Voids investigation in solder joints performed with vapour phase soldering (VPS)

Abstract

Purpose – The main advantages of vapour phase soldering are a non-oxygen environment, the elimination of overheating and the possibility of the vacuum application, which can guarantee undeniably higher quality of solder joints, especially as regards void formation. These features are less affected by the alloy composition. The paper aims to discuss these issues. Design/methodology/approach – The quality of solder joints made in two VPS options (with and without vacuum) was investigated in terms of voids formation. Solder alloys of 37%Pb63%Sn (PbSn) and 96%Sn3.5%Ag0.5%Cu (SAC 305) were applied to an etched Cu layer on a glass-epoxy substrate using the screen-printing method. 1206 SMD resistors were placed on the solder pads with a Quadra pick-and place machine. For the inspection of joint structure and void identification, 3D X-ray images of samples were taken using a computed tomography system with a 180 kV/15 W nanofocus. For comparison, traditional cross-sections of the samples were performed using a metallographic polisher. The cross-section analysis was done in a scanning electron microscope (SEM). To confirm the relevance of these data, a statistical analysis was carried out. Findings – The paper shows that alloy composition has less impact on the quality of joints as regards void formation. The tendency for a different arrangement of voids in a junction depending on the distance SMD element and the thickness of the solder layer was investigated using X-ray computed tomography. Originality/value – The use of 3D computed tomography for void investigation gives full information about the internal structure of the joint and allows for precise void identification. Vacuum application during the soldering allows significant voids elimination.