Solutions for Voiding Control of Lead-Free Soldering at QFN Application

The quad-flat no-lead (QFN) package design is widely applied in many new applications due to benefits including small size, reduced lead inductance, and good thermal and electrical performance. However, it remains challenging to control thermal pad voiding under QFN due to the large coverage area and low standoff. Many parameters, such as solder paste flux type, powder size, powder quality, surface finish, and component size can impact voiding behavior. In addition, process parameters such as stencil design, printing, and reflow also have a strong impact on voiding. In this work, solder paste flux type, solder particle size, reflow profile, and reflow atmosphere were evaluated. While most variables showed some effect on voiding performance individually, the strongest effects were seen through specific combinations of variables. Solder paste flux type effects were strong and reflow atmosphere effects were moderate for SAC305 type 4 powder, both of which increased for SAC305 type 5 powder. Peak temperature and the time above liquidus (TAL) of the reflow profile also had a strong impact on the SAC305 type 4 powder.