Analysis of solder mask roughness and stencil shape influence on void formation in solder joints

Voids inside solder joints are empty spaces negatively affecting the mechanical, thermal, and electrical properties of the solder joint. This article deals with this problematics using two novel approaches to reduce voids’ occurrence. The first approach is the change of the roughness of the solder mask to influence the flux spreading around the solder pad. The second approach is the deposition of the same volume of the solder paste to the solder pads using a modified, thicker stencil with smaller apertures. The experiments were performed for solder pastes containing SAC305 alloy and two types of fluxes (ROL0/ROL1). Solder joints were inspected by X-ray imaging, and the shots were subsequently processed by image analysis. For determination of the spreading area of the flux around the solder pad during the reflow process, the tested boards were scanned by a confocal digital microscope. The results showed that more aggressive flux caused less voiding in terms of the average area of voids covering the soldering pad. Analysis of samples prepared with modified stencils showed a much lower proportion of voids than with the standard stencil. Furthermore, the results revealed that the selection of the solder mask type significantly influenced the voids’ formation within solder joints in the case of the unmodified stencil for solder paste deposition. On the other hand, when the modified stencils were used for sample preparation, the solder mask had no further impact on voiding. Therefore, modifying the stencil can be pointed out as a preferable and effective way.