ELECTROCHEMICAL FAILURES AS A FUNCTION OF FLUX VOLUME UNDER BOTTOM TERMINATED COMPONENTS

摘要

The activity of flux residue changes, when trapped under low profile leadless or bottom terminated components. There are three factors to consider: 1.) Standoff gap - Lower standoff gaps block outgassing channels. Low standoff gaps change the nature of the flux residue by leaving behind flux activators, solvents, and functional additives that normally would be outgassed from the residue. 2.) Narrow Pitch -Miniaturized components have a decreased distance between conductors of opposite polarity. There is a higher potential to bridge conductors with flux residue. 3.) Cubic Volume of Flux - Increased I/O in combination with thermal lugs creates a higher cubic volume of flux left under the bottom termination. High flux volumes can block outgassing channels and bridge conductors. A QFN test board was designed to study the research hypothesis that electrochemical failure phenomena are related to the volume of flux residues present under the bottom terminated components. The designed test board (QFN-11) has four quadrants with varying mounting pad dimensions and component standoffs, resulting in varying amounts of flux volume. The test boards were evaluated using surface insulation resistance (SIR) methodologies, followed by visual analysis of the areas under the BTC components. An understanding of the relationship between flux volume and BTC pattern design is expected to aid assemblers in mounting pad design to reduce the risk of electrochemical failures.