Issue link: https://iconnect007.uberflip.com/i/1529988
22 SMT007 MAGAZINE I DECEMBER 2024 Previous experience from industry and research has led to the essential realization that pore formation depends on many factors and that a sustainable reduction of pores in the sol- der joints can only be achieved with the help of a vacuum process. A vacuum of 150 mbar can already reduce the proportion of pores from, for example, 40% to less than 10%. If very low void ratios <5% are to be achieved, other parameters, such as the surface finish, the sol- der paste type, or the design of the pads, must also be considered. In addition to the primary goal of reducing porosity, vacuum soldering offers other advantages that vary depending on the soldering system. A comparison of the contact, condensation and convection solder- ing processes is presented below. Contact Soldering With Vacuum System technology e contact soldering system with vacuum shown in Figure 3 is ideally suited for void-free soldering of various components (e.g., IGBTs) on DCB substrates. e joining of materials, which are usually made of different materials, is carried out under vacuum at temperatures of up to 752°F. Among other things, the vac- uum helps to minimize oxidation on the com- ponents and at the joints themselves. Heat is transferred both by heat conduction and optionally by radiation. A major advantage of this system concept is that the heating, cool- ing, and vacuum process steps take place in the same process chamber so that the soldering material does not have to be moved in the liq- uid phase. e 500 x 500 mm heating/cooling plate and high level of user-friendliness make the Nexus system particularly suitable for use in small and medium batch production as well as in laboratories. Profiling Figure 4 shows a reflow soldering profile with three vacuum process steps. During the first two process steps, the so-called pre-vac- uum is used to change the atmosphere in the chamber. e possibility of providing spe- cial atmospheres, such as forming gas or for- mic acid, predestines these systems for flux- free soldering with preforms. Both the pres- sure curve (pressure level and holding time) and the temperature curve (precisely adjust- Figure 2: X-ray images of the QFN64 solder joint after soldering without vacuum and after soldering at different vacuum levels and different holding times. Figure 3: The contact soldering system.