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54 The PCB Magazine • April 2015 by Michael Carano oMG eleCTronIC CHeMICalS trouble iN your taNk Copper Discoloration and Other Concerns with OSP Column Introduction Yes, it is about process. And that includes equipment operating parameters as well as chemistry. Getting the OSP process to per- form as it is intended requires attention to both. This month's "Trouble in Your Tank" delves into one of the most irritating issues with respect to OSP: discoloration (read "oxi- dation") on critical circuit features. Why did this occur? Is there more than one cause? Will solderability be compromised? These are just a few of the questions raised when such a situa- tion occurs. An example of this occurrence is shown in Figure 1. Discoloration and even oxidation of the cop- per may occur for one of several reasons. Some of those listed may be more or less obvious than the others (Table 1). Some of these causes can be both equipment and chemistry related. Figure 2 shows another example of discol- oration on the copper pad most likely related to oxidation of the underlying copper. Further analysis of this situation pointed to two pos- sible causes. The first was very low thickness of the OSP coating. In this case the thickness was less than the minimum recommended by the supplier (measured OSP thickness 0.08 micron versus recommended 0.20–0.25 micron). Cer-

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