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92 PCB007 MAGAZINE I JANUARY 2018 that is more accurately described as a mouse bite. This defect is seen at the resist sidewall/ copper interface. These pits are more spheri- cal in shape and do not always lead to the base metal. An inspection of the copper plating cells pro- vided some additional information that should be noted: All the copper plating cells in the fa- cility utilize air sparger agitation. An observa- tion by a member of the tech team indicated that small air or gas bubbles in the copper plat- ing cells were a potential cause of the spheri- cal pits along the edges of the traces and pads. Further, the team felt that the air agitation cre- ated very small air or gas bubbles that would lodge onto the resist sidewall leading to the spherical pits. The agitation system for these plating cells required further scrutiny. To test the theory that air bubbles could be causing the defect, the fabricator sent panels to another fabricator for processing through their pattern plating line. The agitation for the copper cells was provided by eductors, not air spargers. (Eductors move the plating solution in a more uniform way and eliminate air bub- bles.) After the panels were plated, inspec- tors noted that there was no evidence of pits or mouse bites along the resist/trace interface. However, there were still some random pits that indicated that residue on the base metal was inhibiting plating. This situation indicated two things: (1) air or gas bubbles at the fabri- cator were playing a role in the mouse bite de- fects and (2) there was a residue or contamina- tion that led to the irregularly shaped pits (the subject of next month's column). Finally, another critical observation provided further insight to the mouse bite defect. Here one engineer noticed that plating solution was entering the filtration housing in two of the cells (Figure 2). As this solution entered the housing, the air bubbles would become homogenized, so to speak, allowing the plating solution to become supersaturated with air bubbles. This situation allowed the bubbles to grow and eventually lodge onto the resist sidewall/trace interface [1] . To prevent the air bubbles from entering the filtration housing, a system of baffles was in- stalled. The baffles will prevent air bubbles from entering the filtration housing. Of course, a much wiser choice is to convert the agitation system to eductors. Conclusion The dominant cause of spherical or coni- cal pits in copper plating is gas or air bubbles, which can be introduced into the plating solu- tion from air spargers or through supersatura- tion of the plating solution with air bubbles. These bubbles grow on the copper traces at the photoresist sidewall, and thus inhibit plating. Some photoresists may contribute to air bub- ble (mouse bite) pits depending on the resist's wetting characteristics, degree of hydrophobic- ity, and surface imperfections within the resist itself. In Part 3 of this case study, the develop- ment process will be reviewed and any possi- ble parameters that may contribute to the pit- ting defects. PCB007 References 1. "Reducing Copper Plating Pits in PWBs," R. A. Olson, CircuiTree Magazine, August 1991, pg. 30. Michael Carano is VP of technology and business development for RBP Chemical Technology. To reach Carano, or read past columns, click here. Figure 2: Solution filtration housing.

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