Analysis of Back Drilling Technology in High Speed PCB Design

Firstly, the components of a high-speed interconnect link are:

(1) Chip packaging: Usually, the PCB wiring width inside the chip packaging substrate is much narrower than that of a regular PCB, making impedance control difficult;

(2) PCB via: PCB via are usually capacitive effects with low characteristic impedance, and it should be the most focused and optimized in PCB design;

(3) Connector: The design of the copper interconnect link inside the connector is influenced by both mechanical reliability and electrical performance, therefore should seek a balance between the two.

The PCB via is usually designed as through-holes (from the top surface to the bottom layer). When the PCB line connecting the via is routed closer to the top layer, a “stub”bifurcation will occur at the via of the PCB interconnect link, causing signal reflection and affecting signal quality. This influence has a greater impact on signals at higher speeds.

Back drilling technology refers to the use of depth control drilling methods, using a secondary drilling method to drill out the Stub hole walls of connector or signal via.

As shown in the figure below, after the through-hole is formed, the excess Stub of the PCB through-hole is removed by secondary drilling from the "back side". Of course, the diameter of the backdrill bit should be bigger than the through-hole size, and the depth tolerance level of the drilling process should be based on the principle of "not damaging the connection between the PCB hole and the wiring", ensuring that "the remaining stub length is as small as possible", which is called "depth control drilling".

Schematic diagram of through-hole BackDrill section

The above is a schematic diagram of through-hole BackDrill section. The left side is a normal signal through-hole, on the right is a schematic diagram of the through-hole after BackDrill, indicating drilling from the bottom layer all the way to the signal layer where the trace is located.

Back drilling technology can remove the parasitic capacitance effect caused by hole wall stubs, ensuring consistency between the wiring and the impedance at the through-hole in the channel link, reducing signal reflection, and thus improving signal quality.

Backdrill is currently the most cost-effective technology which is the most effective for improving channel transmission performance. The use of back drilling technology will increase the cost of PCB production to a certain extent.

Back drilling is consist of 2 types : single-sided back drilling and double-sided back drilling.

Single sided drilling can be divided into back drilling from the top surface or bottom surface. The PIN hole of the connector plug pin can only be backdrilled from the side opposite to the face where the connect is located. When high-speed signal connectors are arranged on both the top and bottom surfaces of the PCB, double-sided backdrilling is required.

1) Reduce noise interference;

2) Improve signal integrity;

3) Local board thickness decreases;

4) Reduce the use of buried/blind via to reduce the difficulty of PCB production.

What is the role of back drilling?

The function of back drilling is to drill out through-hole sections that do not have any connection or transmission function to avoid reflection, scattering, delay, etc. in high-speed signal transmission.

Back drilling process

a. There are positioning holes on the PCB, which are used for the first drilling positioning and first hole drilling of the PCB;

b. Electroplate the PCB after first hole drilling, and dry film seal the positioning hole before electroplating;

c. Create outer pattern on the electroplated PCB;

d. Perform pattern electroplating on the PCB after forming the outer layer pattern;

e. Use the positioning hole that’s been used by the first drilling for back drilling positioning, and use a drill blade for back drilling;

f. Wash the back drilled hole with water to remove any remaining drilling debris inside.