Backdrilling means the process to remove the copper in "stub" that are not functinally in signal connection or transfer by 2nd drilling process from the backside. Because the stub area will cause signal distortion such as delay, reflection, etc.
Backdrill PCB actually is a type of special depth control drilling technology. In the manufacturing process of multi-layers PCB, for instance a 12 layers board, we need to connect circuit from layer 1 to layer 9. Our typical process is to drill a through hole and then plate copper inside to
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connect layer 1 to layer 12. However, we do only need to connect layer 1 to layer 9, the rest of the hole (layer 10 to layer 12) has no circuits, its like a pillar that will impact on signal transfer, so we need to remove the pillar, its called "stub" in industry, from the backside, therefore the process is so called "backdrilling".
Usually it will not be removed "completely" because in the later process of electrcplating some copper will be wipe off. Also as the needle is sharp shape, it can't be "drilled" 100% too, normally PCB manufacturer will leave a bit stub with length around 50-150um.
Advantages of backdrill PCB
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The advantages of backdrill PCB includes the following
reduce noise interferences
improve the completeness of signal
thinner board thickness
reduce the use of blind&buried hole to reduce manufacturing difficulty
Manufacturing process of backdrill PCB
drill the PCB first round as regular process by using of tooling hole
electroplating the PCB, using dry film to cover the tooling hole before electroplating
make the pattern on the outer layer
graphics electroplating for the PCB and still cover the tooling hole by dry film
locate the board by tooling hole, then drill from backside
rinse the hole by water after backdrilling
What's hard in backdrilling technology?
Depth control
The backdrilling process actually is the processing of blind hole with depth control by drilling machine. The tolerance is mainly subjected to the drilling equipment feather and the substrate thickness tolerance. Also the depth are easily to be infected by other factors such as electro resistence of drilling bit, the bit angle, contact area of the cover plate and measuring unit, flatness of the circut board, etc.
2. Precision control
The backdrill is formed based on the first round drill diameter, so the 2nd round drilling size is largely impacted by PCB expansion/shrink, equipment capacity, drilling type and drilling bit size. It is extremely important for PCB manufacturers.
IQE circuits is quite specilized in manufacturing backdrill PCB, you are welcome to send inquiry to inquiry@iqecircuits.com at anytime.
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