Screen Printing

Clive AshmorePredictions on material transfer get a wrinkle.

When you’re pushing the limits of area ratio rules and trying to realize an acceptable result out of a shrinking window, even the slightest dimensional alterations can make a big difference. That was the topic of my last column, as we looked into the impact of challenging area ratios of 0.4 (the accepted standard is 0.66), as well as the aspect ratio influence as the geometric shape of the aperture moved from a square to a rectangle. The work revealed that at an aspect ratio range of 1.6 to 1.8, the Cp values were noticeably higher than for those apertures with a 1.2 or 2.0 aspect ratio (each end of the square to rectangle spectrum). Intrigued by these results, our team pushed on with the miniaturized aperture (0.4 area ratio) shape analysis, this time to understand the effect of moving from a square to a circle.

I should note a few years back, our company also evaluated the printing performance of pure squares versus pure circles when using activated squeegee technology. Ultimately, the square apertures produced better transfer efficiency rates, which improved dramatically with the use of an activated squeegee versus a standard squeegee. However, what this previous study didn’t assess, and what our team set out to investigate this time, were gradual changes in aperture shape when moving from a square to a circle. For example, taking the Y dimension and slowly changing the radius by slight percentage increases to round the square edges and eventually move to full circle (FIGURE 1).

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Clive Ashmore

 Do aperture area ratio and aspect ratio play an equal role?

Miniaturization has been a central theme of this column for a long time. The enduring relevance of the topic speaks to the ever-moving goal posts when it comes to component sizes and printing dimensions. Today the industry is facing the reality of passive metric 0201s, which has prompted another look at the influence of stencil architecture on the printing process. The impact of introducing sub-150µm apertures means heterogeneous assemblies will include next-generation surface-mount devices that push area ratios below 0.5. To understand the bearing this will have on the print process, our team evaluated not only at area ratio – which has been the historic measure of printability – but also at the area ratio’s associated aspect ratios. The results were intriguing, to say the least.

By definition, the aspect ratio relates to the measurement of the shape of the aperture, and the area ratio corresponds to the aperture opening and side wall area. The formulas for both follow:

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Jerome Rousval

Has its time finally come?

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Clive Ashmore

When print results are out of spec, board thickness could be to blame.

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Clive Ashmore

Do the costs “justify” the image location?

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Clive Ashmore

How accuracy parameters change from lab to shop floor.

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