Back Drilling is a way to remove the copper barrel’s stub from the through-hole. A stub is an unused part of the plated-through holes as it does not perform any function on the circuit, so it’s not needed.
For example, while producing a 12-layer PCB you have to make a hole to connect the first to 10th layers. In general, via holes are drilled and then copper plating happens which connects the first layer to the 12 layers, whereas you only have to connect the first layer to the tenth layer.
The part from the 11 to 12 layer is useless as there is no electrical connection, so it is only a pillar. This extra length of via affects the signal passage, making the communication weak. So, that extra pillar or stub has to be removed from the back through another drilling.
Moreover, back drilling is also known as CDD, controlled depth drilling and you can use it in any Printed Circuit Board having weak signals.
The stub is not just a waste, but it affects high-speed signals by distorting them. When these signals pass through a copper barrel having a long stub, it results in a high distortion. Thick or multi-layered PCBs and back panels are more vulnerable to weak signals due to stubs.
Printed circuit boards of high-frequency should have blind and buried vias, as well as back drilling. Besides, you don’t have to consider the layout design in the back drilling. On the other hand, you have to consider the aspect ratio in blind vias.
When the Circuit board fabrication is over, the fabricators redrill the holes to remove the stubs. This process involves a large drill compared to other holes. In other words, you have to back drill the holes to a controlled or limited depth, like it should not reach the last layer including via.
However, the back drilling is not a clean process like other drilling methods, as the copper gets electrolyzed in this case. Besides, the drill point is sharp in this case, so the fabricator leaves a small point. Moreover, the b-value or the remaining stub length should be from 50 um to 150 um.
Back drilling has many benefits as described below;
• It involves fewer bit errors
• It reduces the deterministic jitter
• It provides more data rates
• You get more channel bandwidth through back drilling
• The enhanced independence matching helps reduce the signal attenuation
• It helps reduce the resonance mode’s excitation or noise
• It does not involve the consideration of any aspect ratio
• It reduces a stub’s EMI radiation
• Back drilling reduces the thickness of local plates
• It enhances the signals, making them powerful
• It eliminates the need for blind holes
• It enhances a PCB’s production process
Back Drilling Principles
During the drill bit drilling, the tip of the drill connects with the substrate board’s copper layer. This process produces a low-level current to evaluate the PCB’s surface height, drilling with an appropriate depth. So, drilling stops after the set depth.
Applications of Back Drilling
You can use the PCBs with back drilling for different industries, including aerospace, computers with large servers, medical equipment, communication, and military.
In the case of military or aerospace, only the manufacturers having a background with these industries are eligible to provide the back drilling in PCBs. This means the ordinary PC Board companies can’t get such projects.
The back drilling or CDD process involves a few steps, such as:
• It involves the PCB with tooling holes through drilling
• Copper plating of holes before sealing of the dry film
• Developing graphics on the external surface when the plating process is over
• After graphics, making plating with patterns and providing the sealing of the dry film of positioning holes. However, the sealing is done before the pattern process.
• The tooling hole’s s drilling and vias’ back drill
• Cleaning of vias from the residual that occurs during a back drilling of holes.
• The back drilling of the left via is done from the top surface, whereas the back drilling of the right via occurs from both sides.
The PCB with back drilling consists of certain features, such as:
• Only a rigid PCB can go through back drilling
• PCBs with 8 to 50 layers can have a back drill
• Its thickness is 2.5 mm or more than that
• The aspect ratio of the PCB is also large
• There is less trace in the external surface, like in a square array with press-fit holes.
• PCB dimensions are larger than the boards without back drilling
• The depth clearance for a back drill has to be +/- 0.05mm
• The thickness of the insulation should be at least 0.17mm
• Generally, the back drill hole is larger than other vias, like more than 0.2 mm
So, the PCB drilling involves various stages and back drilling is the second stage that removes the unused plating of vias. It involves a specific depth and the side of copper.
The secondary drilling has to be precise, and that depends on the expansion and contraction of the board, drilling technique, tool accuracy, and certain other factors.
Whether it’s back drilling or some other, you need high-quality tools to bring the best results.
• Drills often wear while making holes in the thick boards, especially, small drills and they can affect the surface finishing, as well as the size of the hole. It also increases the drilling force, so your tool must have no wear.
• A worn-out drill needs more force to operate, besides, its temperature also increases, and eventually, the chemical and physical reaction of the drill wear also increases, causing a poor drill.
• You can reduce the drill wear by keeping the aspect ratios low, however, this is not in the case of back drilling.
• Also, take care of the chip load that depends on the drill diameter. Often drills with a small diameter break faster than the drill have a large diameter.
• You should hire professional services for your PCB design, execution, and assembly.
A Technique to Calculate the Drill Diameter
You can do it through the below formula,
Size of the back drill = size of the via or pad hole + 2 x design rule of an oversize back drill
What Is The Main Purpose Of Back Drilling?
The basic function of the PCB back drilling is to remove a part of the through-hole that does not transmit the current. If you don’t remove a long hole, it will affect the high-frequency signal transition, causing reflection or distortion.
What Is A Stub In PCB Fabrication?
The stub is an unused part of plated-through holes, as it does not perform any function on the circuit, so it’s a waste. It will affect the signals if you don’t remove the stub.
What Are Plated-Through Holes?
The PTH or plated-through holes connect multiple layers of copper in a PCB to provide an electrical connection to the components.
What Is CDD In PCB?
CDD stands for controlled drilling depth or back drilling that removes stubs or wasted copper parts from the PCB holes.
What Are Other Factors Affecting The Integrity Of The Signals?
Not only stubs, but some other factors also affect the transmission of the electric signals in PCB components. Like, PCB material, connector type, chip package, vias, transmission lines, etc.
Why PCB Should Have Drilling?
The printed circuit boards have become more advanced and their demand has also increased that needs high quality. PCB involves through-hole drilling to create a transition path for the electric signals in various circuits.
PCB drilling process has different stages and back drilling is the second stage that removes the unused copper plating. It involves a specific depth and the side of copper. It has to be precise, and that depends on the expansion and contraction of the board, drilling technique, tool accuracy, and certain other factors. Printed circuit boards of a high-frequency should have blind and buried vias, as well as back drilling.
The stud or extra length of via affects the signal passage, making the communication weak. So, that stub has to be removed from the back through another drilling. The back drilling is not a clean process like other drilling methods, as the copper gets electrolyzed in this case.
PCBs with back drilling is used in different industries, including communication, computers, military, aerospace, etc. you may have to go through some challenges while back drilling, but by following some precautions you can overcome them.
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