FR4 Thermal Properties to Consider During DesignBy ZM Peterson • Oct 13, 2020
All your electronics need to stay cool to prevent component failure and even via/conductor failure during operation. Many boards that don’t run at high temperature or in excessively harsh environments will most likely use FR4 as the PCB substrate. This inexpensive rigid substrate material (or rather, one of its varieties) is the basis of most PCBs deployed in a range of environments.
With that being the case, FR4 thermal properties should be considered when designing a PCB stackup to be deployed in certain environments. The construction of FR4 boards will determine a number of performance aspects, and thermal behavior is only one of the aspects to consider when designing a new PCB. In some cases, you may find other PCB substrate materials that are a better option given the various PCB material properties provided by FR4. If you’re planning to use a generic FR4-grade substrate material, keep reading to see which FR4 thermal properties are most important for different designs.
Important FR4 Thermal Properties
An important point to remember when considering important FR4 thermal properties is this: FR4 is just a NEMA grade of material, it is not a specific substrate material. The electrical, mechanical, and thermal properties of FR4 grade materials depend on the resin content, glass weave style, glass weave thickness, and type of materials used to build up an FR4 substrate. In short, there are many grades of FR4 materials from different manufacturers with slightly different thermal and electrical properties. Some companies (e.g., Isola) produce FR4-grade materials with various electrical losses, thermal properties, standard thicknesses, and glass weave styles.
With that being said, you’ll never find a perfect compromise between all thermal, electrical, and mechanical properties. However, you can often get quite close to satisfying all your design requirements by focusing on the few important materials properties that dominate performance. The table below lists a short summary of important FR4 thermal properties for laminates with Dk from ~4.4 to ~4.8.
Typical FR4 thermal properties.
If your PCB is to be placed in a high power system or a high temperature environment, you’ll need to pay attention to these important FR4 thermal properties.
Thermal Conductivity and Specific Heat
The thermal conductivity of FR4 defines the rate at which heat will move from hot to cold regions in the system; the specific heat defines how much heat is required to raise the temperature of a material by 1 degree (Celsius/Kelvin or Fahrenheit). Together, these values determine the heat constant in a PCB (as well as the values for component packaging and conductors), which is a fundamental input into thermal simulations with 3D field solver packages. These values will then determine how long the PCB takes to come to thermal equilibrium and the specific equilibrium temperature distribution.
Coefficient of Thermal Expansion
The coefficient of thermal expansion (CTE) measures the rate at which the board will expand along different directions. When materials are in contact but they expand at different rates, stress gets induced on one of the materials, which can lead to fatigue after repeated cycling and eventually fracture. This is a primary source of failure in many high power boards, such as LED boards, simply due to an inability to quickly move heat away from hot components (i.e., low thermal conductivity).
Glass Transition Temperature
The glass transition temperature (Tg) of a thermosetting polymer is related to its CTE value. Once the temperature of the material exceeds Tg, the CTE value abruptly increases. For this reason, a PCB should only be placed into operation in an environment where the operating temperature is less than Tg. Note that Tg is also a critical parameter related to mechanical quantities because, when the temperature is above Tg, the material will become rubbery and will be more pliable (i.e., increased moduli).
Which Thermal Properties Matter?
One point to note in the above discussion and table: due to the glass weave style used in PCB laminates (including FR4 grade laminates), all FR4 thermal properties except specific heat capacity are anisotropic. This means heat transfer and thermal expansion are different along different axes in an FR4 substrate. Because the glass weave in the substrate runs parallel to the surface, and because the weave is cross-hatched, the value of these thermal properties along the x and y directions are practically the same, but they are different along the z direction.
In terms of which thermal properties are most important, it really depends on the application. Here are a few simple guidelines:
- CTE. If you’re deploying a board that will be thermally cycled between two extreme temperatures very quickly, you really need to minimize the difference in CTE values between the conductors and FR4 to prevent fracture in vias. Fracture at the neck (microvias) and in the via barrel (high aspect ratio through-holes) are most common, followed by fracture at the top of solder balls.
- Thermal conductivity. If you know you’ll be generating high heat from a group of components and you need to move heat away from them as fast as possible, choose the highest possible thermal conductivity. For specialty auto or aerospace applications, ceramics or metal-core substrates are sometimes used for this reason.
- Tg. You should always try to opt for the highest glass transition temperature possible unless your board will operate at low temperatures.
The experienced PCB design and layout team at NWES understands the importance of FR4 thermal properties and can help you choose the best substrate material for your PCB. We help our private clients, defense & aerospace companies, and the US military stay at the cutting edge with advanced PCB design and layout services. We've also partnered directly with EDA companies and advanced PCB manufacturers, and we'll make sure your next design is fully manufacturable at scale. Contact NWES for a consultation.