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PCB007-Dec2020

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76 PCB007 MAGAZINE I DECEMBER 2020 Introduction In last month's column, I presented various etching defect causes related to equipment pa- rameters. In this month's column, I will dis- cuss various other causes that lead to etching defects. Surface Preparation and Photoresist Adhesion Final etching is often one of the least under- stood processes where downstream root-cause defects are concerned. While it is easy to point the finger at the etching process itself, skilled troubleshooters must be wary of downstream processes and materials. Case in point is sur- face preparation before photoresist lamina- tion, as well as the condition of the copper foil. Figure 1 is a schematic showing a deep depres- sion in the copper foil. Now, while indentations and scratches in the foil and the glass weave can negatively im- A Process Engineer's Guide to Final Etching, Part 2 pact resist adhesion, in a previous column I discussed the importance of surface prepara- tion and overall cleanliness of the copper sur- face. If there are issues with neck downs, re- sist lifting, blistering, etc., it is easy to under- stand how etching solutions (even when under optimum process control) can seep under the lifted resist and etch the copper, creating an open. One can easily see how the depression in the copper foil surface can act as a con- duit permitting etching solution to etch away copper (Figures 2 and 3). Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Figure 1: Hot roll lamination; depression in foil may prevent resist adhesion. Figure 2: Depression in foil surface with possible resist adhesion issue. Figure 3: After etching and resist stripping, copper etched away where the resist did not adhere. (Source: IPC)

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