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58 SMT007 MAGAZINE I APRIL 2023 Immersion tin is a surface finish which is well established in the PCB industry and well trusted in the automotive industry due to its high reliability. In the automotive market, the use of QFN (quad-flat no leads) packages become of increasing interest due to their flexible form factors, size, scalability, and thermal dissipation capabilities 1 . One step for success has been the side-wettable flanks, which provide a reliable solder joint connection to the PCB and enable an automated inspection of the solder joint and solder joint quality. The bottom side of the QFN is usually covered with electrolytic tin, and the exposed areas with the copper flanks require additional treatment to enable the solderability of the flanks. Without further treatment, copper oxides are formed which inhibit the solder wetting so that in such cases only two-dimensional solder joints are formed. To achieve a better reliability, the industry investigates solutions to increase the wettability of the flanks leading to the formation of three-dimensional solder joints. In QFN manufacturing, the final step is a singulation step which can be either dicing or punching. During this step the flanks can get contaminated by tin smear or molding residues which stick on the surface and cannot be removed with standard pre-clean systems for PCBs. Therefore, the cleaner and microetch have to be specifically tailored to remove tin and mold residues and level the copper alloy of the lead frame material. In Figure 1, the copper alloy surface right after the dicing step (a), Immersion Tin Plating for QFN Wettable Flank Connections Article by Britta Schafsteller, Hubertus Mertens, and Gustavo Ramos ATOTECH 58 SMT007 MAGAZINE I APRIL 2023

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