DEK International, a manufacturing processes company in Flamington, N.J., has completed research into screen printing for 0201 surface mount components using SnPb and SAC solder pastes, highlighting new assembly and board design parameters for robust, high volume assembly with minimal application-specific modifications. Among the observations from analyzing a large number of test assemblies post reflow, DEK found that SAC pastes have an inherently wider process window.
Following the research, the optimum pad size for 0201 assembly is now known to be 300 x 380 x 230 micron, with pad-to-pad spacing of 200 micron. No-Clean paste and air reflow atmosphere produce the least tombstone defects. Technologies that increase wetting forces, such as nitrogen reflow or water soluble pastes, induce more defects. Better yields were experienced with SAC-based pastes, because the wetting forces are lower with lead-free formulas.
A DEK statement indicated that after establishing design and process parameters that produced the minimum number of defects, the experiments then applied very challenging assembly scenarios to discover the limits at which defects begin to occur. Pad spacing was also reduced to 100 micron to encourage bridging. In addition, the overlap between the end terminations of the components and the solder paste, which is known as the “grab,” was tested at 150 micron, 100 micron and 50 micron. The results shows that the tombstoning defects cause by increasing stencil offset can be reduced by increasing grab.