Controlled Depth Drilling in PCB Manufacturing: Back Drilling what is back drilling in pcb?

Backdrilling in multilayered printed wiring boards is the procedure of removing the stub to produce vias, allowing signals to move from one layer of the board to the next. (The stubs will create reflection, scattering, delay, and other issues during the signal’s transmission, which will cause the signal to be distorted.) Drilling at a controlled depth requires intricate skill.Making multilayer circuit boards, such 12-layer boards, requires connecting the first layer to the ninth layer. Typically, we drill through holes only once before plating the through vias. The first floor and the 12th floor are hence immediately connected. In actuality, the first floor just needs to be connected to the ninth floor. Since there are no wires linking the 10th through the 12th levels, they resemble pillars. This column has an impact on the signal path and may compromise the communication signal’s signal integrity. Therefore, a secondary hole was bored out of the opposite side of the extra column (referred to as STUB in the industry).

As a result, it is known as back drilling, however it is typically less clean than drilling because the next step will electrolyze some copper and the drill tip is also sharp. We will therefore leave a very small point; the length of this remaining STUB is known as the B value, and it typically ranges from 50 to 150 UM.

Back-drilling PCB technology

Due to the need to reduce signal loss for high frequency applications, a through hole connecting the layers is required for the signal to flow through as it moves from one to the other. It is recommended to remove the surplus copper from this hole for this application because it works as an antenna and affects the transmission if the signal is to flow from layer one to layer two in a 20 layer board, for example.

In order to gain greater signal stability, we drill out the “excess” copper in the hole using back-drilling (controlled depth in z-axis). The ideal outcome is for the stub (or “excess” copper) to be as short as possible. Typically, the back-drill size should be 0.2mm greater than the corresponding via.

The backdrill process removes stubs from plated-through-holes (vias). Stubs are the unnecessary / unused portions of vias, which extend further than the last connected inner layer.
Stubs can lead to reflections, as well as disturbances of capacity, inductivity and impedance. This discontinuity errors become critical with increasing propagation speed.
Backplanes and thick Printed Circuit Boards in particular, can endure significant signal integrity disturbances through stubs. For High Frequency PCBs (e.g. with Impedance control), the application of backdrilling, as well as the application of blind and buried vias, can be part of the solution.
Backdrill can be applied to any type of circuit board where stubs cause signal integrity degradation, with minimal design and layout considerations. In contrast, when using blind vias, the aspect ratio has to be kept in mind.

Features of PCB back drilling

Advantages of Back Drilling

● Reduce noise interference & deterministic jitter;

● Improve signal integrity;

● Local thickness reduction;

● Reduce the use of buried & blind vias and reduce the difficulty of PCB production;

● Lower bit error rate (BER);

● Less signal attenuation with improved impedance matching;

● Minimal design and layout impact;

● Increased channel bandwidth;

● Increased data rates;

● decreased EMI/EMC emissions from the stub end;

● Reduced excitation of resonance modes;

● Reduced via-to-via crosstalk;

● Lower costs than sequential laminations.

Disadvantage of Back Drilling

High signals frequently have issues that might be linked to underutilized via stubs. Let’s take a closer look at a few of the problems with the stubs.

Jitter Deterministic: 
Both clocks are timing, and the amount of time error is referred to as jitter. A deterrent jitter is what is known as a regular (i.e., limited) temporal modification.

Attenuation of the signal:
When a sound is attenuated, its intensity decreases and the pulse becomes weaker.

Radiation from EMI:

A via stub can be used as an antenna, radiating EMI.

General characteristics 

● Mostly are rigid boards on the back;

● Normally used on 8 layers or above;

● Board thickness is over 2.5mm;

● Minimum hold size is 0.3mm;

● Backdrill is 0.2mm larger than the vias;

● Tolerance of backdrill depth+/-0.05MM.

What kind of PCB needs back drilling?

Typically, the PCB board holes are drilled through the board (from top to bottom). If the trace connecting the via holes is close to the top layer (or bottom layer), it will result in stub bifurcation at the via hole of the PCB interconnection link, which will affect the quality of the signal and cause reflections. Signals traveling at a faster pace are more affected by this effect.

