Texas Instruments | Wearable Temp-Sensing Layout Considerations Optimized for Thermal Response (Rev. B) | Application notes | Texas Instruments Wearable Temp-Sensing Layout Considerations Optimized for Thermal Response (Rev. B) Application notes

Texas Instruments Wearable Temp-Sensing Layout Considerations Optimized for Thermal Response (Rev. B) Application notes
Application Report
SNIA021B – August 2015 – Revised October 2018
Wearable Temperature Sensing Layout Considerations
Optimized for Thermal Response
Emmy Denton, Aaron Heng, and Brandon Fisher
ABSTRACT
This application note discusses thermal response considerations for IC temperature sensors in measuring
skin temperature for wearable applications such as fitness bands and medical devices. It will specifically
focus on two devices—the LMT70 and the TMP117 temperature sensors—over the human body
temperature range. This information can be applied, however, to other temperature sensors that come in
similar packages. Contact temperature sensors, such as the LMT70 and the TMP117, need to be placed
in close contact with the surface that must be measured. This can be quite challenging for both DSBGA
(LMT70 and TMP117) and WSON (TMP117) packages if fast thermal response is also necessary.
Experimental results of different PCB layouts for measuring axillary (armpit) temperature and oral
temperature will be presented.
1
2
3
4
5
6
7
Contents
Introduction ................................................................................................................... 2
Small Board Probe Board Description .................................................................................... 5
The Measurement System.................................................................................................. 8
Probe Board Test Results ................................................................................................. 11
Conclusion .................................................................................................................. 14
Appendix..................................................................................................................... 14
References .................................................................................................................. 16
List of Figures
......................................................................................................
1
LMT70 Block Diagram
2
TMP117 Block Diagram..................................................................................................... 3
3
LMT70 PCB Path of Heat Flow (Thermal Conductivity of Materials, k [W/(m×K)], Given in Parenthesis) ....... 4
4
Closeup of LMT70 Mounted on a PCB With Underfill Material ........................................................ 4
5
TMP117 Heat Flow Path
6
Small LMT70 Probe Board Top Side...................................................................................... 5
7
Small LMT70 Probe Board Bottom Side .................................................................................. 5
8
TMP117 Mini Probe Board Top Side L3 .................................................................................. 6
9
TMP117 Mini Probe Board Bottom Side L3 .............................................................................. 6
10
TMP117 Mini Board Cross Section L1 .................................................................................... 7
11
TMP117 Mini Board Cross Section L2 .................................................................................... 7
12
TMP117 Mini Board Cross Section L3 .................................................................................... 7
13
TMP117 Mini Board Cross Section L4 .................................................................................... 7
14
TMP117 Mini Board Cross Section L5 .................................................................................... 7
15
TMP117 Finished Temperature Probe .................................................................................... 7
16
LMT70 Probe Board Thermal Response Measurement System ...................................................... 8
17
Body Temperature Measurement System ................................................................................ 9
18
Still Air Thermal Response Measurement System ...................................................................... 9
19
Moving Air Thermal Response Measurement System ................................................................ 10
...................................................................................................
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5
1
Introduction
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20
Stirred Oil Thermal Response Measurement System ................................................................. 11
21
Comparison of the Small PCB to Other Types of Substrates When Measuring Axillary Body Temperature .. 12
22
Oral Thermal Response With (P4) And Without (P3) Thermal Pad Soldered
23
Underarm Thermal Response With (P4) and Without (P3) Thermal Pad Soldered ............................... 13
24
Oral Thermal Response With Different Board Thickness ............................................................. 13
25
Armpit Thermal Response With Different Board Thickness .......................................................... 13
26
Thermal Response Comparison All Tests .............................................................................. 13
27
Thin PCB .................................................................................................................... 14
28
Flex PCB .................................................................................................................... 14
29
Regular PCB (From LMT70 Evaluation Module) ....................................................................... 14
30
TMP117 BLE Patch Design ............................................................................................... 15
.....................................
