Issue link: https://iconnect007.uberflip.com/i/1104607
84 PCB007 MAGAZINE I APRIL 2019 Two often overlooked performance attri- butes for organic solderability preservatives (OSPs) are the organic film thickness and the topography of the copper after microetch. Film thickness up to an extent is critical. However, the copper topography and surface preparation also play a role. Thus, you should not overlook the criti- cal nature of the overall OSP film thickness. Of course, thickness alone is not sufficient to ensure solderability. The uniformity of the or- ganic film coupled with the film's ability to minimize oxidation are important as well. The topography of the copper surface influences the uniformity of the OSP film as well as its thickness. Microetching Ensuring a clean and properly etched cop- per surface is necessary to affect the uniform deposition of the organic film on the copper surface. Concerns with thinner organic film where there is significant roughness of the copper are well-founded. Internal studies have shown that hydrogen peroxide microetchants impart a smoother topography to the copper surface than persulphate-based etchants. It is recommended that excessive microroughening be avoided. The concern is that with excessive roughness on the copper, there will be areas that have an insufficient thickness of OSP. In these situations, the thin areas will be more prone to oxidation (Figure 1). Some chemical processes impart rather large roughness pro- files, such as those used to enhance solder mask adhesion; these types of etchants should be avoided. The previously described condition becomes more of an issue when the OSP film thickness is thin. But again, the thickness of the overall film does not necessarily imply that thicker is better. It is more about the uniformity of the OSP Performance: Effect of Film Thickness and Microetch Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Figure 1: The bottom image shows excessive roughness leading to areas where the OSP film is thin.