Issue link: https://iconnect007.uberflip.com/i/457216
16 The PCB Magazine • February 2015 OSP for Mixed Metal Finishes With these caveats, the fabricator must also consider the composition of the OSP and how the chemistry performs with mixed-metal fin- ishes. This includes OSP interaction with the SENIG (especially the exposed gold). An exam- ple of gold discoloration is shown in Figure 4. There is a side-by-side comparison of two com- mercially available OSP processes. One OSP is clearly not compatible with gold and should not be used in mixed-metal finish PCBs (OSP 1). These mixed-metal finish boards are becom- ing very common today and the surface treat- ment of such circuits will continue to grow in importance. The demand was such that a water soluble surface-treating agent, that was capable of protecting the bare copper from oxidation without leaving a film on the other metals, needed to be developed and implemented. In other words, the need for an OSP that selective- ly bonds to the copper without adversely affect- ing other metals such as gold or solder is critical for use in selective applications. Conventional OSP technology deposits a significant film on the other metals such as gold, tin and solder. This additional film inter- fered with subsequent operations such as wire bonding, and surface mounting of critical com- ponents. In addition, the contact resistance on the gold increased to unacceptable levels. Thus, when PWBs are fabricated with multiple metal finishes, the metals such as gold or solder would have to be masked to prevent the OSP film for- mation on their surfaces. In some instances, the coating would have to be removed with alco- hol, adding additional labor and cost to the fab- rication process. A factor in promoting this film formation on the metal surfaces in the copper contained in many organic solderability formu- lations. The copper ions form a complex with the active azole ingredient in the OSP chemis- try and actually help to promote film growth. When a copper-gold mixed-metal board under- goes such a process, the OSP forms on the gold and will discolor the finish. On the contrary, the OSP process for mixed- metal finishes do not contain copper ions to promote organic film growth. However, what happens if there are copper ions that are in so- lution simply due to exposure of the copper cir- OSP AND SELECTIVE ELECTROLESS NICKEL continues feature Figure 4: Comparison of conventional OSP versus mixed-metal finish OSP.