13
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
The two major concerns for measuring body temperature in wearables are accuracy and speed. The
temperature sensor response speed is determined by the amount of thermal mass surrounding the
sensor. The accuracy can be addressed by picking a temperature sensor with high measurement
accuracy such as the TMP117 and the LMT70 which are 0.1°C and 0.13°C, respectively. The thermal
response of the temperature sensor is how fast the device responds to a sudden change in temperature.
This application note will discuss PCB layout options to achieve good thermal conductivity, as well as
quick thermal response for the LMT70 and TMP117 as PCB layout can dramatically affect these
parameters. In addition to correct layout, good mechanical and thermal contact are also critical. The
LMT70 and TMP117 are contact sensors, thus making good contact with the surface that is measured is
of primary importance. To achieve the fast thermal response of a temperature sensor, there are a number
of layout technique considerations. There are three methods of heat transfer: conduction, convection, and
radiation. For more detailed information, refer to the Design Considerations for Measuring Ambient Air
Temperature (SNOA966) and Temperature Sensors: PCB Guidelines for Surface Mount Devices
(SNOA967) application notes.
1.1
LMT70
The LMT70 is a 4-pin analog temperature sensor that comes in a DSBGA package measuring 0.88 mm ×
0.88 mm. The small size of the package yields small thermal mass and thus fast thermal response. The
LMT70 also includes internal calibration making it one of the most accurate analog IC temperature
sensors in the market. The LMT70’s typical accuracy of 0.05°C from 25°C to 45°C makes it ideal for
measuring body temperature. The LMT70 temperature-sensing circuitry is based on the transistor base
emitter diode junction thermal properties. The diode voltage is then amplified and buffered as shown in
Figure 1. The sensing element of the LMT70 consists of stacked BJT base emitter junctions that are
biased by a current source. The output of the sensing element is buffered by a precision amplifier whose
class AB push-pull output stage can easily source and sink currents of up to 3 mA. The amplifier output
connects to an output switch that is turned on and off by the digital control input T_ON (see Figure 1). This
switch allows for the multiplexing of multiple sensors on one signal line.
2
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Introduction
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VDD
T_ON
TAO
Thermal Diodes
GND
Figure 1. LMT70 Block Diagram
1.2
TMP117
The TMP117 devices are a family of high-precision digital temperature sensors with integrated EEPROM.
The TMP117 is I2C and SMBus interface-compatible, has programmable alert functionality, and can
support up to four devices on a single bus. These sensors provide an accuracy of ±0.1°C over the -20°C
to 50°C range, and 0.2°C accuracy over the -40°C to 100°C range with 16-bit resolution. The TMP117
comes in a small 2.00-mm × 2.00-mm 6-pin WSON package, operates from 1.8 V to 5.5 V, and typically
consumes around 3.5 µA.
Figure 2. TMP117 Block Diagram
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Introduction
1.3
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Thermal Conductivity
When measuring temperature, thermal conductivity is a key material property that must be considered.
Thermal conductivity (W/(mK)) of several materials that may be used in the production of a PCB are listed
in Table 1.
Table 1. Thermal Conductivity of Different Materials
MATERIAL
THERMAL CONDUCTIVITY k [W/(m×K)]
Air
0.023 to 0.045
Wood
0.04 to 0.3
Epoxy coating on top of LMT70 die
0.2 to 0.3
FR4
0.4
Polyimide
0.5
Mold Compound
1
Thermally Conductive Epoxy
1 to 7
LMT70 Solder Ball
7 to 8
Stainless Steel
16 to 24
Solder (63/67)
39
Nickle
91
Silicon
100 to 120
Aluminum
204 to 250
Gold
320
Copper
400
Silver
425
Diamond
900<
The higher the k factor, the better the thermal conductivity and thus the faster the response time.
