As a kid growing up on desktop computers, getting my first laptop was a watershed moment – taking my games and programs throughout the house instead of being tethered to the work desk was invigorating. While I knew I couldn’t run things as fast as on the family desktop, the relative freedom was a welcome tradeoff. What took me longer to realize was how much more uncomfortable using the computer was when I was resting the laptop somewhere without proper ventilation, as the laptop’s heat, combined with the desert summers, made it nearly unusable for most hours of the day.
Laptop technology has improved by leaps and bounds. However, the central cooling issues remain for electronics, while higher-density assemblies and smaller package form factors continue to underscore the need for a solid thermal design. Alternative thermally conductive substrate material – AKA metal core PCBs – may be necessary for high-power boards to balance design constraints and performance.
Metal Core PCB Advantages and Disadvantages
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Joule Heating – Or Why Heat Is Inescapable in Electronics
Like friction for mechanical systems, the storage and transfer of electromagnetic energy is nonconservative due to resistive losses. The vector for this energy transformation is heat – the classic example is the heat dissipated across a resistor as a product of the resistance and squared current. Since every component and element of the bare board fabrication contains some resistance, losses are inescapable and can quickly snowball, especially in high-current designs (this is the basis for high-voltage/low-current transmission in transmission tower cabling – the power conserves as the voltage increases to compensate for the reduced current, but loss is minimal).
Not only is loss itself inefficient, but a significant amount can also affect the ambient temperature of the surrounding circuit environment. Higher temperatures increase thermal resistance, which further compounds losses as more energy is required to perform the same work at a lower temperature. Furthermore, temperature is far and away the primary agent of board material aging, reducing the overall service life of the board through degradation that eventually renders the circuit parameters out of specification. Broadly, thermal solutions are available in two categories:
- Active cooling consumes energy to regulate heat flow (e.g., forced convention).
- Passive cooling consumes no energy to regulate heat flow (e.g., heat sink).
While active cooling may be part of the larger system assembly, active configurations are far too bulky (considering board area and enclosure space) to be a feasible solution. Passive solutions are preferred: while active cooling performance exceeds anything passive cooling can achieve, the passive elements are more amenable to miniaturization. These features improve thermal routing for the bare board or assembly, but a significant hurdle remains in that the board substrate material is a considerable thermal isolator.
How Metal Core PCBs Alleviate Excessive Ohmic Heating
By convention, the cores of PCBs contain substrate material – FR4 is a common choice, but the suitability will ultimately depend on the circuit needs – but this doesn’t have to be the case. Substrate materials are dielectric, making them poor conductors. While this is beneficial for isolating electrical layers, it decreases heat dissipation due to a high thermal impedance. In high-power circuits, the thermal impedance can become a significant liability to cause damage to the board and components due to heat buildup without adequate thermal routing. The solution is to insert a thermally conductive metal layer of appreciable thickness into the stackup, providing ample thermal conduction.
Thermally conductive metals are also electrically conductive, which complicates the material characteristics of the metal sheet: the adjacent dielectric must be robust enough to withstand the higher temperatures and prevent capacitive coupling that would be nonexistent in a standard stackup. Even normal material descriptors like the thickness, glass transition temperature (Tg), and decomposition temperature (Td) do not provide the minimum requisite explanation. The primary factors designers and manufacturers will weigh will be the insulation resistance (the substrate material is the most significant cost contributor) and thermal conductivity.
Metal core PCB stackups can closely resemble those of standard PCBs, but unique stackups available to metal core designs are also available:
- Single-layer MCPCB – A single-layer board comprises SMD component mounting and a conductive metal sheet on the reverse side.
- Two- or multi-layer MCPCB – Unlike traditional multilayer boards, only one side is available for component mounting. The conductive metal sheet remains on the reverse of the component-mounted side.
- Hybrid MCPCB – A completed PCB assembly with traditional substrate materials that combine a conductive metal sheet with a thermally appropriate buffer substrate.
- Internal MCPCB – The conductive metal sheet approximates a PCB core in a standard stackup design.
The exact parameters of the metal core PCB will vary according to the metal or metal alloy selected as the heat sink layer. Aluminum is exceedingly common for many reasons: it’s readily available, its low cost due to extreme terrestrial abundance, high recyclability/minimal environmental impact, and its comparable weight to FR4 substrate materials. Aluminum can be over 500x more thermally conductive than FR4 materials, but it is not the only metal option available; copper is twice as thermally conductive as aluminum, but several factors such as increased cost and weight may preclude it from all but the heaviest-duty power designs.
Your Contract Manufacturer Keeps High-Power Devices Running Cool
Metal core PCBs grant designers and manufacturers additional materials to realize board designs with extreme heat generation. Not only does the addition of a metal backing improve performance by successfully drawing away excess heat, but it also improves the long-term reliability of the device by ensuring that the thermal aging of board materials remains minimal. The impact of the metal sheet can significantly affect fabrication, however, and designers will want to stay on top of their DFM by communicating with their manufacturer. Here at VSE, we’re a team of engineers committed to building electronics for our customers, including device thermal optimization. We’ve realized life-saving and life-changing devices alongside our valued manufacturing partners for over forty years.
