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72 PCB007 MAGAZINE I MARCH 2018 Solving the Problem At this point the troubleshoot- ing team moved to the develop- ing area in order to ascertain what was required to solve the residue problem shown in Figure 1. The team looked at several plausible areas including: • Developer concentration • Break point • Rinsing time and rinsing effectiveness • Spray pressure in developer In Table 1, an extensive list of developer variables is shown along with the respective pro- cess effects. However, the four areas listed above showed the most promise as to the root cause of the pitting defect. One of the first areas studied was the actual spray pressure within the developer chamber. It was recommended by the resist supplier that the minimum spray pressure be 25-35 psi. During the routine check, the spray pres- sure was reading less than 20 psi. Upon further examination by the engineering team, it was discov- ered that some of the spray noz- zles were plugged with resist resi- dues and others were badly worn from extended use. These two conditions reduced the effective spray pressure. A second area that needed at- tention was the seemingly poor ability to rinse away the solubi- lized unexposed resist during de- velopment. It was also discovered that while the fabricator was us- ing city water for post-develop- ment rinsing, that incoming wa- ter was too soft to ensure effec- tive removal of resist binder salts. A word to the wise: Use of soft Table 1: Developer variables and corresponding process effects. Source: IPC 5001 Handbook and author's personal experience.

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