To understand the back drilling offset,It is necessary to understand how to PCB board process.then know the details of any affects of product performance from design to production process.For “back drilling offset”, let us talk about what is “back drill”, “back drill” is a process for vias,the aim is shorten the via stub and prevent the resonance by via stub appeared in signal concerned frequency band to ensure or improve the performance of the product.
The first to see how the vias that need to be backdrill are implemented step by step, as shown in the following figure:
In view of the “back drill” above drawing process, since it is one of the production process, there must be a deviation, and the following are the errors caused by the back drill
Back drill depth deviation
The perfect situation for the back drilling is to drill down all the stub below the outlet layer.But the production is hard to 100% precision, in order not to hurt the line layer, must keep a certain distance from the line layer, we call it a safe distance.Therefore, there is no perfect situation, and the stub will remain 2mil~12mil after drilling.
Assuming that via outlet layer is in layer4, the back drill will be set from bottom-to-layer5 in the PCB file.It means to do back drill from the bottom layer to layer4 but not to hurt layer4,after doing this, the length of the stub is up to 12mil.At this point, even with FR4 material, the resonance caused by stub is also over 80GHz, and there is no effect on the current 25Gbps/28Gbps signals.
However, if the product is not designed correctly, the back drilling layer is not set correctly, or it can’t be set from bottom-to-layer5 for other factors.For example, avoid the back drill to damage the other layers, or to ensure the connection strength of the connector, only set the back drill from bottom-to-layer7.The over hole stub in the design itself has 25mil, the back drilling deviation of the processing is 12mil, and the final product’s over hole stub may be 37mil.The resonance of the 37mil’s via stub will be about 30GHz.Does the depth deviation of the back drill have a fatal deterioration to your product at this time?Does the system have sufficient margin to resist the deterioration? This needs to be prejudged at the stage of product design avoid panic after the event.
Back drilling diameter deviation
The deviation of back drilling diameter is the deviation of the size of the hole formed by the back drill.In addition to deviation from the depth of the drilling process, the back drilling diameter is also deviation, it is easy to understood.The drilling tool is like electronic drills, and they have many types in size.For example, the via diameter of finished product 0.2mm needs backing drilling. Generally, 0.45mm specification drill is selected, and the drill bit itself has certain tolerance range, such as 0.45mm+/-0.025mm.What is the effect of the deviation of the back drill diameter on the product, we will answer later.
Back drill position deviation
Although back drilling is based on location information, drilling holes through holes one by one, however, the alignment accuracy of backdrilling is less than 100% by affected accuracy.It’s like you drill with an electric drill on the wall, although you have made a positioning mark on the wall before, but 100 holes are drilled, and there are some holes that will deviation a little bit.
We will consider the effects of the deviation of back drill diameter and the deviation of back drilling position on the product get together.Because the deviation of diameter and the position deviation will occur at the same time as long as the back drill,when the via need to be drilled is open around, then the backdrill is easy, The only requirement is drill down the via stub.But if there is a line near via, it is possible to drill down the line.The line is partially drilled makes the line width become small, and then the impedance of the line is raised.
It is impossible to avoid the deviation from production, but we can avoid the effect of deviation on the performance of the product at the early stage.For example, the deviation of the depth of the back drill: the reasonable arrangement of the overlay of the via and the optimization of the stack to minimize the stub caused by the design itself;The back drilling diameter deviation + position deviation: set a constraint design stage, the safe distance from the other signal line to the distance is more than back drilling production and processing requirements.