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NOVEMBER 2022 I DESIGN007 MAGAZINE 51 Lead-free PCBs require that the standard HASL surface finish cannot be used. ere is still a significant amount of discussion on what the long-term surface finishes will be. Currently, the immersion silver and OSP sur- face finishes are the most prevalently specified surface finishes for solderable PCBs. Immer- sion tin is the prevalent surface finish for press-fit backplanes. Please contact your PCB fabricator for current information on where industry specifications are heading. Final Finishes ImmSn (Immersion tin) Immersion tin is a single layer metallic coat- ing of 15.7 µin to 23.7 µin [0.4 to 0.6 µm] of immersion deposited Sn.* is process coats a thin layer of tin directly on top of the copper surface. e tin produces an extremely flat surface for mounting of sur- face mount components with ultra-fine-pitch devices. is also provides a thicker, uniform surface that provides lubrication for press-fit pins. ImmSn is inexpensive and typically used for flat surface. ImmSn is OK for fine-pitch devices with solder mask dams greater than 5 mils [127 µm]. ImmSn is the top choice for press fit pin insertion, for the following reasons: • Careful handling required • Poor environmental resistance of intermetallic • Not great for multiple reflow cycles *Per IPC-4554 Nominal pad size of 0.060" x 0.060" (1.5mm x 1.5mm) Understanding the cost drivers in PCB fab- rication and early engagement between the designer and the fabricator are crucial ele- ments that lead to cost-effective design suc- cess. Following your fabricator's DFM guide- lines is the first place to start. DESIGN007 Anaya Vardya is president and CEO of American Standard Circuits; co-author of The Printed Circuit Designer's Guide to… Fun- damentals of RF/Microwave PCBs and The Printed Circuit Designer's Guide to… Flex and Rigid-Flex Fundamentals; and author of The Printed Circuit Designer's Guide to… Thermal Management: A Fabricator's Perspective. Visit to download these and other educational titles. He also co-authored "Fundamentals of Printed Circuit Board Technologies" and provides a discussion of flex and rigid flex PCBs at RealTime with… American Standard Circuits.

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