In order to acquire a high quality of signal transmission, it is generally understood that the circuit track on the PCB board with its Signals at a rate of about 1Gbps needs to be considered to include back-drill design. Of course, designing high-speed connectivity lines requires system engineering and is not as straightforward as it may seem. If the system interconnection links are not too long or the chip’s drive capability is strong enough, the signal quality may be faultless without any back-drilling. Therefore, system interconnection link simulation is the most accurate method for determining if back-drill is required or not.

You may be aware that, in addition to using back drilling technology, various building methods can also be employed to reduce or optimize the length of the stub. These include different stack-up arrangements, where circuit track traces are shifted to layers closer to the end of the via stub, laser-drilled vias (microvias) holes, or blind and buried vias. Additionally, as other methods are employed to lessen signal reflection on high frequency (higher than 3GHz) boards, back-drilling is not required.

However, these approaches are not always practical from a manufacturing facility and cost standpoints to reduce the signal loss in many high-density PCBs or backplanes/mid-planes. So, the only practical choice is to back drill the via stub out. When blind vias holes are not an option, back drilling becomes imperative for high frequency (more than 1GHz inside 3GHz) boards.

And since the PCB has so many layers, some holes cannot be designed as blind holes (for instance, on a 16-layer PCB, some via holes must connect to layers 1 through 10 and another via hole must connect to layers 7 through 16; this design is not suitable for blind holes but is suitable for back drilling).

How to do PCB back drilling?

The Process of Back Drilling

1.To locate the first drilling hole, use the positioning hole on the PCB that is provided.

2.Before plating, use dry membrane to seal the position hole.

3.Powder the hole with copper to create a guide circuit.

4.Create an outer graphic on the PCB.

5.After creating an outer layer pattern, the graphic board will be executed on the PCB. Before this process,It is crucial to seal the placement hole with a dry membrane before beginning this process.

6.Use the first drilling’s placement holes to align the back drilling, then use the drill bit to drill the electroplated holes that call for this procedure.

7.After the final drilling, the board needs to be cleaned in order to get rid of any potential leftover drills

8.After the board has been validated and the signal integrity has been improved, pay close attention to whether the drilling operation is being carried out appropriately.

Test the Back Drills 

Once the routing is finished, we must ensure that the back drills have been set up properly. To validate this, turn on all layers. You’ll see that the vias’ rim has two colors on it. The first or the starting layer is shown in red, while the final layer is shown in blue. It is simple to tell the back-drilled vias apart from the other vias. Only the back drilled vias are visible with two colors.

Click Drill Table after selecting the location from the main menu to find out how many Vias, PTH, and other trolls have been carried out.

Technical difficulties of back drilling process.

1.Back drilling depth control
For accurate processing of blind vias, back drilling depth control is crucial. The back drilling depth tolerance is largely influenced by the back drilling equipment’s precision and the medium thickness tolerance. Back drilling accuracy, however, can also be impacted by outside variables such drill resistance, drill tip angle, contact effect between cover board and measurement device, and board warpage. To get the greatest results and manage the accuracy of back drilling, it’s crucial to choose the right drilling materials and techniques during production. Designers can guarantee high-quality signal transmission and avoid signal integrity problems by meticulously managing the depth of back drilling.

2.Back drilling accuracy contro 
Accurate back drilling control is crucial for the quality control of PCB in subsequent processes. Back drilling entails secondary drilling based on the initial drill’s hole diameter, and secondary drilling precision is crucial. The precision of the secondary drilling coincidence can be impacted by a number of variables, such as board expansion and contraction, machine accuracy, and drilling techniques. It is essential to make sure that the back drilling procedure is precisely controlled drilling in order to minimize errors and maintain ideal signal transmission and integrity. 

The most important and challenging step is drilling because even a slight error might result in significant damage. Before placing an order, you should consider the PCB maker’s skills.Richfulljoy offers back drilled boards at affordable pricing and specializes in PCB prototype assembly. Our benefits include quick delivery times and high reliability.Richfulljoy, a well-known PCB manufacturer in China, has all the knowledge and abilities required to assist you. If you have any suggestions for a PCB assembly or prototype, please contact us : mkt-2@rich-pcb.com