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DEK concludes research on lead-free pastes

Posted: 05 Jun 2006 ?? ?Print Version ?Bookmark and Share

Keywords:DEK? 0201? surface-mount technology? solder paste?

DEK said it concluded research into screen printing for 0201 surface-mount technology components using SnPb and SAC solder pastes, featuring new assembly and board design parameters for 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-by-380-by-230m, with pad-to-pad spacing of 200m. No-Clean paste and air reflow atmosphere produce the least tombstone defects, said DEK. Technologies that increase wetting forces, such as nitrogen reflow or water soluble pastes, are said to induce more defects. According to the company, 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, global applied process engineer from DEK 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 challenging assembly scenarios to discover the limits at which defects begin to occur. Pad spacing was also reduced to 100m 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 150m, 100m and 50m. 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, 1m and 1.5m 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 minimize the cumulative defects arising from tombstoning, bridging and solder balling," explained Ashmore.

After printing SAC-based paste at the 4mil 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," commented Ashmore.

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