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:

While many factors can affect the print process, it is area ratio that fundamentally defines what can and cannot be feasibly printed. I had no preconceived notions about the aspect ratio variable, but was quite interested to understand the possible implications. Using a Type 5 solder paste material – which can be a bit tricky – the DoE indicated the following inputs: an activated squeegee, an 80µm-thick stainless steel stencil, high print pressure, low print speed and high separation speed. Analyzing printing results for multiple area ratios, it was determined from the Cp comparisons that the minimum printable feature size was an impressive area ratio of 0.4. With this measurement as the constant, the analysis then turned to the impact of different aspect ratios on the 0.4 area ratio.

Our team evaluated the following aspect ratios: 1.2 (almost square), 1.4, 1.6, 1.8 and 2 (rectangular). Graphing the Cp – the repeatability – from these different aspect ratios, it became apparent there was a trend. (Incidentally, this occurred with all the area ratios analyzed as part of the DoE, not just the 0.4 area ratio that was settled on as the minimum printable feature size for extended testing.) At around the 1.6 to 1.8 aspect ratio range, the Cps were noticeably higher, indicating that when the aspect ratio approaches this range even for the same area ratio, it produced a significantly tighter data set.

Interestingly, once the aspect ratio moves beyond the 1.6 to 1.8 range and closer to 2, the improved performance drops off.  The relationship is not linear. Of course, the next question is why?

Why should a 0.4 area ratio aperture that’s almost square not perform as well as one that’s more rectangular? Why do only certain rectangular shapes – those in the 1.6 to 1.8 aspect ratio range – perform better within a given area ratio than others?

Clearly, much more study and analysis is required for this observation. However, our team does have a theory. Let’s underscore the word “theory”: this is our hypothesis based on our findings and printing experience. Here goes:

One theory is that as the aspect ratio increases, so does the aperture opening area. Could the larger opening help with the filling process? The answer is most probably yes, so this is plausible. The other premise is based on the fact that as the aperture is increasing in aspect ratio, the east and west aperture wall surface areas are reducing. Likewise, the north and south aperture wall surface areas are increasing (think moving from a square to a rectangle). This difference in surface area between the east/west and north/south is causing an imbalance in the adhesion between the solder paste and aperture walls. During the separation phase, the solder paste has to break free from the aperture wall. If the east/west walls have less surface area, then these are possibly breaking their adhesion first, creating an opportunity for a better-controlled and constant release process. For aspect ratios squarer in shape, the aperture walls are more equal in length, which results in a tug-of-war, and the paste releases in an unpredictable manner. On one aperture, it may be the east wall; on another, the north.

So, our team’s hypothesis is that if the east/west (Y direction) walls are shorter than the north/south (X direction) walls, they will always release first. So, why the drop-off in Cp after the 1.8 aspect ratio then? It could be because this challenges the five ball rule; although there may be five solder particles within the aperture, the narrower vertical walls may cause the particles to jam and not release as well. Again, this is only theory at this point, but a good possibility.

To be sure, area ratio is still the determining factor for printing feasibility. However, as the finish line continues to move and the industry is faced with ever-smaller devices, aspect ratio measurements appear to be increasingly important.

But, that’s not the end of the story; more study is underway on this and other factors. Watch this space for additional analysis!

Clive Ashmore is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.

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