Effective thermal management is a crucial aspect of multilayer printed circuit board fabrication. As circuit density and power requirements continue to increase, so does the potential for heat buildup within a board’s layers. Without proper control, excessive temperatures can cause delamination, warping, component failure, and signal instability.
To prevent this, PCB engineers must carefully select materials with appropriate thermal ratings, design layouts that promote even heat dissipation, and verify performance against recognized industry standards. Each of these considerations plays a role in maintaining the mechanical integrity, reliability, and electrical performance of the final assembly.
Material Selection
Material selection plays a critical role in manufacturing PCBs and a greater role in thermal management. As most may know by now, most PCB base material or core is made of a mix of fiberglass and resin. The mix of materials provides a rigid board on which copper can then be applied, and circuits can be designed into it.
With this said, how can we measure the board’s ability to withstand the heat generated by the current running through the copper? There are several ways to measure, but for this blog post, we will be using glass transition temperature (Tg). If you are familiar with circuit boards, you’ve already heard of this, but why is this important?
Glass transition temperature rating can be found in the form of this example: FR-4 170tg. This is one of the higher-rated core PCB materials, as well as popular for its properties.
Let’s break FR-4 170tg into sections to better demonstrate:
- FR– Flame Retardant: This establishes that the material meets UL94V-0 requirements, one important feature of this requirement is that the material is self-extinguishing.
- 4 – Distinguishes material from other core materials. FR-4 is constructed using woven fiberglass cloth with epoxy resin as a binder. This helps with heat resistance as well as adding rigidity to the PCB.
- 170tg – This is the Glass Transition rating (170 degrees Celsius). At 170 degrees Celsius, the transition makes the board softer and prone to warping, delamination, and loss of electrical insulation ability.
To add a reference, let’s see how they are compared to other FR levels:
- Type Base Material Typical Tg (°C)
- FR-1 (Paper + Phenolic Resin ~105 °C
- FR-2 (Paper + Phenolic Resin ~105-130 °C
- FR-3 (Paper + Epoxy Resin ~130 °C
- FR-4 (Woven Glass Fiber + Epoxy Resin 130-180 °C (varies by formulation: standard, mid-Tg, high-Tg)
- FR-5 (Glass + Epoxy Resin ≥ 170-200 °C
- FR-6 (Glass + Polyester Resin ~130 °C
There are also higher ratings for PCB material that come with higher price tags, such as Rogers material, which uses hydrocarbon ceramic and Teflon; the values for Tg can reach up to 280 degrees Celsius. This is critical for higher voltage boards that generate more heat due to their having stronger electronic fields that can cause dielectric insulation loss; this lost energy turns into heat inside the dielectric.
Heat Spread Dissipation
Heat is a known factor in how a PCB functions in an assembled product, and a fantastic way to manage heat with multilayer circuit boards is to dissipate the heat away from the source. This can be accomplished thanks to thermal vias, thicker copper weights, and thermal design layout.
Thermal Vias
These are great ways to increase the surface area that heat must travel. By drilling several plated vias (Plated drill holes < 0.021”) you are adding more copper; the distance between layers provides, when all combined, more surface area. This takes the heat more space to travel away from the heat source, which is critical for PCBs under load and with the assembling of components allows for the board itself to remain at a stable temperature for soldering.
Printed circuit board with thermal vias to draw heat away from component.
Copper Weight
Thicker copper weight (OZ of copper used with Fiberglass resin to make a PCB) has better electrical resistance. The higher the copper weight, the less electrical resistance; the more electrical resistance means there is more heat generated.
Thermal inertia plays a crucial part in copper weight. The higher heat capacity and thermal mass of an object (copper) allow for more heat to be absorbed. Heating slower as there is more material to heat up. Like having a thicker towel when cleaning up a liquid, copper acts the same with heat.
Thermal Design Layout
The way that copper is laid out on a sheet of resin-impregnated fiberglass can also account for the way heat travels through a PCB. A printed circuit board designer must apply this practice to avoid decomposition of fiberglass, loss of signal, and poor operation of the assembled multilayer PCB. This is why copper planes must be used.
