Experimental study of bump void formation according to process conditions

Experimental studies of void formation were performed using a solder paste with a small particle size and a thin substrate with a small solder resist opening (SRO) size. Two kinds of Sn–Ag–Cu solder paste, screen printing mask opening (MO) size and pad finishes, and three kinds of SRO sizes were used as process variables. In this study, paste type and MO size had little influence on the formation of voids. In general, fewer voids were formed using the electroless nickel electroless palladium immersion gold (ENEPIG) pad finish than the organic solderability preservative (OSP) pad finish. However, the suitability of the pad finish was different depending on the type of paste. Void formation reduced with decreasing SRO. Both focused ion beam cross-sectioning and a thermal video system were used to ascertain the mechanism of void formation when using the paste with a small particle size, and the entrapment of flux was identified as the main cause for the formation of voids.

► Void formation reduced with decreasing SRO.
► Void area ratio reduced with decreasing SRO.
► Voids were formed in the form of double-walled and ball shape.
► Void generation by the entrapment of flux in the molten solder.