Examination of soldering defect formation at QFN components

In everyday assembling practice, at the Quad Flat No lead (QFN) components, voids commonly appear at the central large termination; big solder balls can be formed underneath the package; open joints and short circuits might be present at the edge terminals. These defects are caused mainly by printing problems and unevenness; nevertheless, other factors can play a role in the forming of these. As a bottom terminated component, QFNs are difficult to rework or repair, so these defects should be avoided. As vacuum soldering is an expensive technology, in my research, I have investigated the joints on 1.25 mm thick FR4 circuit testboards having ENIG (electroless nickel, immersion gold) surface finish, mounted with SAC305 Type IV solder paste and soldered by vapour phase. Stainless steel, lasercut, 125 and a 130 um thick stencils were applied with divided central termination aperture patterns. The aperture reduction was 80 %. The testboards were checked by optical and X-ray microscopy as well. As a result, the most critical factors in defect formation were the moisture content of the flux, the parallelism of the assembled component to the board (tilting) and the assembling force - resulting in the final component-to-board distance and paste distribution.