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October 2014 • The PCB Magazine 65 OxIDE ALTERNATIVES TO ENHANCE LPI ADHESION TO COPPER continues copper is typically a fine-grained amorphous deposit (due to the use of organic grain refiners and brighteners) and is more resistant to etch attack as opposed to the foil (laminate) copper represented by a columnar grain structure. Certainly it is advantageous to use a solder- mask that has good adhesion properties as well as resistance to final finish plating solutions such and ENIG and immersion tin. In Table 1, one can see the surface roughness profile as measured with profilometry. The data supports the assertion that oxide alternative process technology provides an overall surface roughness that is an improvement over conven- tional means. Possible Objections to Using Oxide Alternatives With so many different solderable finishes in use today, it is critical that the soldermask adhere to the surface regardless the final finish process used. It is also true that with a number of different firms marketing soldermasks, not all LPI's are created equal with respect to adhesion. So it is even more critical to ensure that the sur- face of the copper offers an effective roughness profile. However, there are concerns that exces- sive roughening may lead to unexposed solder- mask residues that are not able to be effectively removed in the development process. A second concern is that due to the action of the oxide al- ternative, there is an organic layer co-deposited on the copper surface. Thus, the non-coated sol- dermask features (pads and vias) show the effect of the organo-metallic coating. This coating is beneficial in that it enhances the bonding of the soldermask to the surface. The objection may be that the organic coating is light to dark brown in color. Not to worry, as the coating is easily removed in an acidic chemical step as is used during the final finish plating process. Even if the coating is left intact, there is no evidence that the coating adversely affects the final finish plating process. With respect to concerns of soldermask resi- dues remaining embedded into the roughened copper surface thereby interfering with plating processes, this is not an issue. The key, like all chemical and mechanical processes for PCB fab- rication, is process control to ensure that you don't have trouble in your tank. PCB michael Carano is with omg electronic Chemicals, a devel- oper and provider of processes and materials for the electron- ics industry supply chain. to read past columns, or to con- tact the author, click here. figure 4: no peeling of lPi after enig. the panel was treated with oxide alternative chemistry (figure 3). table 1. Process Rp-v Rms rough Ave rough Median Ht Mean Ht oxide alternative 3.52 um 0.54 um 0.438 um 1.44 um 1.48 um scrubbed surface 2.9 um 0.298 um 0.235 um 1.44 um 1.43 um