● Second drill
How much do you know about the drilling in PCB design? We all know that drilling occurs when several boards are stacked side by side and drilled at the same time. But this is not enough for a PCB buyer or designer, as you have to send manufacturers a clear description of the requirements of design. Thus it is very important to learn more about the drilling in PCB design.
In fact, the manufacturers have limits of how small a drill may be used per board thickness. Here is the rule: the thinner the board is, the smaller the drill bit should be used. This value can be expressed by the aspect ratio including the starting aspect ratio and the finished aspect ratio. The starting aspect ratio is the hole size actually drilled with, while the finished one is the hole size after standard plating. The manufacturer deals with the former aspect ratio, and a designer refers to the finished aspect ratio. Considering this difference, the designer must be sure what value is being referred to when designing a PCB. What’s more, whatever small the calculation of the aspect ratio is, it may be no smaller than the finished minimum drill, as the aspect ratio is limited to the minimum drill.
When the holes are placed in a copper area and should not be plated, the second drill is necessary, this need to be considered to the drilling in PCB design. And the pads around a hole with no plating are known as unsupported pads; they must be larger than pads with plating in the hole for several reasons. The plating prevents the pad from twisting off, dissipating heat, and lifting when soldered. But such holes added plating process will need more cost.