How 3D Printed PCBs Can Help with Impedance-Controlled Routing

How 3D Printed PCBs Can Help with Impedance-Controlled Routing

If you're a designer of ultra-high-speed PCBs or high-frequency RF devices, you'll take advantage of impedance-controlled routing in your PCB design software. These tools are designed to ensure that the impedance of the transmission line is consistent over its length, allowing termination at both ends to prevent reflections. Consistent impedance also ensures consistent propagation delay along the interconnect, allowing parallel high-speed PCB signals (such as those in PCIe) to be precisely matched in length to prevent skew.

Because impedance-controlled routing requires precise fabrication of PCB interconnects, manufacturers have spent considerable effort refining the etch process to ensure trace geometries match standard geometries used in PCB design software. Using 3D printing to manufacture PCBs allows designers to go beyond the standard trace geometries typically enforced in PCB design tools, while still ensuring precise impedance control. This gives designers more options for impedance-controlled routing and design interconnects than standard planar manufacturing processes.

What is Impedance Controlled Routing?

In all electronic devices that require precise design and manufacturing specifications, some high-speed and high-frequency PCBs can be sensitive to impedance changes. EDA tools now provide impedance-controlled routing functionality, where the impedance of an interconnect can be calculated by a number of possible methods. Tools such as electromagnetic field solvers are implemented in many standard routing tools, enabling designers to calculate precise impedances and propagation delays throughout the interconnect.

In this design method, the geometry of the interconnect and its position relative to the reference plane are designed so that the impedance of the interconnect takes on a specific value. When designing traces on a PCB, the distance between the trace and its reference plane is usually fixed by the thickness of the core or stackup. This limits the designer to a specific trace width and thickness during impedance-controlled routing on a flat PCB substrate.

Since impedance control is heavily dependent on the precise conductor geometry, the manufacturing process must be precisely controlled to ensure that the traces being fabricated match the design data. In the planar PCB process, the challenges of precise manufacturing on planar substrates are largely addressed for various interconnect architectures. However, this severely limits the designer's freedom to create impedance-controlled interconnects with unique geometries.

In contrast, the layer-by-layer deposition process offered by inkjet 3D printing systems removes traditional DFM constraints and allows designers to implement virtually any impedance-controlled interconnect architecture.

Unique 3D printed interconnects without standard vias

Much research has been done on modeling the impedance of vias and designing vias with accurate impedances in multilayer PCBs. Vias can cause impedance discontinuities that can cause reflections along the interconnect. Vias are basically inductors, so they also create inductive crosstalk on the interconnect. Although vias are critical for routing between layers in a multilayer board, both properties of vias can lead to signal integrity issues. In many densely routed PCBs, or when using components with high pin count/ball count, vias are often unavoidable.

Therefore, many PCB design guidelines recommend minimizing or eliminating the use of vias on high-speed and high-frequency interconnects. In mmWave PCBs or where edge rates are very fast, some standard through hole geometries such as plated through holes create some insertion loss in the interconnect. This reduces the signal level along the interconnect and causes slight reflections towards the source, reducing the signal level seen at the receiver. In low-level digital components, this can cause the signal to drop below the level required to latch into the ON state. Similarly, in analog components, this reduces the SNR of the interconnect.

These signal integrity issues can be avoided if vias can be avoided during impedance-controlled routing. When you use 3D printed interconnects in a PCB, you can design a unique interconnect geometry that does not require vias for layer transitions. Here are two example interconnect structures that do not require interconnect layer transitions.

Example: Coaxial Interconnect

A good example is the coaxial structure. This construction is naturally shielded and thus isolates internal signal lines from external sources of EMI. This architecture does not require typical via patterns, thereby eliminating potential insertion loss during layer transitions.

This type of interconnect architecture provides a unique level of physical layer security that can often be provided by stripline routing in planar PCBs. However, the layer-by-layer printing process allows these structures to be deposited without the constraints of standard etching and pressing steps in planar PCB manufacturing processes.

Example: Integrated Circuit-Style Interconnect Architecture

Another example is the use of integrated circuit-style interconnect structures and the use of VeCS (vertical conductive structures) for layer switching, as these structures have much lower parasitic inductances.

Other benefits of 3D printed PCBs

The use of 3D printing offers other manufacturing advantages beyond impedance-controlled wiring. Because the manufacturing time involved in 3D printing does not depend on the complexity of the device, the printing time is highly predictable, and a fully functional circuit board can be printed within hours. The same applies to printing costs, which are independent of motherboard complexity. This makes it easier to scale the manufacturing of high-mix, low-volume complex PCBs for any application.

As more materials are available for different printing systems and processes, designers will have greater freedom to adapt their products to highly specialized applications. Inkjet 3D printing systems can currently co-deposit conductive traces and substrates from nanoparticle inks, and standard components can already be embedded in 3D printed PCBs. A wider range of insulating and semiconducting polymers will enable a wider range of devices to be 3D printed directly on highly complex PCBs.