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Texas Instruments How to monitor board temperature Application notes
How to monitor board temperature
Introduction
Thermal issues in circuits could impact system
performance and damage expensive components.
Measuring the temperature of a PCB in sections where
there are “Hot Spots” or “Power Hungry ICs” can help
identify these issues to take preventive or corrective
action in real time.
The Die-attach pad (DAP), if present, provides the
most dominant thermal path between the PCB and the
die.
Systems designers may want to monitor the die
temperature of a power hungry IC—such as a CPU,
ASIC, FPGA, or DSP—to dynamically adjust its
performance, or the designers may want to monitor
“Hot” sections around power stages to either control
the fan speed in a system or initiate a safe system
shutdown.
The ultimate goal is to optimize performance and
protect costly devices. Figure 1 shows a temperaturemonitoring system on a high-performance computer
board.
Figure 2. Package With DAP
The leads and pins provide the most significant
thermal path if the package type does not include a
DAP.
Figure 1. Temperature Monitoring on a Complex
PCB
Heat Transfer From PCB to the Temp Sensor
Local temperature sensors measure their own die
temperature to determine the temperature in a specific
area. Therefore, it is important to understand the
dominant temperature conduction paths between the
die and the object or environment around the sensor.
Heat is conducted primarily through two path types:
through a Die-attach pad (DAP) attached to the
package, or through the package lead pins.
Figure 3. Package Without a DAP
The mold compound provides an additional thermal
path, but due to its low thermal conductivity, any heat
transfer through the mold compound itself is slower
than the heat transfer through the leads or DAP.
Thermal Response
The package type choice determines how quickly the
temperature sensor can respond to changes in
temperature. Figure 4 shows the relative thermal
response rates of different classes of selected SMT
package types that are used for temperature
measurements.
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Packages without a mold compound (CSP, DSBGA)
and packages with a DAP (QFN, DFN) are designed
for applications that require a fast thermal heat transfer
from the PCB, while packages without a DAP are
designed for applications that require slower response
rates. A fast thermal response rate allows the
temperature sensor to respond to any temperature
changes quickly and therefore provide an accurate
reading.
Design Guidelines - Ground Plane Considerations
If it is not practical or cost-effective to place the
temperature sensor on the opposite side of the heat
source. Place it on the same side as close to the heat
source as possible as shown in Figure 6.
The most effective way to create thermal equilibrium
between the heat source and the temperature monitor
is through the use of a ground plane. Use a solid
ground plane that extends from the heat source to the
temperature sensor.
Figure 4. Thermal Response Chart
Design Guidelines - Bottom Side Mounting
The sensor location should be as close as possible to
the heat source designers want to monitor. The
designer should avoid perforations or slits in the PCB
between the heat-generating IC and the temperature
sensor, because they could reduce or stop the thermal
response. If possible, mount the temperature monitor
on the bottom side of the PCB directly below the heat
source as shown in Figure 5.
TI recommends that designers use vias to transfer
heat quickly from one side of the PCB to the other,
because vias have a better thermal conductivity of
copper compared to FR4. Therefore, a designer can
use as many parallel vias or filled conductive vias as
feasible to transfer heat from the heat source to the
temperature monitor to create a fast thermal
equilibrium between the two ICs. A QFN or DFN
package with a DAP can further help decrease the
thermal resistance path between the vias and the
sensor die.
Figure 6. Shared GND Plane Helps With Thermal
Equilibrium
Summary
Temperature monitoring is critical in PCB design that
has “hot” power sections or “power hungry” ICs. The
systems designer must evaluate whether the selection
of local temperature sensors will meet the system
requirements and protection schemes of their design.
The designers must consider the sensor location and
the high thermal conductivity path to create a fast
thermal equilibrium between the sensor and heatgenerating element.
This tech note discusses the basics for package
selection and sensor placement in the board and
layout. More critical PCB and layout guidelines are
covered in the Temperature Sensors: PCB Guidelines
for Surface Mount Devices application report
(SNOA967).
Table 1. Related Documentation
COLLATERAL
DESCRIPTION
Application Report
Temperature Sensors: PCB Guidelines for
Surface Mount Devices
Application Report
Ambient Temperature Measurement Layout
Considerations
Figure 5. Sensor Mounted on Opposite Side of
Heat Source
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How to monitor board temperature
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