If you’re a designer of extremely high-speed PCBs or high-frequency RF devices, then you’ll be taking advantage of impedance controlled routing features in your PCB design software. These tools are designed to ensure the impedance of a transmission line is consistent along its length, thereby allowing termination at each end to prevent reflections. Consistent impedance also ensures consistent propagation delay along an interconnect, thereby allowing parallel high-speed PCB signals (such as in PCIe) to be accurately length matched to prevent skew.
As impedance controlled routing requires precise fabrication of PCB interconnects, manufacturers have spent significant effort perfecting etching processes to ensure trace geometries match the standard geometries used in PCB design software. Using 3D printing to fabricate PCBs allows designers to go beyond the standard trace geometries that are typically enforced in PCB design tools while still ensuring precise impedance control. This gives designers more options for impedance controlled routing and designing interconnects than standard planar manufacturing processes.
This mixer board for RF signals requires precise impedance control.
What Is Impedance Controlled Routing?
Of all the electronic devices that require precise design and fabrication specifications, certain high-speed and high-frequency PCBs can be sensitive to impedance variations. EDA tools now offer impedance controlled routing features, where the impedance of an interconnect is calculated with a number of possible methods. Tools like electromagnetic field solvers are being implemented in many standard routing tools, allowing designers to calculate precise impedance and propagation delay throughout an interconnect.
In this design methodology, the geometry of an interconnect and its location with respect to reference planes are designed such that the interconnect’s impedance takes a specific value. When designing a trace on a PCB, the distance between the trace and its reference planes is normally fixed by the thickness of the core or laminate layers. This constrains designers to specific trace width and thickness during impedance controlled routing on planar PCB substrates.
Because impedance control depends heavily on precision conductor geometry, the manufacturing process must be precisely controlled to ensure fabricated traces match design data. In planar PCB processes, these challenges around precise fabrication on planar substrates have largely been solved for a variety of interconnect architectures. However, this severely limits the freedom of designers to create impedance controlled interconnects with unique geometry.
In contrast, the layer-by-layer deposition process provided by inkjet 3D printing systems eliminates the traditional DFM constraints and allows designers to implement nearly any impedance controlled interconnect architecture.
Unique 3D-Printed Interconnects Without Standard Vias
Much research effort has gone into modeling the impedance of vias and designing vias with precise impedance in multilayer PCBs. Vias can present an impedance discontinuity that causes reflections along an interconnect. Vias are basically inductors, so they also create a source of inductive crosstalk along an interconnect. While vias are critical for routing between layers in a multilayer board, both characteristics of vias create the potential for signal integrity problems. In many densely routed PCBs or when using components with high pin/ball count, the use of vias is usually unavoidable.
For this reason, many PCB design guidelines recommend minimizing or eliminating the use of vias on high-speed and high-frequency interconnects. In mmWave PCBs or with very fast edge rates, certain standard via geometries, such as plated through-hole vias, can create some insertion loss along an interconnect. This reduces the signal level along an interconnect and causes a slight reflection back toward the source, which reduces the signal level seen at a receiver. In low-level digital components, this can cause the signal to decrease below the level required to latch to the ON state. Similarly, in analog components, this reduces SNR along an interconnect.
If you can eliminate the use of vias during impedance controlled routing, then you can avoid these signal integrity problems. When you use 3D-printed interconnects in your PCB, you can design a unique interconnect geometry that does not need vias for layer transitions. Here are two example interconnect structures that do not need standard vias for layer transitions.
Example: Coaxial Interconnects
One great example is a coaxial structure, as shown below. This structure is naturally shielded, thus it provides high isolation of the internal signal line against external sources of EMI. A typical via style is not needed in this architecture, which eliminates potential insertion loss during a layer transition.
This coaxial routing style provides precise impedance controlled routing and shielding without the use of vias.
This type of interconnect architecture provides a unique level of physical layer security that would normally be provided by stripline routing in a planar PCB. However, the layer-by-layer printing process allows these structures to be deposited without being constrained by the standard etching and pressing steps in planar PCB manufacturing processes.
Example: Integrated Circuit-Style Interconnect Architecture
Another example is the use of an integrated circuit-style interconnect structure with the use of VeCS (vertical conductive structures) for layer transitions as these structures have much lower parasitic inductance.
The interconnect architecture in a 3D printed PCB can resemble the architecture in an Integrated circuit, which also eliminates the use of standard vias.
Other Advantages of 3D Printed PCBs
The use of 3D printing provides other manufacturing advantages beyond impedance controlled routing. Because the fabrication time involved in 3D printing does not depend on the complexity of the device, the printing time is highly predictable, and a fully-functional board can be printed in a matter of hours. The same applies to printing costs, which are independent of the board’s complexity. This allows much easier scaling of manufacturing for high-mix, low-volume complex PCBs for any application.
As a more diverse array of materials becomes adapted for different printing systems and processes, designers will have even greater freedom to adapt their products to highly specialized applications. Inkjet 3D printing systems can currently co-deposit conductive tracks and the substrate from nanoparticle inks, and standard components can already be embedded in 3D printed PCBs. A broader array of insulating and semiconducting polymers will enable a broader array of devices to be 3D printed directly on highly complex PCBs.
You can implement impedance controlled routing for nearly any interconnect design you can imagine when you use the right inkjet additive manufacturing system. The DragonFly LDM system from Nano Dimension is ideal for the production of complex additively manufactured electronics (AMEs) in-house with unconventional interconnects on planar or nonplanar AMEs. Read a case study or contact us today to learn more about the DragonFly LDM system.