by Dr. Chingchen S. Chiu, Dr. Ning-Cheng Lee , Kimbela Randle, Christopher Parrish
Voiding in BGA at Sn63 solder bumping stage typically occurs at the interface of eutectic solder and the BGA pad, due to the tendency of forming minimal molten solder surface area at bubble surface . At low voiding level, Pb90 bump systems exhibit more voiding than eutectic Sn-Pb bump systems, primarily due to the sandwich effect which entraps fume bubbles for Pb90 systems. However, at high voiding level, Pb90 bump systems exhibits less voiding than eutectic Sn-Pb bump systems, due to the radius of curvature effect which compresses the bubble size of Pb90 bump systems. In general, the voiding in BGA at solder bumping stage increases with decreasing flux activity, decreasing flux or paste deposition thickness, increasing oxide level of spheres or pads, increasing pad dimension, increasing reflow profile length, and increasing metal content. The sphere oxide effect is more pronounced for Pb90 bump systems than for eutectic Sn-Pb bump systems, due to the immobilized oxide for the former systems as well as the sandwich effect . Voiding also increases with decreasing flux/paste viscosity, presumably due to a decrease in the flux capacity. No correlation can be identified between voiding and flux volatility. The mechanisms of voiding unveiled suggest that the preferential location of voids at interface is inevitable, and use of high melting point sphere for solder bumping helps confining the void size. Surface tension is the most crucial property dictating voiding. It influences the voiding phenomena via tendency of forming minimal liquid surface area at bubble surface and radius of curvature effect .
through-hole connectors, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
