PCB007 Magazine

PCB-Dec2017

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60 The PCB Magazine • December 2017 Case Study: Solving Plating Pits and Mouse Bite Issues, Part 1 TROUBLE IN YOUR TANK by Michael Carano RBP CHEMICAL TECHNOLOGY Introduction It happens when you least expect it. You are processing a highly critical job for a key custom- er. Everything is looking good so far. You have your best team on this job. And then, after all the plating is complete and expensive boards are be- ing inspected, the QC team calls your attention to pitting and what looks like someone had tak- en a bite out of the copper traces and pads. This is, in a nutshell, a significant process indicator at the very least and perhaps a non-conforming defect (depending on whether trace widths and annular rings are reduced below requirements). So, let's look at an actual situation that occurred and how the problem was resolved. In Figure 1, an example of the defect is shown. Background The PCB fabricator had been using the same photoresist for at least two years and had not had any reported issues with pitting or mouse bites. The company employed a standard pat- tern-plating process. After electroless copper was applied, the boards are laminated with pho- toresist, exposed and developed. From here, the boards were processed through the pattern plat- ing process which included an acidic soak clean- er, followed by micro-etch, followed by acid copper and tin plating. The remaining process- es included resist strip, etch and tin strip. It was during the inspection that pitting and mouse bites were noted. So, with the group now brain- storming as to possible causes, several possibili- ties as to the origin of the pits and mouse bites were discussed. These are: 1. Review of lamination procedures • Vacuum draw-down • Lamination pressure and temperatures • Unintended exposure Figure 1: Pitting and mouse bites on copper circuit pattern. Example of two defects shown: Pitting on left photo around area of resist sidewall and on the right example of a pit that goes down to the base laminate.

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