Designing In Thermal Management


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In the case of mixed-signal, analog, and mostly analog boards, special attention must go to careful thermal analysis. The PCB designer performing the thermal analysis must best to deploy certain design steps and techniques to maximize heat dissipation through component and board levels.

By Zulki Khan, NexLogic Technologies Inc.

Thermal management determines and implements methods to transfer the heat PCB components generate away those devices to other sides of the board or eventually into the ambient. Historically, the well-proven heat sink has been the stalwart for board heat dissipation. Today, thanks to new technologies, it continues to dissipate even greater heat more efficiently.

However, with the complexity of today's boards, the PCB layout engineer must think well beyond implementing state-of-the-art heat sinks. For example, take a board with considerable analog circuitry. The PCB designer must first focus on special board requirements, conduct a thermal analysis, then design the board considerating factors that will increase heat dissipation.

Thermal Analysis

Let's take a board design with analog circuitry intended for a DC-to-DC conversion application. Included are a power amplifier, high-performance LEDs, and analog ICs that demand considerably more power. Analog signals carry heavy loads of current, as well as high voltages. When this current passes through physical conductors, it creates considerable heat. Increasing integration of analog functions on a system-on-a-chip (SoC), for example, continues to escalate the need for thermal dissipation.

Figure 1. PCB ground pour is a means of spreading heat to the atmosphere.

LEDs pose yet another heat dissipation challenge. LEDs in automobile, industrial, commercial, medical, and other applications continue to grow dramatically. LEDs by their very nature create considerable heat. Special thermal management must be applied to properly dissipate this heat. Here, the experienced PCB designer places special emphasis on a "thermally challenged board." This means the board has a number of areas where heat is being generated and needs to be dissipated.

The PCB designer focuses on the component and board levels to dissipate heat. For the past several years, component manufacturers have significantly contributed in this regard by producing thermally friendly packages. They're taking a page from PCB fabrication structures to build up packaging laminates to increase thermal performance superiority.

For example, a BGA may have 2, 4, or 6 laminate layers. These multi-layer laminates contain internal ground planes connected to thermal vias under the silicon die. In effect, component manufacturers are helping to dissipate the heat before it goes further into a board.

A TO72 package is another good example of how packaging can increase heat dissipation. An alloy used as a heat sink dissipates the heat directly off the component body. However, in most cases, even with these packaging advances, thermal management continues to be challenging to the contract manufacturer (CM) or EMS provider, especially when it comes to analog components.

Beyond component heat dissipation techniques, the traditional heat sink attached to a thermal-intensive component is the best-known method for dissipating heat. Heat sinks are continually improving with different materials and alloys.

When the PCB designer plans component placement during the layout phase, he needs to review the amount of power that is being generated by these components and then perform the critical thermal analysis and develop a strategy for implementing it.

Thermal analysis involves defining areas of the board where extreme heat is being created. A straightforward rule is not to physically localize heat-generating components in one PCB location, but to spread them out evenly on the board, if possible. An experienced CM or OEM PCB layout engineer ensures that digital and analog components are properly separated.

The designer takes one further step to ensure all analog components are not placed right next to each other. Otherwise, the amount of heat those clustered analog circuits generate will create individual problems, and have a ripple effect throughout a system, especially when used in rugged environments. Therefore, heat being generated in an analog segment must be dissipated to minimize or at least effectively spread it out.

Evaluating and selecting the right heat sinks is a major part of this analysis. In some cases, special heat dissipation considerations are factored into thermal analysis. For example, the PCB designer may not have the luxury of using tall heat sinks if it's a small board going into a handheld product. In those cases, designers must deviate slightly from this heat dissipation method and search for other techniques to cool the PCB.

Figure 2. Metalcore PCB with an aluminum base.

Another important factor to consider is what application the PCB targets. Is it purely analog? Is it only digital? Or is it mixed signal? If it's purely digital, there aren't too many components generating inordinate heat, so thermal analysis can be fairly straightforward. The challenge comes when it's a mixed signal, completely analog, or mostly analog PCB. At this point, the PCB designer performing the thermal analysis considers how best to deploy certain design steps and techniques to maximize heat dissipation through component and board levels.

Designing In Thermal Management

While there is a number of considerations an experienced PCB designer applies for thermal management, the essential design steps for effectively dissipating heat are:

  • Properly distributing analog circuitry throughout the board.
  • Effectively using ground pour on the circuit board.
  • Strategically deploying thieving, when it is possible.
  • Considering a metal core (MC) board, if applicable to the situation.
  • Creating more solid planes for transfer of heat.
  • Selecting the proper heat sinks and attachment process.
  • Using thermally conductive grease, when applicable.

Here, the experienced PCB layout engineer is extremely valuable for designing circuitry in such a way that it can effectively dissipate generated heat. This is especially true for PCBs loaded with analog. First off, the designer distributes analog circuits on the board so there is no thermal concentration in a particular area.

