Aperture shape and size have measurable impacts on material volume.

While the surface-mount printing process is well-defined and established, ongoing demand for expanded product capability in a shrinking device footprint continues to challenge conventional rules. Ensuring robust print deposits for ultra-fine pitch dimensions, printing in tighter side-by-side configurations for high-density products and accommodating high-mix assemblies that require both small deposits and large deposits are all factors that must be overcome as the industry migrates toward more advanced products as standard. What’s more, achieving these priorities must be done cost-competitively and at high yield.

Stencil printing capability is dictated by the area ratio rule and, in my view, we are sitting on the proverbial edge of the cliff in terms of the limits of the printing process. To accommodate future technologies, current printing rules will have to be broken. The area ratio is the central element of a print process that dictates what can and cannot be achieved. Historically, the area ratio has hovered at 0.66, and over time, with better stencil technologies, solder paste formulation advances and improved printing capability, the stretch goal area ratio sits at about 0.5 (a 200µm aperture on a 100µm thick foil). What is critical at these finer dimensions and smaller ratios is a tight tolerance – in other words, the deviation in aperture size that is acceptable. Take our example of a 0.5 area ratio: If the tolerance is +/-10% (the generally accepted standard, incidentally) and aperture size is at the edge of that tolerance with a 190µm aperture on a 100µm thick foil, all of a sudden the true area ratio is now 0.475, which is beyond the edge of the cliff for most processes. Results from testing at our company revealed that a 10% deviation on a relatively large 550µm circular aperture nets 4% less material volume than if the aperture were cut to size. This same scenario on a 150µm or 175µm aperture can result in as much as 15% material volume reduction. It’s a double whammy; not only is it harder to print, but if you can print, the material volume will be less.

Given this, how does the industry move forward? We’ve previously discussed active squeegee technology in this space and its viability for breaking past existing area ratio rule limits and enabling robust transfer efficiency for miniaturized devices. In addition to this, however, new work undertaken by our company has shed light on the increasing importance of aperture shape in relation to improved transfer efficiency. Not only will the aforementioned 10% aperture size deviations factor greatly in transfer efficiency capability, so will aperture shape. The testing revealed that square apertures significantly outperform circular apertures, with the greatest impact being realized at smaller aperture dimensions. To fully understand the effect of aperture shape on volume, however, standard deviation must be analyzed. Aperture transfer efficiency volume numbers in a process are just numbers, say 75% as an average, but establish nothing in terms of maximum and minimum boundaries. But, if the transfer efficiency is 75% with a standard deviation of 10%, then there is a tight band of data behind the process. If, on the other hand, there is 75% transfer efficiency with a standard deviation of 40%, that’s probably not a process I’m going to be shouting about.

With this as the basis, we analyzed the standard deviation in relation to aperture shape and size, as well as the impact of active squeegee technology on transfer efficiency. On the larger aperture sizes (>250µm), standard deviation was approximately 5% on both the circular and square shaped apertures. But, when the area ratio moves below 0.5, this is where significant differences were noticed. The smallest area ratio that could be printed with a standard squeegee and a round aperture was 0.5. With a square aperture and a standard squeegee, the achievable area ratio was 0.47. When active squeegee technology was introduced, good transfer efficiency was realized on circular apertures at an area ratio of 0.45, but on the square apertures, it was a remarkable 0.34. To put it in percentages, 1.2% transfer efficiency was the result on a round 0.34 area ratio aperture, while this same aperture had transfer efficiency of 50% with an active squeegee process implemented. On the square apertures, 7.3% transfer efficiency occurred at a 0.34 area ratio with a standard squeegee and jumped to 60% when active squeegee technology was used. What’s more, this was with a standard deviation of less than 10%, indicating process stability.

As I’ve said before, moving toward more highly miniaturized assemblies is upending many factors of the traditional print process. Ensuring a successful outcome with high yields means taking a holistic approach, incorporating all technologies and best practices available. Aperture shape is but one more piece in this increasingly complex puzzle.

Ed.: The author will present more detail on the findings shared in this column during IPC Apex in San Diego, CA. The session, titled “Printing II,” will take place Feb. 21.

Clive Ashmore is global applied process engineering manager at DEK International (dek.com); cashmore@dek.com. His column appears bimonthly.

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