With engineers pushing the limits of manufacturing by introducing higher layer counts, smaller packages, high density interconnects, and high speed signals, they are also pushing laminates to their processing limits. As the designs become more and more dense, there is a need for materials that can withstand multiple thermal cycles without compromising function / performance. The selection of available materials today provide designers the ability have Low CTE, low loss, thickness control, flatness, high heat resistance, high cycle process.
Aspect ratio is the ratio of board thickness to via diameter and allows judgment of manufacturability. The larger the aspect ratio, the more difficult it is to achieve reliable plating.
Y= Via Diameter, Z= Board Thickness
The Maximum board thickness divided by the smallest selected drill diameter will give you your aspect ratio.
Recommend using 8 mil vias or larger as a standard practice, this will help keep cost down, reliability up, and ease the pains of manufacturing.