How to Do Controlled Depth Drilling (Back Drilling) in Altium Software

 Controlled depth drilling, also known as back drilling, is a technique used in printed circuit board (PCB) design to optimize high-speed signals by removing unused portions of through-hole vias. This helps eliminate stub effects that can reflect signals and cause signal integrity issues.

In Altium designer, back drilling can be defined by creating blind/buried vias that connect to the desired signal layers but do not go all the way through the PCB stackup. This article will go through the entire process step-by-step.

Overview of Back Drilling

• Back drilling removes unused bottom portions of through-hole vias to eliminate stubs
• Stubs can cause reflections and disrupt signals, especially high-speed ones
• Creates a controlled-depth interconnect instead of a full through via
• Done by creating blind/buried vias in PCB design software

Benefits of Using Back Drilling

• Improves signal integrity by eliminating via stubs
• Allows higher routing densities around vias
• Superior high-frequency performance
• Allows thinner PCBs for same number of layers
• Reduces manufacturing costs

When Should Back Drilling Be Used?

Back drilling should be considered when:

• High-speed signals are present (e.g. >1 Gbps)
• Via stub length is > 300 mils
• Reflections/ringing are causing signal integrity issues
• Space is limited around vias for routing traces

Next, we will go over the detailed steps for creating back drilled vias in Altium.

Creating Back Drilled Vias in Altium

The following are the steps to properly define and create controlled depth vias with back drilling in Altium PCB designs:

Step 1: Determine Signal Layers Needed

First, analyze your schematic connectivity and PCB stackup to decide which signal layers each via needs to connect to. This will determine how “deep” each via needs to be.

For example, if signals only route on layers 1–4 in an 8 layer board, you only need vias to connect layers 1–4.

Step 2: Define New Via Types

Once desired via depths are known, create custom via types that incorporate blind/buried settings:

1. Open the Via Types Editor under Design -> Via Styles
2. Select the default via type row
3. Click the Create Custom Via Type button
4. Name the new via (e.g. “4-layer backdrill”)
5. Under Properties, uncheck “Start/Stop On” for unused layers
6. This will create a buried via through layers 1–4 but not entire stackup

Repeat to create other required via types.

[Altium Via Types Editor]

Step 3: Set Up Via Rules

The next step is to set up via rules to map the new back drilled via types to the appropriate signal nets:

1. Open the Via Rules Editor under Design -> Rules
2. Map each new via type to the signal nets that need controlled depth
3. For example, map “4-layer backdrill” to high speed buses that only route on layers 1–4
4. Ensure other nets still use the default via type

This automatically assigns the correct vias.

[Via Rules Mapping]

Step 4: Place Back Drilled Vias

When routing traces that use the via rules, the mapped vias will automatically be placed instead of default ones.

• Simply route traces and place vias like normal
• The via rules will swap them to defined back drilled vias
• Visually verify they are correct type before finalizing designs

Step 5: Generate Fabrication Drawings

The last step is to have fabrication drawings clearly show vias that require back drilling:

1. Under File -> Fabrication Outputs, adjust settings to include:

• Mechanical drill drawing — shows hole sizes and locations
• NC drill file — back drill locations and sizes for manufacturer

1. Verify these files clearly display the blind/buried vias to be back drilled

The PCB designer and fabricator can now easily implement precise back drilling based on these outputs.

Real-World Example

Let’s go through a practical example of doing back drilled vias for a high speed bus in Altium:

The PCB Stackup

An 8 layer board is designed with the following signal layer distribution:

• Layers 1–2: low-speed digital circuits
• Layers 3–6: high-speed differential trace routing
• Layers 7–8: power delivery

Several single-ended 50 ohm traces also route on layers 3–6.

Determining Back Drill Depth

Since the high-speed signals and matched traces only occupy layers 3–6, we create new via types to target those layers:

• Default via: thru-hole all layers
• “HS_BKDRILL_A”: blind from layers 1 -> 6
• “HS_BKDRILL_B”: blind from layers 7 -> 6

This removes stub to bottom power layers.

Creating Custom Vias

Under Via Styles, Define:

• “HS_BKDRILL_A”: unchecked layers 7–8
• “HS_BKDRILL_B”: unchecked layers 1–2

Now two back drill-capable vias are created.

Mapping Vias to Nets

Under Via Rules:

• Map “HS_*” vias to chosen high speed nets
• Other signals keep default via

Placing Vias

Finally, place vias while routing — back drill types auto-assign!

FAQ

Here are some frequently asked questions about back drilled vias in Altium:

Q: Do back drilled vias affect routing density?

A: They actually increase routing density since no stub means more room around the via. The hole size is still the same on outer layers.

Q: Can I get away with just one back drilled via type?

A: It really depends on the board layout — separate types help maximize stub removal where needed. Always check if one type works first.

Q: What fabrication notes are needed for back drilling?

A: The NC drill and fabrication drawing files need to clearly indicate vias requiring back drilling, including locations, sizes, and target depths.

Q: Is there any signal integrity difference between back drilled and regular vias?

A: Back drilled vias can significantly reduce reflections, ringing, and losses compared to a full-depth via, especially at high frequencies.

Q: Do I have to use blind/buried vias or can through-hole work?

A: You lose most of the stub removal benefits with through-hole so blind/buried vias are basically required for proper back drilling.

Conclusion

Implementing precision controlled depth vias enables back drilling that dramatically improves signal performance through via stub elimination. This article walked through how to easily set up back drilled vias types and rules within Altium designer based on your board’s layer stackup and signals.

With the custom vias built, it takes just a simple via rule mapping to assign them appropriately. This makes it easy to integrate back drilling on your next high-speed design. Proper documentation also allows the fabricator to accurately create these vias.

Back drilling requires careful planning but pays major dividends for signal integrity. Altium provides the advanced tools to make this process smooth and error-free. Please reach out with any other questions on how to best utilize back drilling in your complex PCB projects!