DEK has concluded research into screen printing for 0201 SMT 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 analysing 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 µm, with pad-to-pad spacing of 200 µm. 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 formulae. "Assembly conditions that promote slower wetting speeds create a larger process window and produce good yield, and this is largely independent of the component orientation," said Clive Ashmore, and one of the leaders of the research team.
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 μm 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, 100 and 50 μm. To provoke tombstone defects, stencil registration was deliberately offset to create an imbalance in the grab between the two terminations of the same component. Registration errors of 0, 1,0 and 1,5 μm in both x- and y-axes were deliberately applied, to determine tolerances for printing accuracy for successful 0201 assembly.
The results showed that the tombstoning defects caused by increasing stencil offset can be reduced by increasing grab. On the other hand, a large grab increases bridging defects and solder balling. "High printing accuracy and low grab are required to minimise the cumulative defects arising from tombstoning, bridging and solder balling," explained Ashmore.
After printing SAC-based paste at the 4-mil print offset condition, post-reflow yield generally surpassed the performance of the tin-lead eutectic solder assemblies. Tombstoning and solder-balling levels were also significantly lower. "These results show great promise for assembling 0201s with SAC-based pastes," concluded Ashmore.
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