Maintaining a small thermal mass will improve the thermal response time of the circuit. This is where good
thermal modeling software becomes a necessity. Shown in Figure 3 is a cross section of an LMT70
mounted on a PCB. As can be seen in the Thermal Conductivity of Different Thermal Materials table
copper is a very good thermal conductor when soldered. The red arrow shows the heat flow path from the
back side of the PCB to the LMT70 active circuitry (cross hatched area) through metal portions of the PCB
(the traces, pads, and solder balls). There is air surrounding the part and the solder balls in this example,
so the thermal conductivity is compromised. Better results can be obtained if underfill material is added
surrounding the LMT70 die (package) as shown in Figure 4. This can improve the conductivity to the
actual silicon which has very high thermal conductivity. FR4 and Polyimide insulators do not have very
good thermal conductivity, thus their thickness should be minimized.
Top Coat (0.2-0.3)
xxxx
LMT70
Via
PCB
Silicon (100-120)
Active Circuitry
Solder Ball (7-8)
Top Layer (400)
Insulator (0.4)
Bottom Layer (400)
SKIN
= Heat Flow
Figure 3. LMT70 PCB Path of Heat Flow (Thermal
Conductivity of Materials, k [W/(m×K)], Given in
Parenthesis)
4
Figure 4. Closeup of LMT70 Mounted on a PCB With
Underfill Material
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Small Board Probe Board Description
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Figure 5 depicts the cross section of a typical PCB stack up using several common materials, with the
TMP117 mounted on top. TI recommends two vias per landing pattern for this particular package. The
construction of the landing pattern helps to improve the thermal performance. The die contacts the large
area of the exposed pad which provides the most the dominant heat flow path. The heat flows from the
skin directly to the stacked materials and into the exposed pad through the two vias thermal and to the
die. This allows the die to respond quickly to any skin temperature changes.
Mold Compound
Bond Wire
(Thermal Conductivity k [W/(mK)])
Die
Epoxy
Pin
Exposed Pad
Via
Heat Flow
PCB
Mold Compound (1)
Pin (400)
Top Layer (400)
FR4 (0.4)
Bottom Layer (400)
Stainless Steel (16 to 24)
SKIN
= Heat Flow
Figure 5. TMP117 Heat Flow Path
2
Small Board Probe Board Description
As mentioned previously, both the size and the choice of material impacts total thermal mass. The
following board was designed to optimize the thermal response time of the LMT70 and TMP117. The
board described in this section was made very small and approaches the minimum sizes for PCB
manufacturing. The back side of the board has a large surface area with vias to the top side as shown in
Figure 6 to Figure 9.
2.1
LMT70 Mini Board
All boards are assembled with LMT70 first with the panel intact. The LMT70s are then epoxied to the
board. Forty gauge nickel wire was soldered to the PCB connecting holes for connection purposes as
shown in Figure 16. Nickel was chosen for the wire material as it has lower thermal conductivity than
copper. The small diameter wire was chosen because of its small mass. This type of wire was used in
order to minimize the thermal effects of the wires to the PCB response time. The wires attached to the
PCB can act as a heat sink thus lower thermal mass and lower thermal conductivity would minimize the
heat sink affect. Holes were used rather than pads in order to provide more mechanical strength to the
wire assembly. 4-mil copper traces and 40 AWG copper wire should affect the response time by a very
small amount as the main benefits of this layout are the exposed copper bottom side pads and vias.
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Small Board Probe Board Description
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Figure 6. Small LMT70 Probe Board Top Side
Figure 7. Small LMT70 Probe Board Bottom Side
Probe Board Dimensions
• Width: 115 mil (2.9 mm)
• Height: 85 mil (2.16 mm)
• Thickness: 20 mil with 1-oz copper
• Via size: 12-mil hole, 24-mil diameter
• Wire pad: 8-mil hole, 20-mil diameter
• No bottom side solder mask
2.2
TMP117 Mini Board
To measure the thermal response and the temperature accurately, TMP117 is assembled on a tiny board
based on the layout technique in Figure 10 to Figure 14. The mini board is glued with highly thermally
conductive adhesive to the walls of the stainless steel tube. The thin, thermal epoxy sometimes creates a
bubble of air which will form an insulator and increases the thermal response time. Four insulated nickel
wires are soldered to the through hole for provision of power and I2C communication at the end of the
probe. The mini board is inserted into a fitted stainless steel probe. The bottom side of the mini board is
filled up with highly thermal conductive epoxy to keep the board contact with the probe casing. This allows
the external heat to transfer more quickly to the TMP117.
