Designing Your PCB for Thermal Reliability
Thermal reliability encompasses multiple aspects of your PCB, including substrate materials, material interfaces, and components.
Ensuring thermal reliability requires getting the board temperature as low as possible, which is all about material selection, component layout, and placement of other features like mounting holes and slots.
You’ll need to ensure your components have passed stress tests before they are used in your next board.
These capacitor faults occur over time due to repeated thermal cycling between extreme temperatures.
When your components get hot, so does the rest of your board. Heat moves away from warmer areas of your board and raises the temperature in cooler areas, and the rate at which heat moves away from hot regions of the board depends on a number of factors. When you’re planning a new PCB layout, how can you ensure your temperature does not break through your required limits? It’s all about creating an even heat distribution in your PCB and designing your board to withstand thermal stress.
These ideas are central in thermal reliability, where the goal is to ensure your board lifetime is maximized despite repeated exposure to heat (thermal cycling). Thermal reliability is also a key component of surviving thermal shock, both at the board level and component level. Understanding these aspects of thermal and thermomechanical behavior can help you extend your board lifetime and make smarter design decisions to reduce temperature rise and gradients.
What Influences Thermal Reliability?
There are three perspectives from which thermal reliability must be considered:
Thermal shock reliability. Near-instantaneous exposure to high temperature causes the board to heat up very quickly, which can place stress on solder joints, vias, material interfaces, and other elements.
Thermal cycling reliability. The board may not heat up to an extremely high temperature, but it will cycle between two extreme values repeatedly. Over time, conductors can become fatigued (e.g., through micro-cracking, corrosion, or pitting) and elastic materials can become embrittled.
Component failure. Components can fail due to thermally-assisted failure mechanisms, such as electromigration in ICs. Failure mechanisms are component-specific and need to be examined on a per-product basis.
Materials scientists working in the PCB industry have spent a significant amount of time identifying failure mechanisms in different structures, both due to thermal shock and thermal cycling. Once you consider the influence of the external environment, such as humidity, dust, or other airborne contaminants, other failure mechanisms can become prominent. Components are something of a wildcard; you have to rely on the manufacturer to thoroughly test components and only send out reliable products.
Once a board is fabricated, it may need to undergo thermal shock testing to ensure it will comply with relevant standards. Basic IPC and MIL-STD testing standards exist for evaluating thermal and mechanical shock to ensure reliability. If you want to ensure thermal reliability, you need to design solutions to both general and specific reliability problems in your PCB. This, in turn, requires considering the environment where the board will be deployed and the thermal behavior of the environment.
What Influences Thermal Reliability?
With this in mind, here are some steps you can take to ensure thermal reliability and the problems these design choices help solve. Without a doubt, many thermal reliability problems encountered in practical systems can be solved with some basic design and layout choices, as shown below.
Use Your Layout to Control Heat Flow
There are many aspects of your board’s layout that influence how heat dissipates from hot components. These features include the arrangement of plane layers, mounting hole and slot arrangement, component placement, and substrate material, which all influence heat flow from hot to cold regions in a PCB.
This unplated mounting hole acts as a barrier to heat conduction away from other components.
Holes and slots can act like thermal barriers that hold in heat. Therefore, mounting holes or other voids in a board should be placed around the edge and, preferably, connected back to a large chassis with screws. Hot components should not be clustered in one area. This helps spread out the thermal load across the board and creates a more even temperature distribution.
Don’t Stack Too Many Blind/Buried Vias
Repeated thermal cycling is known to cause fracture at the base and necks of microvias. Fracture at the blind/buried or buried/buried interface is one problem that is much easier to solve. Simply stack no more than 2 or 3 of these microvias and stagger the remainder (so-called “staggered microvias” in the HDI community, see below). In any case, the via aspect ratio should not be too large as larger vias have thinner plating near the center of the via neck.
Staggered and stacked blind/buried microvias. In general, you should try to stack no more than 2 layers of microvias to prevent fracture at the base of the structure. The internal layers can then be connected with a standard buried via.
Use the Right Substrate and Solder Materials
Embrittlement in PCB substrate materials is a long-term thermal reliability problem that occurs during continuous exposure to high temperatures. Over time, outgassing and resin degradation cause the board to become brittle, where it can easily fracture. Conformal coatings can help with outgassing at lower temperatures, but the best choice is to use an alternative substrate material for boards running at high temperatures. Ceramics are ideal for these applications, although they carry their own fabrication difficulties.
Solder materials can also have long-term reliability problems due to thermal cycling. When brought up to high temperature, stress can concentrate at the ends of solder balls, leading to fatigue and failure. Diffusion of different elements in the alloy can also cause depletion in certain regions of the solder, leading to reduced mechanical strength and eventual fatigue failure.
Some other important substrate material parameters to consider are:
Substrate CTE values: The z-axis CTE value of FR4 substrates is much larger than that of copper; consider an alternative material with a CTE value that is closer to that of copper.
High-Tg substrate materials: The CTE value can be kept lower over a broader temperature range when the Tg value is larger.
High thermal conductivity: Using a high thermal conductivity PCB substrate (e.g., metal-core) will help heat move away from components and other sources quickly, giving a lower equilibrium temperature.
These parameters in mind will help ensure your circuit designs will be vastly more thermally reliable. Always utilize safe thermal design practices within your electronics design.
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