If the circuitry allows it, the next step is to perform so-called ground pour. This involves pouring copper on the PCBs unused surface area. Basically, the PCB designer is spreading copper over the surface area; copper will conduct the heat to dissipate. So, instead of using one small surface mount (SM) pad for heat dissipation, the PCB designer is now increasing the surface of the board, for example, to a half-inch by half-inch pour surface area. This ground pour area would then be able to dissipate the heat a lot more quickly compared to the SM pad alone (Figure 1).

An astute PCB layout engineer will also carefully consider thieving and metal-core (MC) boards as other ways to design in thermal management. Thieving is a process used to distribute copper on a board to improve the etching process. It also increases thermal dissipation because now there is more copper area. So, now the expanded copper area can dissipate the heat from the main section or sources into the air using those extra, non-functional copper pads.

As for the MC PCB, a base metal material is used as a heat distributor and is an integral part of the circuit board (Figure 2). A single-layer MC PCB provides a highly thermally conductive base material for spreading heat.

The PCB layout engineer also creates solid planes, when possible, as a way to reduce thermal challenges and increase heat dissipation. A board with a number of planes allows greater heat dissipation through those planes, although they are sometimes internal planes and not in direct contact with the ambient temperature. Nonetheless, solid planes offer increased surface area to dissipate heat.

Figure 3. Heat sink with short fins dissipates heat to the ambient.

Last, but not least, is the important task of selecting the right heatsink and attachment process in designing for thermal management. A heat sink's purpose is to conduct heat away from the thermal-generating devices to other parts of the silicon tree to the ambient.

The more efficient heatsinks provide greater capacity to spread the heat out, and higher thermal conductivity means higher rate transfer. Therefore, if a heat sink with lower thermal conductivity is selected, it won't dissipate as much heat compared to one with a conductor demonstrating a higher thermal conductivity.

Heat sinks are made of extremely high thermally conductive materials like aluminum and copper. When a PCB designer opts for a heat sink with fins, it helps to increase heat dissipation into the ambient atmosphere (Figure 3). Also, an important aspect to factor in a design is to over-specify a heat sink by at least 15 to 20 %. This buffer prevents problems that can occur due to exposure to extremely hot conditions, mil/aero applications, rugged environments, or because of thermal mismanagement at the design stage.

Figure 4. Thermal grease is used as a way to dissipate heat, mostly in LED applications.

As part of the design process, the PCB designer places special emphasis on the interface between heat sink and associated component or to the board itself because it is critical for effective thermal transfer. Normally, thermally conductive aluminum filled epoxy is used for bonding the fins of the heat sink to the component or to the board. It's important that the right kind of alloy or substrate is used for attaching those heat sinks.

That requires considerable calculations and the right amount of aluminum or copper content within the epoxy and alloy substrate. The substrate material connects the heat sink to its associated component. For instance, as shown in Figure 4, a thermally-conductive grease is used to attach an IC with its heat sink. SMT

Zulki Khan, founder and president, NexLogic Technologies, Inc., 2085 Zanker Road, San Jose, CA 95131, may be contacted at zk@nexlogic.com, www.nexlogic.com, (408)436-8150 ext 102.

Thermal Management and Miniaturization: Collision Imminent

Dr. John Parry, Mentor Graphics

There is a strong case to be made for considering thermal management from the outset of almost every electronic product design project. Why? Because most everything electronic is getting smaller, and thermal management and miniaturization are on a collision course.

Historically thermal management has been the realm of the mechanical engineer, deploying thermal management solutions after the electronic design has been completed. At the enclosure level, cooling fans can be included in the chassis. At the PCB level, small clip-on heat sinks can be attached selected packages. The afterthought approach has been generally satisfactory until recently.

Power density is increasing as miniaturization reduces the size of silicon chips and their packages, and PCBs and their enclosures. The traditional thermal solutions hamper potential miniaturization. Heat sinks take up space (especially when keep-out regions are considered), fans too use space and consume power from the power supply, and both add weight and cost to the product.

Considering thermal design from the outset can save a lot of time and cost later on, and ultimately lead to quieter, cheaper, and more reliable products. If fan cooling can be avoided, then operation will be silent and the product more reliable, as fans are prone to failure. Clip-on heatsinks may offer a convenient afterthought solution for components that are just a bit too hot, but become an Achilles' heel for reliability as their size increases; they can actually unclip under shock and vibration and increase solder joint fatigue.

Leading electronics companies consider thermal design from a project's outset, typically using computational fluid dynamics (CFD) software dedicated to electronics cooling. CFD analysis can involve 'what-if' simulations to evaluate different cooling strategies for the enclosure before the architectural design is complete, and PCB-level simulations prior to layout closure. Instead of performing multiple PCB re-spins to achieve thermal verification, most companies do just one re-spin and complete their thermal verification in about 1/3 of the time taken by other companies. Thermal modeling and simulation tools can attenuate the verification process, arguably the number one cause of late deliveries for new products. Timely thermal design in the overall product creation process is the best way to avoid costly collisions between shrinking devices and their cooling solutions.

Dr. John Parry, CEng, business development manager, Mechanical Analysis Division, Mentor Graphics Corporation.

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