Author: William E. Coleman, Ph.D.
Abstract: QFNs present several assembly problems. The QFN can float during reflow if there is too much solder. Aperture size is a problem because with aperture widths as low as 0.175mm and aperture lengths as low as 0.4mm there can be a problem with the percent of paste transfer. Another challenge is the type of solder mask that’s employed on the printed circuit board. The article explores three types of solder mask designs – SMD where the pad opening on the board is defined by the solder mask; NSMD, where the pad itself defines the boundary of the pad and the solder mask is pulled back off the pad (typically 0.05 to 0.075mm per side); and NSMD–Window – and gives stencil design considerations and guidelines for each type. QFN repair is also covered. It shows how with proper stencil design, proper stencil technology selection (laser, electroform, and nano-coat), and proper PCB solder mask layout, the challenges that quad flat no-lead packages (QFNs) present to the assembly process can be overcome.
Released September 2012