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76 SMT Magazine • May 2017 TAPE AND REEL SOLDER PREFORMS after the screen-printing process and the posi- tioning of the preforms. As Figures 2 and 3 suggest, the preforms must be in contact with the paste for two essen- tial reasons: (1) they must be held in position; and (2) they must be reflowed using the flux contained in the solder paste, which is more than sufficient. Additionally, it is not absolutely necessary for the preforms to be immersed com- pletely in the paste. In fact, as seen in Figures 2 and 3, only a minimum of 20% of the pre- form must sit in the paste deposit. During the reflow process, the molten alloy of the preforms is entirely drawn into the hole. It has also been proven that there is no risk of short circuit, even though the preforms are so close to each other. See X-ray in Figure 4. Figures 5 and 6 show the other side of the board after the reflow process. It is evident that the hole has been completely filled. Further- more, it is clear that there is no residual flux on the board, which would require washing or cleaning. The Alpha solder preforms allowed SECO to simplify the assembly process, reduce costs, Figure 3: Preforms shown in position. Figure 4: X-ray of PCB after reflow. Figures 5 and 6: Bottom side of PCB after reflow showing no residual flux.

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