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November 2014 • The PCB Magazine 45 LEAD-FREE COMPATIBLE OSPS: WHAT DOES THIS REALLy MEAN? continues 1. Make up a 0.1 N solution of silver nitrate in water. 2. Select either test coupons or actual PCB treated with OSP. 3. Apply silver nitrate solution with eye dropper (one drop) on area to be tested. 4. Allow 20 seconds to pass and inspect visually. If there is no discoloration on the copper surface (up to the 20 second interval), the coat- ing is deemed sufficient thickness. I have per- formed this test many times and consider it re- liable check on the process. Of course, as has been presented in past columns by this author, there are other important factors that are part of the OSP reliability equation. The Role of OSP Film Thickness It is worth mentioning that the author is not advocating increasing the thickness of the OSP film as a means to improve solderability. It is true however that a sufficiently thick and uni- form organic film is required at the very least to minimize oxygen penetration to the base met- al. The photo in shown in Figure 3 shows spo- figure 2: next-generation osP molecule—higher temperature resistance than benzyl-imidazole shown in figure 1. figure 3: sporadic discoloration of an osP treated surface.