Electronics Assembly Knowledge, Vision & Wisdom
Selecting Stencil Technologies to Optimize Print Performance
Selecting Stencil Technologies to Optimize Print Performance
Research has been done to identify individual factors in stencil performance; this paper discusses the real-world application of numerous findings.
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Authored By:
Chrys Shea
Shea Engineering Services
Burlington, NJ USA

Summary
The SMT stencil is a key factor in the solder paste printing process. It has been shown repeatedly that print quality has the largest impact on end-of-line quality, and a good print process can make or break the profitability of building a PCB assembly. A good print process relies on a good stencil.

Much research has been performed to identify individual key factors in stencil performance; this paper and presentation discuss the real-world application of numerous findings. They review the numerous considerations in design, material, manufacturing and coating considerations, and how to best choose them based on PCB layout.

Conclusions
Optimizing stencil performance based on a PCB layout is a straightforward process, but requires a number of decisions based on the features of the layout. The first, and most critical, choice is on foil thickness. Typical SMT processes use 5mil (125um) foils, but some components require larger deposits that drive thicker foils and some require smaller, more precise deposits that require thinner foils. The stencil designer needs to make sure the foil thickness and aperture sizes do not violate area ratio rules, and stencil design analysis software speeds the calculation process while preventing errors.

Sometimes foils must be stepped to accommodate multiple thicknesses. Stepping guidelines are available to help insure the best possible print quality; if the guidelines are compromised, the print quality is likely to suffer. Steps of 1mil or less may be addressed by using an incremental-size nickel foil; steps or 2mil or greater should use Fine Grain SS. Other considerations for using Fine Grain SS include foil thicknesses of 5mil (125um) or less, devices with pitches of 20mil (0.5mm) or less, high density or highly miniaturized layouts, or area ratios less than 0.66.

Secondary processes like nickel plating over SS or electropolishing have not recently been shown to improve overall print performance and typically should not be a factor in stencil design or manufacturing decisions. Rather, the cut quality that a supplier is capable of providing should be a larger consideration. The smoother walls created by the combination of specialized SS and modern laser cutters have shown to produce the best print quality in successive tests, consistently outperforming every other stencil fabrication technology available. The finer the feature, the more important cut quality becomes - 1206s are more forgiving than 0201s; as are QFPs compared to QFNs. If secondary processes are employed, users should understand why they are required and what specific impact they have on the process.

Nanocoatings improve quality and cost by keeping the PCB side of the stencil clean, reducing underwipe frequency and improving print definition. They can positively impact any solder paste printing process, regardless of PCB layout.

Initially Published in the IPC Proceedings

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