Routing Challenges of High-Temperature Designs

Key Takeaways

• Looking at what industries high-temperature circuit boards are being used in, and what some of the different thermal challenges are.

• Some design methodologies that will help with high-temperature boards.

• How your PCB design tools can help with your high-temperature PCB design.

How can you utilize setting up design rules to ensure your routing goes as smoothly as possible?

More and more we are seeing circuit boards that operate hot and/or operate in hot environments

Anyone who has walked barefoot down a sidewalk on a hot summer day, is aware of the importance of finding the coolest spots of the path to step on. You will avoid the obvious hot spots, try to land on some grass, and if someone is watering their yard, head for the cool puddles of water that are collecting. When designing a printed circuit board that will operate at high temperatures, we need to approach it with the same mentality – finding the coolest way to place and route the board.

As modern electronics require faster circuitry and more power, they are also being used in increasingly harsher environments. The combination of these scenarios will push the operating tolerances of the board materials, and it is in the best interest of the designer to find new ways to beat the heat. Here are some routing challenges of high-temperature designs, and some ideas on how to create a PCB that can handle these higher temperatures.

Understanding High-Temperature Circuit Boards

For years most electronics have operated at standard temperatures that have not required special thermal consideration. That has been changing though with devices that have higher performance and current specifications. Even products that have not been considered as high-temperature electronics before, such as computers, are now beginning to cross the high-temperature threshold. In addition, there are also industries where the operating environment for electronics is higher than the rest. Some of these include:

• Gas and oil: The needs for high-temperature electronics in this industry are three-fold. First, there is the drilling part of the operation where the drilling equipment is monitored and directed by sensors and geosteering electronics. Second, electronic instruments are used to measure properties such as radioactivity and magnetic resonance for geologists to assess the condition of the drilled hole. Third, during production additional instruments are used inside the hole to monitor pressures, temperatures, and vibrations in order to control the pumping of the materials. These are critical path electronics, and a failure can be extremely costly in both time and money.

• Avionics: As aircraft development continues to evolve, so does the need for the next generation of electronics. One common enhancement is that the heavy wire harnesses and connectors that used to be used in planes are now being replaced with direct control electronics. While this reduces the weight of the plane considerably, it also puts the electronics closer to the engines and other sources of heat.

• Automotive: The car industry is also using more and more electro-mechanical and mechatronic systems in place of the older mechanical and hydraulic systems.  As with aircraft, this puts the electronics closer to the sources of heat then they used to be.

Circuit boards that run at higher operating temperatures or in hotter environments, or both, need special consideration when it comes to their manufacturing. This can include using different solder pastes that won’t melt at the higher temperatures and different board materials.

Circuit board operating temperatures are defined by their Tg rating (glass transition temperature). FR-4, the standard material that is used in most circuit boards, is Tg rated to operate up to 130 to 140 degrees centigrade before the glass begins to change from a solid to a liquid state. As circuit board operating and environmental temperatures exceed those temperatures, materials other than FR-4 such as Shengyi S1000-2, ARLON 85N, and ITEQ IT-180A need to be considered instead. These materials will offer resistance to the higher temperatures as well as to thermal stress and shock.

In addition to manufacturing processes and materials though, engineers need to also design for higher temperatures. Let’s take a look at what that means.

Component placement, power traces, and power planes are all important in PCB thermal management

Basic Design Principles to Meet the Routing Challenges of High-Temperature Designs

The first thing PCB designers need to consider with circuit boards that operate at and in higher temperatures is the type of components that they will be using. Many standard components are not designed to function in extreme heat, and will not perform up to specification or simply fail at those temperatures. Designers need to include temperature ratings when they search for components to use in these designs to prevent board failures.

The next step is to consider how the components are placed on the board. It is important to keep as much distance between parts that run hot as possible to prevent the creation of high heat zones on the board. Power supply components such as high powered resistors and voltage regulators create a lot of heat, and when multiple parts are bunched up together it will generate an extra area of heat. While the individual parts of a power supply have to remain close to each other, different power supplies should be spread out as much as possible to create a thermal balance on the board. Here is where using a PCB design system like Cadence Allegro can really help by setting up specific keepout zones and other design constraints to govern the placement of your components.

There are other areas of our design to consider as well. Using vias to conduct heat into the power and ground planes of the board is one while modifying the system enclosure to include thermal insulation is another. Adding heat sinks into the design to help with components that run hot is also important, and cooling fans is another way to help dissipate the heat. Finally, there are routing considerations that can help as well.

Trace Routing on a High-Temperature Printed Circuit Board

A lot of the trace routing that you will do on a high-temperature PCB will be similar to what you are already doing. You need to prepare your design for routing, set up the layer stackup requirements, add the design rules, and route the board as you can see in this link. The high-speed design rules will still need to be followed, which will require setting up design rules and constraints. 

What will need more attention is how you route power traces. You will want to make sure to keep high current paths as short as possible and keep them from other sensitive circuitry. High power circuits require more copper to conduct this power as well as to dissipate the heat. When it comes to power and heat, wider is better. Traces that are too narrow may result in performance that is degraded.

Another area to look at for PCB routing on high-temperature designs is the construction of the power and ground planes.  Large copper planes in a circuit board with vias help to remove heat from hot devices. When these planes are connected to the external layers of the board, the heat has an even greater chance of being pulled out of the board and into the surrounding environment. 

The key though is to remember that your planes also need to serve the signal and power integrity needs of the board by providing clear return paths as well as isolated grounds for noisy power supplies as depicted in this white paper. Here is where the skill of the designer is needed the most to design the power planes to accomplish all of these goals while also dealing with the heat. Fortunately, there are features in your PCB design tools that can help.

Tools like Cadence’s IR Drop Vision will help in the creation of power and ground planes

How Your Design Tools Can Help

The first step that you can take to help design a circuit board for high-temperature operations, is to use your design rules and constraints to their fullest capacity. Make sure that your power and ground traces are specified at the widths you need to conduct heat as well as power. Another great tool that you can use is an online component browser such as the Unified Component Search in Cadence’s schematic systems. With this tool, you can define different component parameters in your search filters to find the parts that will best fit the thermal needs of the board.

You should also work with simulation and analysis tools to help you with the creation of your design. The IR Drop Vision analysis tool in Allegro will help as you craft your power and ground planes in order to make sure that you get the power coverage in your board design that you need.  In addition, there are plenty of other features and tools available to you to help you manage the thermal aspect of your design before you commit to manufacturing.

You need the best PCB design system to answer the call of today’s electronics that operate at higher temperatures and in hotter environments. Allegro PCB Designer is the design system that you have been looking for that will give you the tools you need to get the job done right.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.