Heat cannot simply be ignored; it must go somewhere. If there is space for a heat sink or fan, thermal management can often still be addressed using conventional methods. In many modern electronic assemblies, however, that space is simply not available due to weight, cost or space constraints. In these cases, the PCB itself becomes a key element of the thermal management concept.
This is precisely where this article begins. We deliberately go one level deeper than a conventional introductory article and examine which technology is actually best suited to which thermal challenge, where the limits lie and what needs to be considered during implementation.
When does the PCB itself become the cooling solution?
As long as external cooling components can be used, the PCB usually plays a secondary role in thermal management. Once space, weight or cost constraints rule out this option, however, the PCB itself must take on the thermal function. This is typically the case when external cooling components such as heat sinks or fans cannot be used at all due to space, weight or cost constraints. In such cases, designers should assess at an early stage whether the PCB requires a fundamentally different thermal design rather than trying to compensate for heat dissipation later by adding external components.
The right technology for each challenge
In practice, there is no single solution for every thermal challenge. Instead, different technologies are particularly well suited to specific problems.
Thermal Vias
Challenge:
Localised hotspots
Benefits:
Conduct heat through the PCB away from the hotspot, into inner layers and/or to the reverse side
Cost effective
Heavy copper PCB (105 to 400 µm)
Challenge:
High currents
Benefits:
The increased copper cross section rapidly spreads heat across the board area
Combines high current carrying capacity with very effective heat spreading
IMS PCB
Challenge:
Extremely high heat on one side
Benefits:
The metal base acts as an integrated heat sink
Copper inlays
Challenge:
Localised hotspots where Thermal Vias do not provide sufficient heat dissipation and heavy copper is not technologically applicable
Benefits:
The inlays conduct heat away directly beneath the component
Which of these technologies is the right choice in a specific application depends on many factors, including the design, the specific requirements and the cost. Copper Coins are the most cost intensive option, while Thermal Vias are the most economical. The real engineering expertise therefore lies not in offering as many technologies as possible, but in defining the right architecture for each thermal challenge and combining technologies intelligently where a single solution is no longer sufficient. This is exactly why PCB engineering should be involved early in the project so that all relevant aspects can be considered together with the designer.
How to conduct heat away from the hotspot as directly as possible
When a high-performance IC, FPGA, RF power amplifier, LED module or power semiconductor generates significant heat locally, the objective is to make the thermal path from the component through the PCB and any metal structure to the heat sink or enclosure as short and efficient as possible. The key is to minimise the overall thermal resistance. Every interface and every material layer act as a physical bottleneck. For this reason, the focus is not on a single material in isolation, but on the interaction between the component, PCB, copper structures, material system and enclosure as one integrated thermomechanical system.
In practical terms, this means keeping the solder mask clear in the area of thermal pads, placing Thermal Vias as laser drilled microvias directly in the solder pad and filling them with copper, using so called via in pad technology. Through hole vias in pad are plugged and overplated. For IMS PCBs, a dielectric layer that is as thin as possible, for example around 50 µm, and has high thermal conductivity is also recommended.
Thermal Vias or Copper Coin: how significant is the difference?
A Copper Coin reduces thermal resistance by approximately a factor of ten compared with Thermal Vias. The reason lies in the thermally conductive cross section. With Thermal Vias, the copper plating in the hole is typically only 20 to 25 µm thick, while the via core remains either empty, and therefore filled with air, or is filled with epoxy resin. As a result, the thermally conductive cross section is extremely small. Copper filled laser microvias perform better than conventional Thermal Vias, but still fall far short of the performance of a solid Copper Coin. Designers dealing with very high local power dissipation should therefore take this difference into account when selecting the technology, even though a Copper Coin is the more cost intensive solution.
Why effective thermal management directly influences the service life of electronics
The relationship between temperature and reliability is more pronounced than many assume. Reducing the operating temperature by just 10 degrees Celsius doubles the service life of semiconductors and capacitors. More than 50 percent of electronic failures are thermally induced.
