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62 The PCB Magazine • December 2017 CASE STUDY: SOLVING PLATING PITS AND MOUSE BITE ISSUES, PART 1 2. Effectiveness of developing step • Developer pH • Developer loading • Breakpoint in developer 3. Rinse water quality after developing • Hardness of water • Rinse water temperature 4. The electroplating process (pattern plating, cleaning) • Effectiveness of acid cleaner • Acid copper plating solution (organic contamination, air bubbles) • Solution filtration Unintended exposure is often overlooked as a potential cause of pitting and mouse bites. During the exposure process, there can easily be some amount of UV light that may find its way to areas that are not intended to be exposed. Unintended exposure will lead to partial polymerization of the resist. Consequently, this leads to incom - plete development of the resist in the areas that require complete de- velopment. Resist residues left be- hind will increase the risk of mul- tiple plating defects such as pits, mouse bites, domed cir cuit traces, ragged edge plating, and can even result in the plated copper lifting from the base. The factors contributing to this variable are degree of collima- tion and declination of the UV ra- diation, the distance between the phototool and the resist, which in turn depends on the vacuum draw- down, the coversheet thickness, the presence or absence of a protec- tive coversheet over the phototo- ol, the degree of the UV scatter that the beam experiences while travel- ing through the media, and how much, and at what angle, UV radia- tion is reflected by the substrate un- der the resist. More on this later. The Troubleshooting Approach The team felt fairly certain the root cause of these defects rested in one or more of the pro- cess areas listed above. So a divide and conquer approach was taken in order to speed up the process of identifying and correcting the root cause. When using this approach, list all process steps in sequence. Examine product halfway through process for defects or its possible cause (is the problem there yet?). Keep on dividing until the exact process step causing the prob- lem is located. Also, make use of other infor- mation sources, such as suppliers and available literature. This may also include cross-check- Table 1: Process variables for dry film lamination and exposure. (Source: author's personal experience and IPC-5001 document.)