Maximum Board Thickness and Aspect Ratio

Recommended Design Rules and Strategies for BGA Devices User Guide (UG1099)

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The maximum board thickness is a function of the minimum drill diameter and aspect ratio, both of which are provided by the PCB manufacturer. A typical aspect ratio of 15:1 indicates that the board can be no thicker than fifteen times the drill diameter. A drill diameter of 10 mils, for example, would lead to a maximum board thickness of 150 mils. Apart from the CP package, Xilinx recommends finished drill diameters to be 10-15 mils, which translates to an actual drill diameter of about 13-18 mils (plating typically reduces the diameter by about 3 mils). A 10 mil drill would lead to a maximum board thickness of 100 mils for a 10:1 ration, or 150mils for a 15:1 ratio. Advanced manufacturing technologies can support from 17:1 to 22:1 ratio, but at increased costs.