removes resid- ual unexposed, partially poly- merized resist and developer solution. The remaining exposed resist is "fixed" by stopping the development action through rapid removal of the developer solu- tion in the water rinse. This is a critical and often overlooked step. Any residual develop- er solution that remains and is not adequate- ly rinsed away will potentially lead to sidewall damage as well as contaminate the copper sur- face. This in turn will lead to pitting and other plating anomalies. One idea worth exploring (to ensure devel- opment is stopped) is to rinse after develop with hard or acidified water. This action turns the binder molecules within the resist to insol- uble entities. A final drying step removes resid- ual moisture to harden the exposed resist for better survival in etching or plating solutions. An example of what can happen when re- sist residue is not effectively removed is shown in Figure 1. Figure 1: Note pitting of plated copper down to base. Residue is evident on the base copper inhibiting complete electroplating of the copper, resulting in a pit.

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