Investigating the increment of deposited solder paste due to uneven PCB Surface

The level differences on the PCBs´ surface can keep the stencil away from the PCB during stencil printing, which causes large deposited solder paste volume, and can result in solder bridges after reflow soldering. The level differences can be caused by identity decals, or by other artifacts on the PCB. In our experiment the effect of these level differences was investigated. A testboard was designed with solder pads and copper tracks. The thickness of the copper tracks were enlarged selectively by electroplating, thus those kept the stencil away from the PCB. In this way we could investigate the effect of PCB level differences. In the same time another test pattern was designed as well, where small pads were placed in continuously growing distance from an electroplated area. With this test pattern we could investigate the stencil bending, and a minimum distance is determined which should be kept in mind to enable the stencil to bend down to the PCB. The detailed results are presented in the paper.