Figure 8. TMP117 Mini Probe Board Top Side L3
Figure 9. TMP117 Mini Probe Board Bottom Side L3
Probe Board Dimensions
• Width: 150 mil (3.81 mm)
• Height: 170 mil (4.31 mm)
• Thickness: 20 mil with 1-oz copper
• Via size: 8-mil hole, 20-mil diameter
• Wire pad: 8-mil hole, 20-mil diameter
• No bottom side solder mask
6
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Figure 10. TMP117 Mini Board Cross Section L1
Figure 11. TMP117 Mini Board Cross Section L2
Figure 12. TMP117 Mini Board Cross Section L3
Figure 13. TMP117 Mini Board Cross Section L4
76.2mm
5.08mm
Figure 14. TMP117 Mini Board Cross Section L5
Figure 15. TMP117 Finished Temperature Probe
All the materials used on the finished probe are constructed to sense any temperature change and can
detect the temperature instantly. Placing the IC temperature sensors into a closed-ended stainless steel
protective probe may help prevent contamination or potential damage. The board layout and construction
for wearable applications is similar to the design procedure for the temperature probe in Figure 15. The
same techniques can be applied to make wearable applications such as watches by placing the bottom
mini PCB between the thermal-sensing contact and the skin.
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The Measurement System
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3
The Measurement System
3.1
LMT70 Setup
Axillary
Temp
LMT70EVM
3.3V
Small LMT70
Probe Board
x
x
MSP430
P2.5_VREF
1.5V
Vref
VDD
x
USB
Cable
x
T_ON
x
P2.3
LMT70
M
U
X
TAO
GND
12-bit
ADC
Laptop
Computer
Running
LMT70EVM GUI
12"
40AWG
Nickle
Wire
Figure 16. LMT70 Probe Board Thermal Response Measurement System
The LMT70 Probe board is connected to the LMT70EVM through 40AWG wires as shown in Figure 16.
The LMT70 output temperature is recorded using the LMT70EVM GUI. The LMT70EVM is USB powered.
See the Temperature Sensor for Wearable Devices Reference Design (TIDA-00452) and LMT70EVM
User's Guide (SNIU024) for more information on the GUI and firmware source code and performance of
the system.
3.2
TMP117 Setup
The purpose of these experiments is to investigate how long it takes for the TMP117 to respond to a
sudden change when the temperature is set to 70°C or human body temperature. Speed and accuracy
are very important when measuring human body temperature. The speed of the thermal response is
dependent on the materials used and the thermal mass of the probe that the TMP117 have been
assembled into. Any thermal mass such as stainless steel metal, PCB materials, and thermal compound
will slow the response time. The thermal response time for the probe result may differ, perhaps due to the
different types of systems used for each probe.
There are five types of tests for thermal response performed: oral, underarm, stirred oil, still air, and
moving air. The setup for the first method, human body temperature thermal response, is shown in
Figure 17. The second method uses the oven chamber for still air and moving air. River rocks are placed
inside the oven chamber in order to increase thermal mass to help maintain temperature stability and
uniformity, although these are not depicted in Figure 18 and Figure 19. Finally, the stirred oil, which has
the most thermal conductivity and therefore the fastest thermal response time, is shown in Figure 20.
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The Measurement System
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Figure 17. Body Temperature Measurement System
The TMP117EVM comes in a USB stick form factor with an onboard MSP430F5528 microcontroller, which
interfaces with both the host computer and the TMP117. TMP117EVM uses the +5-V input power supply
of the USB connector to power the EVM. The TMP117EVM is designed with a perforated breakaway
portion on the board, which was removed for these experiments. The finished temperature probe is
connected to the EVM headers for remote temperature measurements. The simplest way to take a
temperature measurement from the human body is in the mouth or under the arm.
