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14 The PCB Magazine • February 2015 Selective Electroless Nickel-Immersion Gold (SENIG) Several years ago SENIG offered the promise of reduced gold cost due to the selective nature of the process. In this case gold would only be deposited on circuit features that would require wire bonding or act as touch pads or contacts. This type of process has become very popular for flip-chip applications. The remainder of the board would be processed through OSP (a.k.a., the mixed-metal finish). This promise has been met for the portable devices that are cost sensi- tive but require high reliability against shock- drop. The process sequence requires a secondary imaging step after soldermask. The photoresist protects the area on the circuit card from the ENIG processing steps. After the ENIG is plated the resist is stripped exposing the remaining cir- cuit features which are bare copper. Here, the board is processed through the OSP line to com- plete the selective process. There are a few criti- cal issues that the engineer must be aware when working with SENIG. First, it is best to employ a higher phosphorous content nickel (i.e., 10– 12%) to provide the corrosion resistance neces- sary to withstand leaching from the photoresist as well as the acidic preplate chemistry of the High I/O packages will test the process limita- tions in imaging, etching, solder mask and sur- face finishes. 3 Chip scale packages and BGA will find in- creased use as an interconnect medium. The trends listed above are only a snapshot of the many issues that fabricators and assem- bly companies face. However, those listed are the ones that most closely reflect the trends in- fluencing the solderability of components and the bare board surface. This fact relates most- ly to the actual selection and performance of the surface finish. Regardless, the finish must be able to perform under a variety of condi- tions, consistently and reliably. What is really interesting is the trend towards building circuit boards with mixed-metal finishes. Some refer to this as selective finishing. Here circuit boards are selectively finished with SENIG (electroless nickel-immersion gold). Other circuit features are finished (in most cases) with OSP. This pres- ents several challenges both from an OSP stand- point and the proper selection of the electroless nickel and pre-plate surface preparation. The process sequence for SENIG with OSP is shown in Figure 1. OSP AND SELECTIVE ELECTROLESS NICKEL continues feature Figure 1: the process sequence for SEnIG and OSP-mixed finish.