The nickel content of print stencils appears to be the figure of merit when migrating to lead-free assembly. This is the latest observation to emerge from DEK’s investigations into paste-printing performance with next-generation Pb-free solder. This recent study investigated the impact of stencil materials and manufacturing technologies on paste-volume repeatability, paste-on-pad registration and process window.
Comparing results from stencils manufactured using several materials and fabrication techniques, produced to match a test PCB featuring a range of popular pad sizes and shapes for modern assemblies, demonstrated paste-volume repeatability close to 90% for pure nickel stencils, with electro-formed versions just slightly ahead. After printing with nickel electro-form stencils, as well as pure nickel and high-nickel-content ones cut by a laser, the experiment showed pure nickel electro-formed to be marginally ahead of pure nickel laser cut. Both types showed better results than other stencils, including acrylic and conventional stainless steel versions. These observations indicate that nickel plays a significant role in enhancing paste release.
The research also showed that, even though the Sn96.5Ag3.0Cu0.5 solder appears to be the most widely recognized alloy for Pb-free assembly, these formulations continue to display differences in performance. “It is important to know how these differences will impact each stage of SMT assembly as our customers implement new Pb-free processes,” said Clive Ashmore, process engineering group manager. “In the longer term, we can expect Pb-free paste development to converge on a set of more or less universal properties as the technology matures, just as happened with Pb-rich pastes at the beginning of the SMT era”.
The differences in behavior mainly concern the effect of the rheology on the aperture-release characteristics, which has a critical impact on the deposited volume repeatability. High repeatability is related to high yield and productivity, as well as product uniformity. Pb-free pastes also have a higher solids content. The effect of this on excursion speed and volume repeatability was countered effectively by switching to the enclosed ProFlow head in place of ordinary printing with squeegees. This is because the improved aperture filling and adjustable transfer pressure achieved with ProFlow enables a wider process window and also improves volume repeatability.
By setting up careful experiments to enable direct comparisons between leading Pb-free solder pastes and a cross section of widely used stencil technologies, DEK was able to collect over five million data points. From these, the process experts have been able to draw a number of conclusions about the interactions between the pastes and stencils expected to be adopted after the 2006 deadline for Pb-free assembly to take over. The results show it is important to evaluate a candidate solder paste extensively before adopting it for production. Some assemblers may need to consider migrating to a high nickel-content stencil. Other factors affecting release efficiency of Pb-free solder pastes include aperture dimensions and aspect ratio.
EPP EUROPE 415
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