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The Heat is on For Product Design: How to Prevent Devices from Overheating

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Ziad Naboulsi | October 2, 2020

Design engineers have more to worry about these days than ever before. The demand for smaller electronic devices with more capabilities is exponentially growing, causing a similar growth of engineering challenges. More integrated circuits, resistors, diodes, transistors, and capacitors must be packed into a shrinking space, increasing a device’s susceptibility to overheating due to a reduction in thermal flow.

Done improperly, poor design and engineering can be a direct cause of overheating, leading to decreased reliability, premature failure, and damages resulting in avoidable cost and brand damage. All electronic devices and circuitry generate excess heat as current flows through a circuit, causing the need for better thermal management. As each component has a rated maximum operating temperature, and heat dissipation is critical to avoid overheating and exceeding the temperature limit.

How is Heat Removed from Electronics?

Heat must be dissipated to ensure that devices remain safe, reliable, and perform as intended. This process can happen through many different methods, including:

  1. Convection, a transference of heat through moving fluids
  2. Conduction, a process in which heat dissipates throughout a material
  3. Radiation, allowing heat to dissipate through electromagnetic waves

Mechanism

Conduction
Convection
Radiation

Definition

Heat transfer between objects that are in physical contact.
Heat transfer between an object and its environment due to fluid motion (liquid or gas).
Heat transfer through the emission of electromagnetic waves.

Heat Dissipation Path

External pins of component to the printed wiring board. Then, convection into the atmosphere.
Top surface and sides of the package into the atmosphere.
Radiative heat from the package surface area. Low contribution to total heat dissipation unless package surface area is relatively large.

Percentage of Heat*

~80%
~10%
~10%

*Estimates For A 352-pin BGA Type Component

Heat Sinks and Thermal Conductivity

In many electronic designs, ~80% of heat energy is dissipated through conduction to some form of a heat sink designed to dissipate heat by spreading it over a larger surface area, removing the heat to the atmosphere through convection.

The heat flow between a component and ambient air is modeled as a series of resistances to heat flow. The sum of these is the total thermal resistance (°C/W). The goal here is high surface area in contact with air and a material with high thermal conductivity.

That being said, common heat sink materials are materials with high thermal conductivities:

Material

Aluminum Alloy 1050
Aluminum Alloy 6061
Aluminum Alloy 6063
Copper

Thermal Conductivity (W/mK)

229
166
201
400

Conformal Coatings and Heat Dissipation: Considerable Challenges

Overheating is not the only reliability challenge for design engineers. Environmental constituents such as corrosives pose significant threats to reliable printed circuit board (PCB) performance. As devices shrink, engineers are increasingly incorporating conformal coatings into their designs in lieu of enclosures and seals, which can consume precious real estate in miniaturized devices.

These polymeric films are used as a barrier layer to contaminants, corrosives, solvents, and solid materials commonly found in operating environments. Materials such as acrylics, silicones, epoxies, polyurethanes, and Parylene can be applied to substrates, enhancing electronic reliability in the harsh environments that next-generation electronics face today.

These protective coatings can outperform seals and enclosures when it comes to reliability. The electrically insulative properties of conformal coatings prove useful here; spacing between conductors can also be reduced, allowing for the integration of more electronic components.

Unfortunately, traditional conformal coatings have low thermal conductivities, generally lying between 0.125 to 0.335 in W/mK. As the chart above indicates, these values are hundreds to thousands of times lower than those for ceramics or metals. As such, conformal coatings are good heat barriers, and when used in thick layers, can interfere with the heat transfer path, impeding heat dissipation.

Although formulating a coating with metal or inorganic fillers can increase thermal conductivity, the compositions containing metal fillers will become electrically conductive as well, meaning that they are no longer suitable for electronic applications that require electrical isolation.

This presents a paradox to design engineers. There is a need for highly thermally conductive polymers capable of transferring heat that are also electrically insulative, particularly for high-density microelectronics.

For conformal coatings, if thermal conductance is a requirement, design engineers can take either or both of two approaches:

  • Employing a coating filled with as much thermally conductive filler as possible.
  • Integrating the thinnest coating possible, as thermal conduction is inversely proportional to thickness.

Although using a filler can lead to a gain in thermal conductivity, ironically, the benefit is offset with a thicker coating challenging to apply. The coating’s electrically insulating properties may be affected as well. 

Parylene Conformal Coatings for Heat Dissipation

When it comes to reliability, you shouldn’t have to choose between bulkier, less reliable barrier protection, and susceptibility to overheating. Evaluating conformal coatings for heat dissipation entails several considerations: thermal conductivity, emissivity (the surface of a material’s effectiveness in emitting energy as thermal radiation), film uniformity, adhesion to the substrate, and film thickness.

However, of the three, thermal conductivity and film thickness are the most critical parameters to consider for heat dissipation when selecting a conformal coating. This is where Parylene is a distinctive choice.

Coating

Desired
Parylene-N
Parylene-C
Acrylic
Epoxy
Silicone
Urethane

Thermal Conductivity (W/mK)

High
0.13
0.08
0.13-0.25
0.17-0.21
0.15-0.31
0.21

Film Thickness (µm)

Low
2-50
2-40
30-130
30-130
50-210
30-130

Done improperly, poor design and engineering can be a direct cause of overheating, leading to decreased reliability, premature failure, and damages resulting in avoidable cost and brand damage.

Other conformal coating materials are typically applied at a thickness range of 50 to 250 microns. Parylene can be applied at a fraction of the thickness, ranging as low as 2 microns. For context, this is less than the width of a strand of human hair.  

Parylene Performance at Scale

At HZO, we provide Parylene services to industries of all types, notably the industrial, medical, IoT, consumer electronics, and automotive verticals, all areas where device overheating is a serious concern. But when it comes to conformal coatings, Parylene is not just a superior choice for heat dissipation. It exhibits many excellent properties that bolster device reliability, such as chemical resistance and the ability to remain reliable throughout a wide temperature range.

Each Parylene type has distinct characteristics, so there is a Parylene for virtually every application. For example, Parylene C provides the best corrosion resistance of all the conformal coating polymers. My colleague, Dr. Sean Clancy, delivered an excellent webinar on the subject of Parylene’s corrosion resistance, which I encourage you to view. You can also download HZO’s Parylene datasheet to learn more about Parylene’s beneficial properties.

Something my team is incredibly proud of is our customer service. We have formed meaningful partnerships over the years, providing masking and demasking recommendations, offering DFX guidance, and working with our customers to integrate our coating process into many points of their production.

If you are interested in protection for your project and need to better understand the alternatives, or are simply curious to learn how leading OEMs are designing the next generation of mobile, wearable, edge, and general electronic devices, reach out to us today or fill out contact or quote form to get started. We are not simply a job shop, but a team of dedicated engineers that understand your design challenges. 

When it comes to heat dissipation or electronic protection, you do not have to settle. Your device is in good hands when you work with HZO.

When it comes to heat dissipation or electronic protection, you do not have to settle.

Ziad Naboulsi

Ziad Naboulsi is an application engineer at HZO.

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