Figure 18. Still Air Thermal Response Measurement System
In this test, the box is completely sealed, and the only opening is an insert with a diameter of about 0.75
inches. River rocks are placed inside the box, creating a high thermal mass that helps the temperature
remain uniform across the chamber. The TMP117 probe is connected via the TMP117EVM USB
connector to the computer, and the TMP117 probe is powered by the +5-V input power supply of the USB
connector. After several hours, when the oven chamber reaches the set point temperature (70°C), the
probe is quickly inserted into the cavity opening, and the TMP117EVM GUI logs the temperature data.
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Figure 19. Moving Air Thermal Response Measurement System
The next method is the moving air thermal response shown in Figure 19. In this experiment, river rocks
are placed inside the box, creating a thermal mass that helps the temperature remain uniform across the
chamber. The tunnel is assembled with an anemometer metal probe and a fan. The anemometer metal
probe measures the air velocity, and the fan pumps a constant air temperature into the tunnel. The fan’s
speed can be controlled, and the designer can choose various velocities using a signal generator. The test
in this example, however, is using a constant velocity of 2.15 m/s with the TACH pin left floating.
The oven temperature is set to 70°C, and after several hours, the fan’s power supply (12 V) and
anemometer meter are turned ON. Once the fan and anemometer meter are stabilized, the TMP117 probe
is quickly inserted into the tube until the TMP117 probe reaches the set point temperature.
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Figure 20. Stirred Oil Thermal Response Measurement System
The final method is the stirred oil thermal response, as shown in Figure 20. Among the three experiments,
this method has the fastest thermal response time. The oil bath’s temperature is set to 70°C, similar to the
still air and moving air setup. Once the temperature is stabilized, the TMP117 probe is dipped into the oil
bath’s well while the TMP117EVM GUI logs the temperature. The oil bath is a compact chamber that
contains a special fluid that helps obtain stability and uniform temperature; the oil bath also uses the
precision tweener as a reference temperature.
4
Probe Board Test Results
The curve in Figure 21 shows the percent of final value on the Y axis and time in seconds on the X axis.
An initial temperature of about 22°C is the 0% level as shown in the curve. The 100% level is the axillary
skin temperature. It is common to normalize the thermal response time of a temperature sensor in this
manner. Usually thermal time constant is given to the 63% level, similar to RC time constant. This is a
good way to compare the response times of different boards as it normalizes the starting temperature of
the test and allows for easy comparison.
4.1
LMT70 Thermal Response Result
As seen in Figure 21, the Small LMT70 Probe Board (purple trace) improves the thermal response time
performance at 99% of the final value when compared to several other types of PCBs by about 100
seconds. The next best performing PCB is the flex PCB shown in green. More information on the Thin
PCB (red), Flex PCB (green), and Regular PCB (blue) can be found in the Temperature Sensor for
Wearable Devices Reference Design - LMT70 TI Design (TIDA-00452) and Section 6.
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Probe Board Test Results
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Percent of Final Temperature (%)
120
99%
100
80
100 second
Improved
Response Time
60
40
Regular PCB
Thin PCB
Flex PCB
Small LMT70 Probe Board
99% Reference Line
20
0
0
50
100
150
200
250
Time (s)
300
C007
Figure 21. Comparison of the Small PCB to Other Types of Substrates When Measuring Axillary Body
Temperature
4.2
TMP117 Thermal Response Result
The approximation for step response in 1τ is about 63.2%. The plots from Figure 22 to Figure 26 show
how quickly different probe types respond to changes in temperature in different mediums—oral,
underarm, still air, moving air, and stirred oil. The TMP117 has an exposed thermal pad for better heat
transfer through the package. Figure 22 to Figure 26 show the performance with and without the thermal
pad soldered to the PCB. These results shows that the TMP117 with exposed pad soldered responds
more quickly compared to without the thermal pad soldered. Table 2 shows the numeric value for all test
setup at 63.2%.
Figure 24 and Figure 25 illustrate the major difference in response time from different PCB thicknesses.
