What is back drilling in PCB?
One of the challenges that PCB design and manufacturing face is how to maintain signal integrity. Back drilling also called controlled depth drilling (CDD), is used to remove conductive via stubs of the copper barrel in the through-hole in printed circuit boards (PCB). As a part of the via, the stub can result in serious signal integrity problems in the high-speed design. In addition, via stub will cause the signal to be reflected from the stub end, thereby disturbing the original signal. In other words, if the stub is pretty long, the distortion will be severe. Via stub is non-functional in the plated through hole since it does not contribute to signal transmission. It is worth noting that high-frequency PCB boards (greater than 3GHz) do not require back drilling to reduce signal reflections because other alternative strategies such as alternative stack-up arrangements can be used.
So how does back drilling overcome the signal integrity problem? By using a slightly larger drill bit, re-drill holes after plated-through-hole manufacturing to remove these stubs. Back drill the holes to a predetermined and controlled depth, which is close to but not touching the last layer used by the via. The ideal remaining stub should be less than 10mils. The diameter of the back drilling hole is slightly larger than that of the plated through hole. Usually, the diameter of the back drill bit is 8mils to 10mils larger than the original drill diameter. The reason is that the gaps of traces and planes must be large enough to avoid accidental drilling through the traces and planes adjacent to the back drilling during the back drilling process.
Figure 1: Before back drilling
Figure 2: After back drilling
Here is a back drilling example. There is a through-hole from the first through the twelfth layer in a 12-layer stack. While the via is merely used for signals from the first to third layer. Thus, via stubs are applied after the third to twelfth layer. As a result, resonance and reflection will occur at very high frequencies, thereby attenuating the signal at the resonant frequency. Thus, carry out back drilling after the third to twelfth layer to remove copper plating and reduce the stub length. In order to remove undesired copper, the diameter of the back drilling hole should be larger than the original drill diameter. The drilling depth is defined as the sum of the thickness of all layers from the start layer to the end layer minus the thickness of the end layer.
Once the back drilling decision is made, the next step is to decide how much reaming stub length can keep. The decision is determined by the following factors: the required signal integrity performance and actual cost-effective manufacturing considerations and limitations. Generally, the increase in PCB manufacturing costs is due to the increase in the number of back drill vias and the decrease in the maximum remaining stub length.
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