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12 The PCB Magazine • January 2017 There are so many final surface finish options to choose from today. How do you decide which is best? HASL—both tin-lead and lead-free—im- mersion tin, immersion silver, ENIG, OSP, and ENIPIG are the primary finishes used in PCB fabrication. Fabricators and assemblers generally work with the majority of these surface finishes to support their customers' requirements. So the question is, with all of these available, how do OEMs select their preferred surface finish? In the past, the primary function of the surface finish was to protect the copper from oxidation prior to the soldering of components. Today's expectations also include: superior solderability, contact performance, wire bondability, corrosion and thermal resistance, and extended end use life. Designs have changed. Lines and spaces are reduced, solder types and flux chemistries are different due to no-lead requirements, the number of assembly cycles has increased, and the product may need to carry high- frequency signals. Things to think about when selecting a final surface finish: • Does the application require tin-lead or lead-free assembly? • Will the end environment have extreme temperatures or humidity concerns? • What shelf life is needed? Will it be months or years? • Volume and throughput • Does the design have fine-pitch components? • How many assembly cycles will be required? • Is this an RF or high-frequency application? • Will probe-ability be required for testing? • Is thermal resistance required? Once the project requirements have been identified, the surface finish options can be reviewed to find the best fit. Final Surface Finish: How Do You Choose? FEATURE COLUMN: FLEX TALK by Tara Dunn OMNI CIRCUITS

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