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60 SMT Magazine • July 2015 BENEFITS OF SOlDERING WITH VACuuM PROFIlES, PART 2 continues The results in Figure 4 of the extreme com- parison of soldering at ambient pressure and with a vacuum of 10 mbar are also confirmed by the test run at 100 mbar. Void content (< 3%) and the number of voids can be signi- ficantly reduced in this case as well. Beyond this, the selection of aperture geometry and stencil type has a less significant influence on vacuum soldering results. However, one gains the subjective impression that in this case the plasma stencil has the tendency to leave some- what less voiding, which could possibly be tra- ced back to better release of the wet paste and the associated dimensional stability. And thus the stable, uniform paste release would have to be taken into consideration as an influencing parameter. Summary Soldering with the targeted use of a vacuum can contribute to a considerable reduction of the number of voids and void content. In or- der to assure that no negative effects occur, such as damage to sensitive components or solder splashing, it should be possible to profile the pressure curve like a temperature profile. The use of different aperture geometries or variously coated stencils doesn't necessarily result in si- gnificant differences during vacuum soldering. The theory involving outgassing ducts cannot be confirmed in this case, because they disap- pear already in the preheating zone. To a much ArtiCle Figure 2: Soldering profile without vacuum (left); vacuum profile with 10 mbar and a dwell time of 10 s (right). figure 3: comparison of stencil geometry with and without vacuum process.