Figure 26 shows that the oral, underarm, and stirred oil experiments yield faster thermal response time
compared to moving air and still air measurement. The response curve of the stirred oil setup has the
fastest thermal response time, because the heat transfer through the fluid is more efficient than heat
transfer through air.
Table 2. One-time Constant Thermal Response Results
ONE-TIME CONSTANT (τ)
PROBE NO.
12
LAYOUT
BOARD
THICKNESS
256.8
L1
20 mil
No
208
L1
20 mil
Yes
82.2
252.6
L2
20 mil
No
84.8
265.2
L2
20 mil
Yes
23.2
82
253.2
L3
20 mil
No
19.6
82.2
257.8
L3
20 mil
Yes
14.8
24.8
75.8
267
L4
20 mil
No
15
26.6
80.6
252
L4
20 mil
Yes
n/a
STIRRED OIL
(s)
ORAL (s)
ARMPIT (s)
MOVING AIR
(s)
STILL AIR (s)
P1
16
18.6
32.4
74.2
P2
14.8
16.8
22.2
62.2
P3
15
16.8
23.6
P4
15.2
18.8
23.2
P5
13.2
15.2
P6
13.2
15.2
P7
13
P8
9.4
THERMAL PAD
SOLDERED?
P9
12.4
14.4
16.6
73.4
261.6
L5
20 mil
P10
16.6
19.2
26.4
80
267.6
L1
40 mil
No
P11
14.8
19
26.8
80.6
268
L1
40 mil
Yes
P12
15.8
20
24
87
267.2
L2
40 mil
No
P13
18.6
22.6
27.8
91.4
256.6
L2
40 mil
Yes
P14
19
20
30
91.8
270.6
L3
40 mil
No
P15
16.4
20.4
28.2
82
258.2
L3
40 mil
Yes
P16
16
20.4
31.4
84
271.2
L4
40 mil
No
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Table 2. One-time Constant Thermal Response Results (continued)
STIRRED OIL
(s)
P17
P18
ORAL (s)
ARMPIT (s)
MOVING AIR
(s)
18.4
19
26.8
83.2
15.8
19.6
28.2
81.8
P19
22
30.6
37.6
90.8
P20
20.6
26.4
33.2
87.6
120
Percent of (Final - Initial) Temperature (%)
Percent of (Final - Initial) Temperature (%)
ONE-TIME CONSTANT (τ)
PROBE NO.
100
80
60
40
20
P3
P4
0
0
100
200
300
400
Time (second)
500
265
L4
40 mil
Yes
273.8
L5
40 mil
n/a
280
L3
62 mil
No
275
L3
62 mil
Yes
100
80
60
40
20
P3
P4
0
0
120
100
80
60
40
20 Mil
40 Mil
62 Mil
20
0
50 100 150 200 250 300 350 400 450 500 550 600
Time (second)
D003
Figure 24. Oral Thermal Response With Different Board
Thickness
Percent of (Final - Initial) Temperature (%)
THERMAL PAD
SOLDERED?
100
200
300
D012
400 500 600
Time (second)
700
800
900
D013
Figure 23. Underarm Thermal Response With (P4) and
Without (P3) Thermal Pad Soldered
Percent of (Final - Initial) Temperature (%)
Percent of (Final - Initial) Temperature (%)
BOARD
THICKNESS
120
600
Figure 22. Oral Thermal Response With (P4) And Without
(P3) Thermal Pad Soldered
0
LAYOUT
STILL AIR (s)
120
100
80
60
40
20 Mil
40 Mil
62 Mil
20
0
0
100
200
300
400 500 600
Time (second)
700
800
900
D007
Figure 25. Armpit Thermal Response With Different Board
Thickness
120
100
80
60
40
Oral
Armpit
Still Air
Moving Air
Stirred Oil
20
0
0
100
200
300
400 500 600
Time (second)
700
800
900
D008
Figure 26. Thermal Response Comparison All Tests
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Conclusion
5
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Conclusion
The purpose of the thermal response study was to demonstrate how various layout methods could
improve IC temperature sensor thermal response time. Among all sensor layout techniques, the
temperature response data shows that the thickness of the board made the most significant improvement
in response time. However, whether the thermal pad is soldered or not, the temperature response is
nearly identical when the device is immersed into temperature environment where the bottom side design
with copper plane is placed underneath the IC. Overall, stirred oil and oral tests provide the fastest thermal
response because these tests contain fluid that has the ability to transmit temperature quickly.
