Pb-free screen printing study recommends more nickel in future stencils

Citation

(2005), "Pb-free screen printing study recommends more nickel in future stencils", Soldering & Surface Mount Technology, Vol. 17 No. 3. https://doi.org/10.1108/ssmt.2005.21917cad.007

Pb-free screen printing study recommends more nickel in future stencils

Pb-free screen printing study recommends more nickel in future stencils

The nickel content of screen print stencils appears to be the figure of merit when migrating to lead (Pb)-free assembly. This is the latest observation to emerge from DEK's investigations into screen printing performance with next generation Pb-free solder pastes. This most recent study investigated the impact of stencil materials and manufacturing technologies on paste volume repeatability, paste-on-pad registration, and process window. The full results of the research are available by special request from DEK representatives or by visiting the web site: www.dek.com.

Comparing results from stencils manufactured using several materials and fabrication techniques, manufactured to match a test PCB featuring a range of popular pad sizes and shapes for modern assemblies, demonstrated paste volume repeatability close to 90 percent for the pure nickel stencils, with electro- form only slightly ahead. After printing with nickel electro-form stencils, as well as pure nickel and high-nickel-content stencils cut by a YAG laser, the experiment showed pure nickel electro- form to be marginally ahead of pure nickel laser cut. Both types showed better results than other stencil types, including acrylic and conventional stainless steel stencils. 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 recognised alloy for Pb-free assembly, Pb-free pastes using this alloy 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, Global Applied Process Engineering Group Manager, DEK Printing Machines. “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 behaviour mainly concern the effect of each paste's rheology on its 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 than their predecessors. The effect of this on excursion speed and paste volume repeatability was countered effectively by switching to ProFlow^® DirEKt Imaging™ in place of ordinary screen printing with squeegees. This is because the improved aperture filling and adjustable transfer pressure achieved with ProFlow enables a wider process window and also enhances paste 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, DEK process experts have been able to draw a number of conclusions about the interactions between the available Pb-free pastes and stencils expected to be adopted after the 2006 deadline for Pb-free assembly to take over.

Overall, the results show that it is important to evaluate a candidate solder paste extensively before adopting it for production. Some assemblers may need to consider migrating to a nigh nickel- content stencil supplier. Other factors affecting release efficiency of Pb-free solder pastes include stencil aperture dimensions and aspect ratio. The full results of this, and other DEK investigations into Pb-free paste performance during screen printing, are available from DEK representatives upon request, or via the DEK web site at www.dek.com/leadfree