Overview of what goes into circuit routing with an IC
Discussion of tips to help you route trace from complex ICs
Various tools to help you route as smoothly as possible
Circuit routing has become relatively complex, requiring a variety of tools for success
A long time ago, routing used to be a much simpler beast to tackle. In the past, designers relied on basic automated routers or manually did the circuit routing themselves. Nowadays, however, components are more miniaturized and complex. Individual integrated circuits (ICs) have more inputs and outputs, with increasing levels of interconnects. As the component density of ICs increases, the task of routing becomes more challenging. This challenge extends out of an individual IC to designers working on circuit boards as well. The increased I/O count results in more pins that are usually closer together, complicating the task even further.
In other words, from the IC-level to the intra-IC level on a PCB, the task of circuit routing has changed. In this article, we’ll be taking a closer look at circuit routing in each of these levels, from within an IC to between ICs on a printed circuit board.
The Task of Circuit Routing
All routing (whether in ICs or PCBs) follows the same basic structure. In electronic design automation (EDA) software or PCB software, polygons represent individual components or devices. These polygons have associated nets, which describe other polygons that should be routed to them. In the process of routing, all components that share a net are connected to each other from their respective terminals. Throughout this process, specific design and timing rules must also be adhered to in order to ensure functionality.
Furthermore, in successful routing, the design must not have crosstalk issues or antennas, and it must meet specific metal density requirements. Juggling all these various concepts is what makes routing a challenge.
Routing Within an Integrated Circuit
Routing within an integrated circuit requires a variety of specific considerations unique to this topology
Developing integrated circuits (ICs) is a long and complex process. Here, we’ll be doing a surface level overview of what goes into the routing stage of an IC.
In developing integrated circuits, the place-and-route state usually involves a schematic, hardware description language (HDL) files, or pre-routed cells. After routing, the IC layout is converted to a “mask work” in GDSII or OASIS file formats, which is necessary for the next steps of the IC fabrication process.
In routing ICs, specific aspects such as the transistor power dissipation room, interconnect resistances, and interconnect current densities should be monitored. ICs contain extremely small components (as opposed to larger discrete components on PCBs) and therefore can suffer from these elements if left unmonitored.
Special care should be given to the layout of sublocks, as their location plays a role in operation speed, the effect of noisy components on sensitive components, connecting to outside of the IC, and heat generation.
Finally, attention should also be given to other phenomena such as electromigration (when material is transported by the movement of ions in a conductor) within the metal interconnects in addition to electrostatic discharge into the smaller components.
Routing to and From ICs on PCBs
As the need for faster computing power grows, ICs have gotten more and more complicated with increased pin counts. To accommodate the increased number of pins, vendors decrease the pin-pitch as the total pin count goes up, making routing IC chips on PCBs more of a challenge.
There are a number of different methods for routing BGAs. Basics include utilizing diagonally routed escape patterns and organizing signal paths. Place decoupling capacitors close to the pins to reduce inductance and consider using microvias. See our full guide to routing BGA packages for more.
Double-data rate (DDR) memory also requires specific routing for proper functionality. Matching lengths of traces and incorporating specific routing patterns and methodologies such as T-topology and fly-by routing can be effective. See more DDR routing tips here.
Medical IoT Devices
Medical IoT devices are ever-evolving technology - especially medical device ICs. For this reason, routing your board in such a way that your design contains compartmentalized sub circuits is important in minimizing revisions down the line. In regard to traces, keeping sensitive traces away from noise sources is an incredibly important aspect. In other words, route your traces in a way that maintains the best signal integrity. For more information on IoT, look into our post about IoT connectivity.
For routing flexible circuits, there are a couple challenges that you should be aware of. Specifically, be careful with routing around bend areas. Traces should cross the area perpendicular to the bend line. Place vias and pins away to prevent damaging the holes. Minimize the amount of layers around bending areas, as this adds additional stiffness; instead, stagger the traces. See our article on tips for flex PCB design for more information.
PCB Routing Tips for Signal Integrity and Connections
Ensuring signal integrity is vital to the functionality of your circuits
After creating traces that breakout from your IC pins, it’s important to maintain signal quality to the best of your ability. Multiple phenomena can degrade signal quality, including:
- Electromagnetic coupling (crosstalk) - When one trace that is located too close to another couples to it, degrading signal quality. The length of these signals plays a large role in the amount of crosstalk they may experience. This can happen on parallel traces on the same layer or on two different layers, which is known as broadside coupling.
- Switching noise (ground bounce) - If your board contains a lot of ICs that switch often, signals may not return all the way to ground when they switch low. Depending on the value of the “low” signal, components may read this faulty low signal as high, resulting in compromised circuit functionality.
- Electromagnetic interference - Can be radiated by many high frequency signals. This includes trace and via stubs if not properly routed. Furthermore, the return signal path should be located on an adjacent reference plane and have the shortest possible route back to the source to minimize EMI leakage.
- Impedance mismatches - Can occur for transmission lines or in differential pairs, and can impact integrity. To deal with this, ensure that you are using impedance controlled traces with specific widths, clearances, and configurations. Consider using your PCB editor’s trace tuning parameters as well.
Routing Features and Additional Techniques
When routing your PCB board, utilizing a variety of advanced techniques may help. Some of the following features may be key to smoothing your circuit routing challenges:
- Slide routing can help clean traces by grabbing segments and pulling them to any desired location. Vias and other design elements will move out of your way automatically.
- Fanout routing, also known as escape routing, is an automatic feature that creates traces from ICs with high pin counts and connects them to vias - perfect for BGAs!
- Bus routing lets you route traces in groups, and as the name suggests, is perfect for busses.
- Trace tuning can be an invaluable tool when working with design constraints. It sets the trace to the necessary length for a given impedance value.
- Cleanup routing simplifies and cleans up the routing on your board, getting rid of extraneous segments. It also can add teardrop features to vias, which can increase integrity in flex boards.
The Power of an Advanced PCB Editor
Nowadays, PCB designers have a multitude of tools to choose from that are helpful for circuit routing and maintaining best practices. With Cadence Allegro’s PCB constraint manager system, you’ll be able to set up design rules for nets, components, high-speed traces, and propagation delays. This is essential for routing complicated high-speed circuits that may have a multitude of different constraint systems, impedance controlled traces, and complex ICs.
Furthermore, Allegro offers a variety of different tools to help you with complex routing patterns, including all that we’ve mentioned in the section above. With the Sigrity Aurora tool, you’ll be able to do an in-depth analysis on signal integrity, power, and electromagnetic simulations all within your layout environment.
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