PCB turnkey manufacturing

PCB Thermal Analysis From Stackup to Assembly

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All PCBs will generate some amount of heat, but reliability is a matter of determining whether a design will operate as intended during operation despite heat generation. PCB thermal analysis strategies focus on understanding how much heat is generated in a PCBA and how that heat is transferred around a PCB layout. Once this is understood, it’s possible to evaluate heat management strategies should they be needed.

In a PCB, the first place to start performing thermal analysis is in the PCB stackup, specifically regarding material selection, the copper weight used in the design, and the number and thickness of dielectric layers. Components should also be evaluated at their operating conditions as part of thermal analysis in order to identify potential overstressing and failures. We’ll discuss these points in this article, as well as some additional design steps that can be taken to help prevent excessive thermal stresses acting on a PCBA.

Basics of PCB Thermal Analysis

Thermal analysis in a PCB focuses on understanding where heat will be generated and how that heat can be transported to a cooler area of the system. Methods for determining the temperature distribution in circuit boards are well known and can be accessed in simulation packages. However, there are some simple design considerations that can be implemented before running simulations, which will help ensure success on the first prototype.

Components and Thermal Resistance

All components generate some heat, and it’s a designer’s responsibility to identify where excessive heat generation will occur and cause the board to reach high temperature. In components, the temperature rise can be accurately calculated using a component’s thermal resistance. The thermal resistance of a component defines the expected temperature increase above ambient per unit power dissipated in the component.

The thermal resistance specification for a component can be found in datasheets. The specifications from die-to-package and die-to-air are the most important as these realize two important operating conditions. The latter is what one would expect to measure from the surface layer on a PCB when a component is dissipating a specific amount of power without a heat sink, forced-air cooling, or other heat dissipation mechanism. An example for a typical LDO is shown below.

 

PCB thermal analysis

The thermal resistance specification for components can be found in datasheets. This example is for the NCP5500 from ON Semiconductor.

 

IPC-2221 and IPC-2152

One area where designers sometimes start to look at thermal reliability and equilibrium temperature is the IPC-2152 standard. This standard addresses equilibrium temperature rise in a PCB trace or plane carrying some current, and the standard succeeds the original IPC-2221 standard for PCB thermal analysis, and it provides some estimates of equilibrium temperature in a bare board for a given current draw and copper weight in the PCB. Some important context is needed when interpreting this standard as the values generated from nomographs can be overly conservative when we consider the balance between power dissipation and minimum temperature.

The original standards were developed using a single nomograph for a single type of board, where a hypothetical trace spans across an unpopulated board and is run with DC current. The resulting equilibrium temperature was measured for various currents, trace dimensions, and copper weights, and the results were compiled in the IPC-2221 and IPC-2152 standards. Real boards are much more complex, having planes, copper pour, and varying densities and weights of copper throughout the layer stack.

When we consider real boards with multiple copper layers, different dielectric thicknesses, and copper weights, we can extract some trends that help keep high current traces operating within specification:

  • Higher copper weight traces can withstand more heat dissipation. Similarly, heavier copper planes can receive more heat from a hot trace and transport it throughout the board.

  • A thinner dielectric between a hot trace and a plane will pull away more heat from the trace. This is not considered in the IPC-2152/IPC-2221 standards.

  • Using a thinner conductor will create greater DC power losses, and using a higher current will allow for more heat dissipation via DC resistance.

Unfortunately, simulations or measurements are the best tools to use to evaluate thermal reliability in copper as the IPC standards only provide overly conservative trace width and copper weight requirements, and following these can make design and layout impractical. To see why the IPC-2221 and IPC-2152 standards are inaccurate, the table below shows an example comparison of standards-based temperature predictions and measurements for a trace in a PCB carrying 7 A DC current.

 

PCB thermal analysis IPC-2152

These results reveal how the IPC standards can overestimate temperature rise when applied to real boards.

