Process considerations for lead free assemblies

Often, challenges occur when transitioning legacy designed products from leaded solder materials to a RoHS compliant material set. The increased process temperatures for SAC or SN100C
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solder materials are often limited by the PCB laminate or the thermal sensitivity of the assembled components. In the case of Plated-Thru-Hole (PTH) applications, the initial legacy designs may have been tolerant to the tin-lead solder process but not so for the Pb-free wave materials. For example, thicker PCBs with high board thickness to hole diameter ratios may experience insufficient "Hole-fill" defects. This problem is further magnified by updated IPC "Hole-fill" standards? The intent of this paper is to highlight some of the process considerations necessary to achieve a highly reliable assembly process when converting from tin-lead to Pb-free solder. This paper will discuss: 1) The thermal delta necessary to achieve successful hole-fill. 2) Differences in wave solder fluxes. 3) Assembly preheat methods and equipment. 4) Wave solder pallet design considerations. 5) Component and connector rework strategies. This paper will explain what can be done on the production floor to transition a product from tin lead materials to Pbfree materials.