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60 SMT007 MAGAZINE I APRIL 2023 process dedicated for QFN plating compared to a standard immersion tin process for PCB applied on QFNs. The pads with a solder coverage of minimum 50% and larger are acceptable, the pads with a coverage of less than 50% are rejectable. Exemplary images of the soldered side flanks are shown in Figure 4. The image shows the soldered pads after 16 hours of baking at 150°C and after storage for 8 hours at 93°C and 93% relative humidity. The results confirm that the immersion tin process designed for the QFN side flanks can achieve a significantly better soldering performance than an immersion tin process for PCBs adopted for QFNs. As during the aging of immersion tin layers, the thickness of the intermetallic compound (IMC) increases and leads to a lower content of free tin; this often is considered as critical for the solderability of the tin surface. Storage tests have been conducted to study the growth of the intermetallic compound over time depending on the substrate alloy. For this the tin layer has been plated on EFTEC 64T and C194 alloy. The com- position of the two alloys is given in Table 1. For all conditions, an overall thickness of 1.5 µm tin was deposited and subjected to thermal and steam aging at 150°C and 93°C, and 93% relative humidity. The results of the IMC and free tin thickness are shown in Figure 5—the thickness is stated in µm, where pale blue reflects the free tin and dark blue the IMC. The results show that the IMC formation is driven by temperature in the first place. The IMC growth is linear over time for the storage temperature of 150°C. After four hours at 150°C, approximately half of the tin thickness is consumed by IMC. The IMC formation is slower for the humidity aging at 93°C where, at the same time, about 40% of the tin layer is converted to IMC. It also shows that even after 16 hours at 150°C, 25–32% of free tin still remains to ensure good solderability. The impact of the alloy material is low and leads to approximately 10% more free tin on the EFTEC 64T material. The tin deposit on the side flanks provides good solderability even after thermal or steam aging which correlates with sufficient free tin remaining after the IMC formation. The single Figure 4: Exemplary images of the solder wetting after 16 hours of baking at 150°C (left) and eight hours at 93°C/93% RH (right). Table 1: Alloy composition in % w/w