by Dr. Ning-Cheng Lee , Paul A. Jaeger, Wanda B. Hance
Solder beading is a special phenomenon of solder balling when using solder paste in certain SMT applications. In brief, solder beads are large solder balls near components with very low stand-off (see scheme below). With more attention being drawn to no-clean paste applications due to CFC concerns, a better understanding of this event becomes indispensable. In this study, the data indicate solder beading was caused by flux outgassing which overrode the paste cohesive force during the preheat stage. The outgassing promoted the formation of isolated paste aggregates underneath the low clearance components. At reflow, the isolated paste melted and , once emerged from the underside of the components, coalesced into solder beads. Processingwise, this problem can be remedied by slowing down outgassing via a milder preheat profile, or by reducing print thickness. Materialwise, solder beading can be corrected by enhancing the paste cohesive force via cold welding of solder powders during the preheat stage. This in-turn can be accomplished through the use of lower activation temperature flux, coarser solder powder, higher metal load, and solder powders with lower oxide content. Other parameters which could affect the performance will also be discussed.
lead-free, pb-free, solder balling, SMT, flux, solder paste, beading, solder beading
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
