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Thermal Design basics for CPU modules

Thermal Design Overview

Thermal design is a method of keeping the device junction temperature within the operating range. As we all know the Potential Difference (V) between two points is the product of Current (I) & Electrical Resistance (R). We can correlate the thermal parameters with the electrical parameters for better understanding of basic thermal equation. Here Temperature difference (∆T) between two pints can be compared with the Potential Difference (V), Power Dissipation (PD) with Current (I) & Electrical Resistance (R) with Thermal Resistance (Rθ).

Thermal Design basics for CPU modules

The symbol θ is generally used to denote thermal resistance. Thermal resistance is in units of °C/watt (°C/W). Thermal resistance is a measure of a materials ability to conduct heat. Materials that are good conductors of heat (metal) have a low thermal resistance & Materials that are poor conductors of heat (plastics) have a high thermal resistance. The total thermal resistance determines how well an integrated circuit can cool itself. As a summary a good thermal resistance will lower the Integrated Circuit junction temperature & keep the circuit functioning within the operating temperature range.

Since in a design the Ambient temperature & the power dissipation are fixed, only way we can control the junction temperature within operating range is by reducing the Junction to Ambient Thermal Resistance JA). 

Basic Thermal Model

The Most basic thermal Model is shown below. All thermal effects are reduced to a series of thermal coefficients, which can be summed. This type of model is sometimes called a ‘single-resistor’ model.

Thermal Design basics for CPU modules: Figure 2

Theta-JC (Junction to Case Thermal Resistance)

Junction-to-case thermal resistance measures the ability of a device to dissipate heat from the surface of the die to the top or bottom surface of the package. It is applicable for packages used with external heat sinks and only applies to situations where all or nearly all of the heat is dissipated through the surface in consideration. 

Theta-CA (Case to Ambient Thermal Resistance) Case-to-Ambient thermal resistance measures the ability of a device to dissipate heat from the top or bottom surface of the package to Ambient. 

Theta-JA (Junction to Ambient Thermal Resistance) Junction-to-air thermal resistance is a measure of the ability of a device to dissipate heat from the surface of the die to the ambient via all paths. It is relevant for packages used without external heat sinks and similar PCB as JEDEC test board.

 

The junction temperature TJ can be calculated as: 

TJ = (θ JC + θ CA) x PD + TA = θ JA x PD + TA 

The maximum ambient temperature can be calculated as: 

TA max = TJ max - (θ JC + θ CA) x PD max

Adding a Heat Sink

Adding a heat sink, basically replaces the thermal resistance from the case to the surrounding environment, with the thermal resistance of the heat sink.

Thermal Design basics for CPU modules: Figure 3

Theta-CS (Case to Sink Thermal Resistance) Case-to-Sink thermal resistance measures the ability of an interface material to dissipate heat from the top or bottom surface of the package to Heat Sink.

Theta-SA (Sink to Ambient Thermal Resistance) Sink-to-Ambient thermal resistance measures the ability of a heat sink/heat spreader to dissipate heat from the surface of device to Ambient. 

The Total Thermal Resistance from Junction to Ambient can be calculated as: 

Θ JA = θ JC + θ CS + θ SA

So the junction temperature TJ of a device with heat sink can be calculated as: 

TJ = (θ JC + θ CS + θ SA) x PD + TA


The Heat Spreader

Heat spreaders provide the conduction heat path between the package and the system enclosure, alone, may not be sufficient enough for thermal management of high power component for extended operations. Heat spreaders are used to spread the heat while transporting it from the die to the PCB, product chassis or a heat sink (if the product design form factor permits), which in turn dissipates heat to the local environment.

Below are the details of iWave systems Qseven Heat spreader thermal resistance calculation. 

The Heat spreader material details are shown in below figure.

iWave Systems heat Qseven Spreader

The Heat spreader theoretical thermal resistance are calculated using the below equation. 

Thermal Resistance = Thickness/ Thermal Conductivity X Area 

Item Number

Material

Thermal Resistance (C/W)

Thermal Conductivity

(W/m-K)

Thickness

(m)

Length

(m)

Width

(m)

1

Al-6063

0.002278423

209

0.002

0.07

0.06

2

Copper

0.022792023

390

0.002

0.015

0.015

3

Gap Pad HF48-6

2.777777778

3.2

0.002

0.015

0.015

4

Thermal Pad HF225UT

0.507936508

0.7

0.00008

0.015

0.015

TOTAL THERMAL REISTANCE

3.3107

 

 

 

 

 

Note that the actual thermal resistance will be higher & will dependant on various factors. 

PCB Design Recommendations

Primary path for heat transfer through the PCB is by conduction. Means the planes, due to the high thermal conductivity of copper compared to the rest of the board materials, carry nearly all the heat. Board Thermal Resistance can be lowered by:

  • Increasing the number of vias & via size connecting to Power & GND plane.
  • Increasing the power and ground planes thickness
  • Using full contact via instead of thermal relief for power and ground vias. 

 

In some of the devices there will be dedicated “thermal balls”, which will have improved thermal conductivity to the die. Typically, thermal balls will be placed in a block in the centre of the package, where the thermal connection to the die is best. The number of vias connected to these balls can be maximized by placing them in a ‘checkerboard’ pattern, with wide traces (which lowers the thermal resistance) from the pad to each via.

Finally, the connection from the vias to the planes should not use thermal relief connection traces, as these are designed to prevent the flow of heat through the PCB. The thermal vias should have a solid connection to the traces on the outer layer, and the planes on each layer. 

Heat transfer from the board to the ambient is primarily by convection. Thermal resistance from Board to Ambient can be lowered by:

  • Copper pouring the unused areas of the top and bottom layers of the board and connecting these areas to the power or ground planes with thermal vias.
  • Increasing the airflow across the board, either by using fans, or ventilation through the enclosure, or both.
  • Good thermal connection from the board to the enclosure. Example: placing plated, grounded mounting holes close to the devices with the largest power dissipation, using metal standoffs for these mounting holes, and using a metal chassis.


Conclusion

Carefully estimating thermal resistance is important in the long-term reliability of any IC. Design engineers should always correlate the power consumption of the device with the maximum allowable Power dissipation of the package selected for that device using the provided thermal resistance parameters. iWave systems provides the best thermal solution for its ARM Cortex based CPU modules by considering all this factors into account and make the CPU modules life longer.

Roshan D'souza- Member Technical- Hardware

iWave Systems Technologies Pvt. Ltd.