An important factor is the difference in coefficients of thermal expansion between the PCB material and the component materials. When heated, these materials expand at different rates, which can generate considerable shear stress at the solder joints. Appropriate thermal management prevents local hotspots and thereby reduces thermal stress across the entire assembly, from the solder joint to the via.
The trade-off between miniaturisation and heat dissipation
Electronic assemblies are becoming smaller while computing performance, current density and component density continue to increase. This often makes the thermal situation more challenging. This is one area where HDI structures with very fine geometries and filled blind vias can be combined effectively with other technologies.
One possible solution is the use of T-Coins. The smaller stem, measuring approximately 2.5 by 2.5 millimetres, is positioned directly beneath the component and conducts the heat into the larger section of the coin. Rigid Flex PCBs also offer a design advantage in this context. Heat generating electronics can be placed on the rigid section and thermally coupled to a metal enclosure, while the flexible section routes the signal traces to the next level with minimal space requirements. By contrast, embedding technology has only limited applicability at Varioprint.
When thermal management meets high frequency design
The challenge becomes particularly demanding when optimum heat dissipation must be achieved together with controlled impedances, low losses and high signal integrity, for example with PTFE or other low loss materials. RF materials generally have poor thermal properties, which is where hybrid stack ups come into play.
The critical RF signals are routed exclusively on the outermost layers, for example layers 1 and 2, using a dedicated RF substrate such as the Rogers RO4000 series. The inner layers and the reverse side, by contrast, use standard high Tg FR4 or heavy copper. The surface finish also requires careful selection. ENIG should be avoided because nickel is ferromagnetic and has high electrical resistance, which can significantly attenuate RF signals due to effects associated with skin effect. Finishes such as ENEPIG, DIG or EPIG are more suitable.
The most common mistakes in thermal PCB design
The same issues repeatedly become apparent in customer designs during DFM review. One key point is impedance. Have the trace width and spacing actually been adapted to the specific RF substrate used in the hybrid stack up? The clearance between Thermal Vias and differential high-speed pairs is equally important and must be sufficiently large. The return current path must also be consistently taken into account during design, as must the selection of the appropriate final surface finish, particularly for RF applications.
How early the PCB manufacturer should be involved in the project
For demanding applications, the PCB manufacturer should ideally be involved before the layout is frozen. Many of the key thermal decisions are already made during component placement, stack up definition, material selection, via architecture, copper distribution, Copper Coin positioning, PCB thickness definition and mechanical coupling to the enclosure. Once these decisions have been fixed, correcting them later usually requires considerable effort.
At this early stage, Varioprint contributes its expertise in layout and stack up optimisation and supports the assessment of PCB thermal management. Thermal simulation itself, however, is generally not part of the service portfolio. Particularly in aerospace, defence and high-end electronics, the PCB manufacturer should therefore not be brought in merely as a fabricator but should be involved in the development process early on as an engineering partner.
Where the technology is heading
The future of PCB thermal management does not belong to a single technology. It lies in the intelligent combination of HDI, copper filled microvias, heavy copper, solid local copper structures, metal bases and consistent thermal coupling to the enclosure. As processors become more powerful and RF power, GaN and SiC technologies, higher currents and increasingly compact systems advance, electrical, thermal and mechanical design are becoming ever more closely integrated.
At the same time, another factor is becoming increasingly important, particularly in Europe and the USA: a resilient and controllable supply chain. In the aerospace, defence and space sectors, as well as in other safety critical electronics, unit price is therefore not the only consideration. Technical expertise, IP protection, traceability, reproducible processes, long term availability and short distances between development and production are decisive. High end manufacturing in Switzerland can offer a strategic advantage here. Engineering and production remain geographically and technologically close, processes can be tightly controlled, and critical manufacturing know how remains in the immediate environment of product development.
Conclusion: Mastering technology rather than simply offering it
In high reliability electronics, the decisive question is ultimately not only whether a particular PCB can be manufactured. The question is whether the technology is mastered in sufficient depth to ensure that it can still be manufactured reproducibly and reliably tomorrow. This is precisely the difference between a PCB that somehow manages the thermal load and one that remains reliable under continuous operating conditions.