6
Appendix
6.1
Other Board Descriptions
A quick review of the additional LMT70 boards are briefly described in this section. These boards have a
color-coded boarder to match the traces shown in Figure 21 for easy reference purposes. Figure 27 is 2
mm wide at the right, (8 mm wide at left), and 0.5 mm thick with 0.102-mm (4-mil) traces. The LMT70
mounts at the far right. No thermal vias or pads are on the back side of the board.
Figure 27. Thin PCB
Figure 28 has the LMT70 mounted in the middle. It has a stiffener on the back side of the LMT70 to
provide mechanical stability to the LMT70 mounting.
Figure 28. Flex PCB
Figure 29 is the LMT70EVM which is standard 12-mil thickness but has very small 4-mil traces. The
LMT70 is mounted on the far right. The size of the PCB is 850 mils or 21 mm by 600 mils or 15 mm with
thickness of 1.5 mm.
Figure 29. Regular PCB (From LMT70 Evaluation Module)
14
Wearable Temperature Sensor Layout Considerations Optimized for Thermal
Response
SNIA021B – August 2015 – Revised October 2018
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Appendix
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Figure 30 shows a wearable patch design using the TMP117, which communicates via BLE to an Android
or IOS app. The design is flexible, with a total board thickness of less than 7 mils, meaning the expected
thermal response should be very fast. The TMP117 layout is consistent with L4, and the patch is designed
for use in Axillary (underarm) measurements. The TMP117 extends away from the rest of the board in a
thin flex-cable, which allows the TMP117 to sit under the arm of the wearer while the rest of the board is
exposed to open air. The TMP117 thermal pad is not soldered down in this design.
Figure 30. TMP117 BLE Patch Design
SNIA021B – August 2015 – Revised October 2018
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Wearable Temperature Sensor Layout Considerations Optimized for Thermal
Response
Copyright © 2015–2018, Texas Instruments Incorporated
15
References
7
References
•
•
•
•
•
•
16
www.ti.com
TMP117x High-Accuracy, Low-Power, Digital Temperature Sensor With SMBus- and I2C-Compatible
Interface (SBOS740)
Design Considerations for Measuring Ambient Air Temperature (SNOA966)
Temperature Sensors: PCB Guidelines For Surface Mount Devices (SNOA967)
LMT70, LMT70A ±0.1°C Precision Analog Temperature Sensor, RTD and Precision NTC Thermistor IC
Data Sheet (SNIS187)
LMT70 Evaluation Module Precise Analog Output Temperature Sensor with Output Enable (SNIU024)
Temperature Sensor for Wearable Devices Reference Design (TIDA-00452)
Wearable Temperature Sensor Layout Considerations Optimized for Thermal
Response
SNIA021B – August 2015 – Revised October 2018
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Copyright © 2015–2018, Texas Instruments Incorporated
Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from A Revision (December 2017) to B Revision ........................................................................................... Page
•
Added references to the TMP117 in the document ................................................................................... 1
Changes from Original (August 2015) to A Revision ..................................................................................................... Page
•
•
•
•
•
•
•
Added new content and subsections to Section 1 .................................................................................... 2
Added Section 2.1......................................................................................................................... 5
Added Section 2.2......................................................................................................................... 6
Split Section 3 into LMT70 Setup and TMP117 Setup ............................................................................... 8
Split Section 4 into LMT70 Thermal Response Result and TMP116 Thermal Response Result ............................. 11
Added Section 5 ......................................................................................................................... 14
Added new references to Section 7 ................................................................................................... 16
SNIA021B – August 2015 – Revised October 2018
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Revision History
17
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