The solder paste application and remelting on the connection array of a QFN is called prebumping. Afterwards the exact component placement onto the corresponding connections of the board can be done easily, because the applied lead became stuck to the SMD. Therefore the soldering results of this replacing QFNs are superior.
At the first step of Transfer-Prebumping solder paste is dispensed onto the ceramic transfer plate. Using a high end dispenser allows the preparation for a few components in one process. The operator chooses a program regarding the solder paste pattern. Directly after the application onto the ceramic plate, the new SMDs are placed by the help of a high end positioning system which makes the pads of the QFNs touching the solder paste patterns.
Generating the bumps is the last step. Metallising and melting on the solder paste is done because heating up the ceramic plate by the powerful under heater and additional heating of the QFN bodies by the supply of hot air. Of course the power of both heating systems are regulated by an thermal profile. The preparation of the solder depots stuck to the pads of the QFN with flux is ensuring the optimal wedding during soldering the prebumped SMD onto the board.
At the Hot-Print-Prebumping the operator puts the QFN in a special tool, which fixes the pads of the components congruent with the holes of the corresponding stencil. Solder paste is provided to the box outs of the mask by the help of an applicator. Heating up the special tool metallises the solder paste and it becomes stuck to the connections of the QFN. Before the SMD is soldered onto the board, the bumps ought to be fluxed.
