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APRIL 2023 I SMT007 MAGAZINE 59 the formation of the three-dimensional solder joints, the solder wetting is tested by immersing the QFN coupons into liquid solder and evaluating the surface coverage of the side pads with solder. The coupons are dipped into the solder with an angle of 45° for five seconds. This test is performed after a thermal pre-aging of eight and 16 hours at 150°C and a steam aging test at 93°C for four and eight hours. Figure 3 shows the performance of an immersion tin after the cleaning step is shown. The cleaning includes a pre-clean and the microetch step (b). The last one shows the side flank after tin plating (c). After the precleaning, the copper surface appears even; scratches and smear residues from the separation process are removed. In the immersion tin plating process, the side flanks are covered with the immersion tin forming a dense tin layer covering the full pads on the QFN side wall. After the tin plating process, post-treatment solutions can be applied to prevent any discoloration of the tin layers in the later assembly processes and create a hydrophobic surface for an additional protection of steam aging. To confirm the capability of the tin layers to improve the solder wetting and enable Figure 1: Pretreatment and tin plating of QFN side flanks: a) After QFN singulation; b) After cleaning and microetch; c) After tin plating. Figure 2: Acceptance criteria for solder wetting of QFN side flanks. Figure 3: Solder wetting of the QFN side flanks with different immersion tin finishes. a b c

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