In-situ observation and simulation of the solidification process in soldering a small outline package with the Sn-Ag-Cu lead-free alloy

The formation process of solidification defects in solder fillet of circuit boards with a small outline package (SOP) soldered with Sn-3wt.%Ag-0.5wt.%Cu has been examined primarily by using an in-situ observation system and solidification simulation software. The lead frames of the SOP are two kinds of materials: Cu and Fe-42wt.%Ni (42 alloy). Microstructural observations were made of the SOP solder joints using metallographic cross sections. The β-Sn dendrite structure in the solder fillet of a SOP with 42-alloy lead frames is larger than that of a Cu lead frame. This is attributed to the slower cooling speed of the 42-alloy lead-frame SOP than that of the Cu lead-frame SOP. The solidification of a SOP joint is not uniform and locally time dependent. The solder surface of the slowly cooled region of the 42-alloy lead-frame SOP exhibited shrinkage voids and a rough surface because of the coarsening of the dendrite structure. According to the simulation of the solidification process, the relationship between the solidification process and formation characteristics of solidification defects for a SOP joint can be clarified.