Laser-cut stencils

It is common sense that friction between the paste and the aperture walls has a strong influence on paste transfer. This has implications both for materials and fabrication technologies. For example, the electro-forming process results in inherendy smooth aperture surfaces. This is enhanced by the low coefficient of friction displayed by nickel, from which electro-formed stencils are generated. On the other hand, laser-cut apertures present a relatively high friction surface, even though they are demonstrably adequate for many applications. If market pressures demand, stencils cut by laser from nickel-rich stainless steel may provide a workable balance of outright paste-release efficiency, turnaround time and overall stencil cost. 

Figure 2 shows deposit height and area data, as well as the volume of paste released by individual apertures in a set of test stencils fabricated using electro-form and laser-cut processes. Because paste volume is closely related to the quality of the mechanical joint created between the device interconnect and the land during reflow, this investigation makes valuable data available in the quest to develop high-yield assembly processes based on lead-free materials. 

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