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86 PCB007 MAGAZINE I APRIL 2019 OSP film and thickness over the peaks and val- leys on the microetched copper surface (Figure 2). As a rule of thumb, an OSP film thickness of 1,500–2,000 angstroms (0.15–0.20 microns) is sufficient for lead-free assembly. However, that means minimum thickness over the thin- nest areas of the profile. To ensure sufficient operating window for lead-free assembly with OSP, increase the film thickness to 2,500–3,000 angstroms. Regard- less, do not overlook the importance of sur- face topography and its influence on the sol- derability or surface preparation. Compare the two pictures in Figure 3; on the left is a copper surface is cleaned with scrubbing only, and on the right is the scrubbed surface with OSP. No other surface preparation was used. Note the dark spots on the right image; these dark spots represent areas on the cop- per surface where the OSP is too thin or non-existent, which will lead to poor solder- ability. Again, ensure that the pre-clean line before OSP provides a clean, virgin copper sur- face with a uniformly etched surface. Ideally, 20–40 microinches of copper should be re- moved. Figure 2: OSP film over a peroxide-sulfuric acid etched surface (L) and OSP film over a heavily microroughened surface (R). Figure 3: Improperly prepared surface with (L) a scrubbed surface and (R) a scrubbed surface with OSP.