Selecting Alternative Materials for Multilayer PCB Substrates

May 3, 2019 Zach Peterson

Sometimes, it pays to go against the grain. No matter what someone says, you shouldn’t be forced to settle for the most popular PCB substrate material on the market. If you need to build a device that is specialized for extreme environments, repeated thermal cycling, or high speed/RF devices, there are alternative materials for multilayer PCB substrates that may be a better choice.

 

You Don’t Have to Settle For FR4

While FR4 is by far the most substrate material for single and multilayer PCBs, it has its drawbacks. Like other electrical insulators, it is also a poor thermal conductor compared to other materials that are suitable for supporting printed circuits. Given the thermal demands in modern PCBs that operate at high speed and/or high frequency, and given the harsh environments in which these systems are being deployed, it might make sense to use a different material for your next PCB.

Using a board with higher thermal conductivity allows heat to easily spread throughout the board, allowing your board to operate at a more uniform temperature. In contrast, FR4 boards with high speed/high frequency devices, particularly with devices like high speed CPUs or FPGAs, will develop hot spots. If these boards are cycled between high and low temperature, traces and vias can delaminate or crack due to accumulated stress. The mechanical properties of some alternative materials can be tuned, helping prevent these stress problems under thermal cycling.

Alternative materials for multilayer PCB substrates provide other advantages besides thermal management. As an example, the manufacturing process for ceramic PCBs allows passive components to be embedded in the inner layers of a multilayer ceramic PCB. The mixture of materials required to create a ceramic board allows their mechanical properties to be tuned while maintaining a high ratio of thermal to electrical conductivity. The thermal expansion coefficient of ceramics for PCBs is closer to that of most conductors, which reduces mechanical stress during cycling.

Thermal management in FR4 boards can be complemented by including a metal core in the interior of the board. This nicely allows heat to be dissipated throughout the board, ensuring a more even temperature distribution. This is useful in a number of applications, including boards for LED lighting or boards with high speed ICs.

 

Composite Epoxy Material (CEM)

One popular alternative group of materials, especially in Asia, is composite epoxy materials (CEM), specifically CEM-3. This class of composite materials is made from woven glass fabric surfaces and a non-woven glass core combined with an epoxy synthetic resin. Some manufacturers advocate that CEM-3 should completely replace FR4 as it is cheaper to produce, provides the same level of flame retardance, and is usable with the same manufacturing processes as FR4.

The glass transition temperature of CEM-3 (approximately 125 °C) is similar to that of FR4 (approximately 135 °C). Other CEM-based materials, for example CEM-1 and CEM-2, have much lower glass transition temperatures and should not be used with multilayer boards. Most manufacturers will only recommend using CEM-3 for low layer counts, although it is being used to replace FR4 boards with a similar number of layer counts.

ntegrated circuit on alternative materials for multilayer PCB substrates

An integrated circuit and passive components hand-soldered onto CEM-3

 

High Frequency Laminates

With devices that will operate at high frequency, using materials specialized for high frequency throughout your board may be the best choice. This is especially the case if you will be routing high frequency signals throughout the interior of your board. The two classes of high frequency materials are PTFE (Teflon) and non-PTFE based materials.

High speed/high frequency laminates are often used in the outer layer of high speed/high frequency PCBs in order to reduce signal attenuation. PTFE-based laminates are normally placed on top of an inner core in high speed devices, naturally allowing it to be used with multilayer PCBs. Compared to FR4, Teflon is recommended for GHz and higher frequencies and data transfer rates due to its much lower dispersion and lower dielectric constant, leading to a faster signal propagation speed at these high speeds.

Teflon provides other advantages as well. It is a poor absorber of water, so it is useful in humid or wet environments. It can be used as a laminate on a number of materials, thus it can be used to form a low loss layer specifically for high speed/high frequency signals. However, it is more expensive than FR4 and is somewhat more challenging to work with from a manufacturing standpoint. It also has lower thermal conductivity than FR4, so thermal management in PTFE boards is important.

Teflon frying pan

You can use Teflon for more than cooking your eggs

There are a variety of other materials that can be used for high speed, high temperature, and HDI multilayer boards. No matter which alternative materials for multilayer PCB substrates you use in your next board, you need the right design software to analyze the electrical behavior of your designs.

Altium ® was created with the tools that allow you to build advanced PCBs for any application. The stackup materials compiles relevant electrical and mechanical specifications for a broad range of materials, and this tool interfaces directly with your design, simulation, and documentation features in a unified design environment.

Download a free trial of Altium today to learn more about the board design features and the design environment. You’ll also have access to the industry’s best design features in a single program. Talk to an Altium expert today to learn more.

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