The Thickness of Finished PCBs

To determine the maximum value of finished PCB thickness, you have to measure it from copper to copper. The maximum thickness is very important to the PCB board fabrication facility. It can affect the aspect ratio, the stack heights for drilling and profiling, and, finally, the permanent limits of manufacturing equipment for processing. To get detailed information about maximum finished PCB thickness, please go to the Materials Section.

Rules
For calculating the total PCB thickness you need to include the solder mask thickness. The total value must be between 0.5 mm (0.020 inches) to 6.85 mm (0.270 inches).

Limitations
The following parameters can place limitations on the fabrication of a PCB. These limitation include racks for plating, electro-less baskets, United Laboratories’ rating for flammability, outer layer scrubbing and laminating equipment. There are some boards that require special handling and fabrication processing. One of the special handling specifications at the hot air leveling operation is for PCBs with a thickness that is smaller than 1.3 mm (0.050 inches). This dimension influences the machine capacity in a negative way that drives up fabrication costs.

Definition of the Aspect Ratio
The Aspect ratio is calculated with two measurements: the maximum PCB thickness and the smallest drill diameter that is used. By dividing the maximum PCB thickness by the smallest drill diameter, you will obtain the Aspect Ratio. To get the correct Aspect Ratio, ensure that the maximum PCB thickness is calculated over copper and prior to plating. There is no effect on the Aspect Ratio due to the increase in thickness resulting from plating, hot air solder leveling and the solder mask.