 

Due to the obvious lack of generalization of the IPC nomographs and analysis procedures, measurements and simulations are needed to properly analyze and evaluate a PCBA. I’ll be discussing these points on an upcoming Altium podcast with Mike Jouppi, one of the original members on the IPC 1-10b Task Group. We’ll post more updates on the blog and on our resources page when this podcast becomes available.

Other Aspects of Reliability

An important property of materials in a PCBA that will determine reliability is the coefficient of thermal expansion (CTE), as well as the glass transition temperature (Tg) of PCB substrates. FR4-grade materials can have high Tg values depending on the resin system used in the material, and these are generally preferred in any design where large thermal excursions are expected. When the operating temperature is determined, the expected level of expansion and stress on solder joints in the PCB should be determined as these operational aspects will affect reliability.

Although excessive stress on solder joints and the board itself is a known cause of failure, these problems are more critical during thermal cycling. Whenever thermal cycling occurs, fatigue failure may result after many large thermal excursions to high/low temperature values. Transitions between high operating temperatures and the idle equilibrium temperature should be known, although estimating failure is more difficult and requires its own set of testing. Depending on industry (e.g., in aerospace and military electronics), specialized environmental testing is needed as the thermal excursions and mechanical shocks can be extreme, regardless of the current used in the design.

More specialized designs like a metal-core PCB can provide high thermal conductivity and will sink a lot of heat away from hot components in the PCB. However, these designs can have very low yield depending on the design, and they are not recommended for use in rugged electronics assemblies. Instead, there are other design practices that can be implemented to help maximize reliability through heat management in a PCBA.

More Tips for Thermal Management

Although thermal management begins in the PCB stackup and layout, there are additional measures a design team can take to ensure the PCBA does not get too hot during operation. Some designs will be fine with only using an appropriate PCB stackup with plane layers and sufficient copper, as this will help normalize the temperature distribution across the PCBA. Other designs, such as high current power systems or devices with high I/O count processors, will need additional strategies to aid thermal management.

  • Thermal pads and paste: Advanced materials in a passive cooling strategy can help quickly draw heat from active components and into a heat sink or enclosure wall. Thermal pads and thermal pastes are used to form these bonds to help remove heat from high temperature components.

  • Fan placement: Fans are typically mounted directly to a hot component in order to draw cool air into a hot heat sink. In some systems, mounting a larger fan to the enclosure can provide lower noise, lower power consumption, and greater heat removal. This strategy is becoming more popular in rack-mount networking equipment and servers.

  • Heat sink designs: Heat sink designs can be very unique in order to provide maximum convective cooling via cold airflow through the enclosure. The heat sink should be chosen to accept maximum airflow for a given fan placement so that temperature can be minimized.

  • Higher resin content: Using prepreg layers with higher resin content will aid thermal transport. This is also desirable in high voltage designs in order to ensure reliability, although be careful of conductive anodic filamentation at high DC bias, particularly in laminates with DICY curing agent.

  • PCB layout: Active components in your PCB layout act as heat sources by dissipating some heat in the packaging and nearby copper in the PDN. If you have a large number of active components, try to space these out to create a more even temperature distribution.

For designs requiring airflow, the current methodology for designing these systems involves CFD simulations to visualize and track airflow throughout the PCBA and the enclosure. At some point, PCB thermal analysis and simulations cannot provide the absolute highest level of accuracy, and a design will need to be tested and evaluated with measurements. Prototype assemblies are needed for these evaluations, and designs should be adjusted as needed to ensure PCB thermal management strategies will work as intended.

 

CFD simulation

This simulation was created using Ansys to evaluate airflow throughout an enclosure.

 

 

Whether you’re designing an ultra-rugged aerospace system or feature-rich IoT products, you’ll need to find a design firm that understands PCB thermal analysis and manufacturing challenges. NWES is an experienced design firm that develops advanced IoT platforms, RF power supply designs, data center products, rugged aerospace systems, and much more. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your next rugged electronics system is fully manufacturable at scale. Contact NWES today for a consultation.

 



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