One vital aspect of PCB design is to achieve signal integrity and high-speed data transfer rates. It is often a problem in high-frequency PCB designs where a transmitted signal undergoes distortion due to multiple layers. That is where a back drill PCB comes into play.
Here, we cover how to use back drilling to contain the signal noise, quantization noise, and other undesired effects in high-speed PCB.
What is Back Drilling in PCB?
Also known as Controlled Depth Drilling (CDD), Back drilling involves using a drill bit to remove unused portions of copper barrels or stubs from a Plated-Through-Hole or Via in multilayered PCB boards. It is because, in some PCBs, some stubs are a nonworking part of the via and affect signal integrity.
We need an opening slightly larger than the PTH to drill back to remove the conductive region from the through hole. A perfectly done back drilling does not affect the PCB's performance.
(Different drill bit sizes).
Back Drilling Advantages and Disadvantages
- This technique reduces the number of bit mistakes per unit of time or Bit Error Rate (BER).
- It increases the channel bandwidth and data rates by removing stubs that cause distortion.
- Also, there is a decrease in signal layer distortion problems called deterministic jitters. These include timing errors caused by EMI, noise-related propagation, and via-to-via crosstalk in the PCB.
- Resonance frequency mode excitation is reduced.
- Better impedance matching and reduces stubs EMI/EMC radiation due to reduced signal attenuation.
- Easy to produce and cost-effective.
- Back drilling is only suitable for high-frequency boards between 1GHz and 3GHz with no viable blind vias.
- A unique technique must be employed to avoid damaging traces and planes lateral to the backboard hole.
What Kind of PCB Needs Back Drilling?
Generally, PCB boards with circuit tracks carrying signals at rates greater than 1Gbps are viable for back-drill designs. Through-holes are drilled from top to bottom of the PCB board. As a result, it yields high-quality signal transmission. Most times, stub removal occurs via the hole of the PCB interconnection. It happens when the trace connecting the via holes is close to the top PCB layers. Therefore, it results in signal reflections.
There are alternative construction techniques. Examples include laser-drilled vias and blind and buried vias. These alternate arrangements for stacking also reduce signal reflection. However, these options are costly for many high-density PCBs or backplanes/midplanes. For this reason, the only viable option is back drilling to remove the via stub.
Furthermore, some holes on PCB boards are less suitable in their design than blind holes, especially in multilayer printed circuit boards. If the PCB has deterministic jitters, strong BER, or other EMI issues, then PCB back drilling is essential.
Back Drilling Examples
A through-hole via, in a twelve-layer stack-up, goes from internal layers one through twelve. If the via only has signals from layers one to three, you create the via stub using specific-sized drill bits. The back drill should be at least eight millimeters in diameter over the primary drill size. However, ten millimeters is ideal. Hence, it prevents the back drill from accidentally drilling through adjacent layer pairs.
The via stub begins after layer three to down to layer twelve. It, therefore, creates resonance and reflections at very high frequencies. As a result, the signals attenuate at the resonant frequency. Back drilling removes unwanted copper plating from layer three down to layer twelve, reducing the stub length.
(Plated-through holes on a back drill PCB)
Back Drill Table with Design Values
Gmin. = Thickness Target Layer.
H = Distance To Connected Layer.
Tolerance = Thickness Stub Remainder.
Steps for a Back Drill
Here are the back-drilling steps you need to follow in the manufacturing process of a PCB:
- Locate the first drilling holes using the positioning holes provided on the PCB.
- Place a dry film to seal the positioning holes before electroplating.
- Electroplate the first drilled PCB board layer.
- Transfer outer layer graphics on the electroplated PCB.
- Once there is an outer layer pattern, perform graphic plating on the PCB.
- Ensure dry film sealing treatment on the positioning hole before graphic plating.
- Use the same positioning hole for the first drill for back drilling.
- Back-drill the electroplated holes that need drilling.
- Clean the back drilling holes properly to eradicate residual drill chips in the back drilling.
Difficulties of Back Drilling Process
Back Drilling Depth Control
Utilizing the depth control function of the drill to process the blind vias is essential to back drilling. Usually, the tolerance relies on the accuracy and the tolerance of the medium thickness of the equipment. Also, accuracy depends on external factors like the drill's resistance, the drill bit's angle, etc. For this reason, selecting suitable drilling materials and methods is vital to achieving accurate control. In turn, you achieve better results.
Back Drill Accuracy Control
Ultimately, back drilling depends on the hole diameter of the primary drill. This feature makes the accuracy of the secondary drilling coincidence vital. Therefore, it affects the quality control of the PCB going forward. Also, note the essential design rules like board expansion and contraction, drilling equipment accuracy, drilling methods, etc., before commencing the back drill.
Back Drill FAQs
Why is Back drill also called control depth drilling?
It is because a back drill requires specific calculations. Hence, it achieves the depth necessary for accurate results during the drill.
How much should the dielectric thickness design be?
The ideal dielectric thickness design is at least 0.2mm.
Name alternate construction techniques that are useful to minimize stub length other than back drilling.
They are laser-drilled vias, Blind and Buried vias, and alternate stack-up arrangements.
(A digital high-speed signal analyzer).
A back drill is a useful drilling process for removing problematic stubs from solder masks on a PCB. Problematic stubs often cause deterrent jitters like signal crosstalks. It is essential in improving signal integrity in hole technology boards.