Copper Planes
A ground plane is a perfect way to disperse heat. It is an inner layer of copper that can be used to provide constant power to the component and to aid in the transfer of heat from outer layers. Essentially acting as a heat sink, with the help of vias as micro vias as a path to travel from layers, ground planes also provide plenty of surface area for thermal inertia and thermal equilibrium to take place.
Thermal Relief
Thermal Relief Pads are used on copper layers to control the heat flow during soldering. Ground planes might work a bit too well at heat dissipation during soldering and oven reflow. During soldering, the copper must reach a certain temperature to properly bond with solder, thermal pads allow copper to reach an ideal temperature. The copper is designed to allow heat to build up on the thermal pad by adding 0.003-4” spokes to a pad, making the thermal pad.
Standards, Testing, and Qualifications
A way to make sure that a PCB can withstand heat and effectively manage is by making sure the materials meet the requirements necessary to operate at higher-than-expected temperatures.
UL Testing
Underwriters Laboratories conducts and evaluates products. They are an Independent Global Safety organization that specializes in ensuring products meet specific safety, performance, and flammability ratings.
Here is a list of some of the UL ratings available:
- UL 94: Flammability of plastic materials. Classifies materials by their ability to extinguish a flame after ignition. Common ratings: V-0, V-1, V-2, HB, and 5VA / 5VB.
- UL 94V-0: High flame-retardant rating. Material stops burning within 10 seconds on a vertical specimen with no flaming drips. This is the minimum standard for most PCBs (like FR-4).
- UL 796 Printed Wiring Boards: Safety standard covering construction, performance, and thermal endurance of PCB laminates and coatings.
IPC Standards
These standards define performance, testing, and design requirements for PCB Materials.
Common IPC Standards:
- IPC-2221: Generic Standard on Printed Board Design. Provides thermal design rules, including trace width vs. current, spacing, and heat dissipation guidelines.
- IPC-4101: Specification for Base Materials for Rigid and Multilayer Printed Boards.
Defines thermal performance, Tg, CTE (Coefficient of Thermal Expansion), and decomposition temperature (Td) for laminates like FR-4 and FR-5. - IPC-6012: Qualification and Performance Specification for Rigid Printed Boards.
Includes tests for thermal stress, solder float, and delamination resistance.
Summary
Understanding how materials, copper distribution, and design layout contribute to thermal behavior is essential in modern multilayer PCB fabrication. Selecting the proper Tg-rated substrate ensures heat tolerance, while design features such as thermal vias, copper planes, and relief pads help maintain stable operation under load.
Adhering to UL and IPC standards provides further assurance that the PCB will meet required safety and performance benchmarks. Together, these factors create a robust foundation for high-reliability applications where temperature management is critical to long-term functionality.
Key Takeaways
- Thermal management begins with material selection: Choosing the right PCB base material, such as high-Tg FR-4 or advanced substrates like Rogers, is the foundation of heat control. A higher glass transition temperature (Tg) improves resistance to delamination, warping, and dielectric breakdown under high heat.
- Thermal vias and copper weight are key to heat dissipation: Plated vias create conductive pathways for heat to move away from critical components, while thicker copper layers reduce resistance and improve the board’s thermal mass, allowing for more stable operating temperatures.
- Copper planes and thermal relief pads improve performance: Ground and power planes act as built-in heat sinks, dispersing thermal energy across layers. Thermal relief pads help balance heat during soldering, preventing uneven heating or cold joints in high-density layouts.
- Testing and certification validate reliability: Compliance with UL 94V-0 for flame retardance and UL 796 for thermal endurance, along with IPC-2221, IPC-4101, and IPC-6012 standards, ensures that materials and designs meet proven industry requirements for temperature stability and long-term durability.
- Integrated thermal design extends product life: Managing heat through thoughtful material selection, smart copper distribution, and adherence to performance standards helps maintain signal integrity, mechanical strength, and reliability in demanding multilayer PCB applications.
