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In the design of multilayer printed circuit boards, via-holes are used to electrically connect the PCB patterns on the different layers.

They come in several types:
Reverse drilling in circuit boards
Blind vias. They connect the outer layer to several inner layers
Buried vias. They connect several inner layers to each other
Microvias. Laser-drilled, designed to connect two adjacent layers in the most dense constructions
Normally, the first three types are used on high density printed circuit boards (HDI) which can be combined with each other if necessary.

There is however a technique which allows the use of via-holes for dense mounting - reverse drilling.
What is reverse drilling
Reverse drilling is not the most complex technique and can be used at almost any stage of board manufacture if required.
However, it is rarely used by manufacturers.
Reverse drilling or backdrilling is a technique where an existing metallised via-hole is re-drilled using a slightly larger diameter drill bit. This is done to remove a column of metallisation that was not involved.
Re-drilling is performed with a controlled depth, and just below the layer that uses the transition hole. This drilling can be applied on both sides of the PCB as the "reverse" means in this case "re-drilling".
But why drill the hole first and then get rid of it, using additional time and effort?
That is the answer to the question of why this technique is rarely used and is usually required in cases where tests have shown that distortion and losses have occurred due to unused holes.

Let's take the example of a twenty-layer card with a thickness of 2.6 millimeters. The via-hole was created to go from the outer top layer Top to the inner layer int2, the signal does not need to go further. But it will, on reaching the destination layer (int2), follow the metallized transition below - or rather, part of it. Upon reaching the lower layer Bot, the signal will be reflected and, in a distorted way, go back and re-split on the int2 layer.

Such "walk" of signal is eventually expensive and creates noticeable losses. Of course, this is acceptable for simple cards and the loss is almost imperceptible. But for high-end ultra-dense cards it is a good reason to get rid of the extra metallized layer to reduce distortion.
Why reverse drilling is necessary.
But why use reverse drilling when it is possible to create not-go via-holes, which have many advantages compared to through-holes, especially for high-density boards?
To implement not-go holes, they have to be considered as early as at the drafting stage, since there are a number of nuances involved in making them: for example, increased thickness of the metallization on the inner layers, which can lead to board warping if the holes are not considered at the design stage and resorted to at a later stage.

In this case, reverse drilling can be applied at any stage of PCB design — even, if required, after pressing. All that needs to be done is adding the drilling to the documentation. Of course, ideally, reverse drilling should be applied at the design stage, but things can be different in production, and the cost of multilayer complex PCBs can be so high that creating them from scratch because of one annoying transitional hole error is not the best option.
Modern PCB companies carry out reverse drilling without too much time and also at low cost, as the process is not considered to be particularly complex. Saifon Technologies implements backdrilling in-house at any stage of your PCB design.