Texas Instruments | Effects of Soldering on High Precision IC Temperature Sensors | Application notes | Texas Instruments Effects of Soldering on High Precision IC Temperature Sensors Application notes

Texas Instruments Effects of Soldering on High Precision IC Temperature Sensors Application notes
Effects of Soldering on High Precision IC Temperature
Sensors
Introduction
Solder-Shift
When within recommended operating conditions, every
IC temperature sensor has some small range of
expected outputs for a given fixed temperature. This
range is what is referred to in temperature sensor data
sheets as device accuracy. For example, the TMP117
ultra-high accuracy digital temperature sensor at 25°C
has an output expected to be within ±100m°C of 25°C.
Even with the TMP117's extremely fine temperature
resolution of 7.8125m°C, this range is only around 26
possible digital outputs. This means that all TMP117
sensors should output one of these 26 values when
the temperature of the die is assured to be 25°C.
The term solder-shift refers to the loss in absolute
accuracy of a temperature sensor due in most cases
to the soldering of the thermal pad. Using the example
of the TMP117 again, a device at exactly 25ºC without
the thermal pad soldered down may output code
number 20 of the expected 26 values, and it will do so
reliably within ±1 code based on the TMP117's typical
±1 LSB of repeatability. If the same device then had
the thermal pad soldered down, it cannot be expected
to output code number 20 at 25ºC. Instead the device
accuracy will most likely shift, even possibly outside
the ±100m°C expected range. This shift in accuracy
has not been measured for a wide array of
temperature sensors, but where it has been measured,
typical shifts are only on the order of 10's of m°C. This
makes solder-shift an effect that is virtually
undetectable in all but the most precise devices.
However, for the TMP117, the resolution and precision
of the device make the solder-shift effect reliably
measurable.
Attaining this level of accuracy across a widetemperature range requires not only a detailed
understanding of analog CMOS design, parasitics, and
calibration, but also considerations for the mechanical
and thermal environment that the die of the device will
be sensing in. With such high levels of sensitivity, it
should come as no surprise that improper device
soldering can produce some amount of unpredictable
behavior. This document covers the findings on how
this stress from soldering an IC manifests itself on the
TMP117. The goal being to inform tradeoffs between
absolute accuracy and mechanical stability that come
with soldering the thermal pad of the device.
Specifically, it will cover the phenomenon of soldershift, which refers to the shift in device accuracy postsoldering.
Figure 1, taken from the Temperature Sensors: PCB
Guidelines for Surface Mount Devices (SNOA967),
shows a cross section of a die inside a WSON
package, just like the one used for the TMP117DRV.
The die itself is adhered to an exposed pad,
sometimes referred to as the Die Attach Pad (DAP),
using an epoxy to allow for efficient heat transfer
between the surface of the PCB and the sensor. Due
to this contact, when the thermal pad is soldered to the
PCB, some amount of mechanical stress from the
hardening of the solder is also applied to the die.
Findings
To try and quantify the important effect of this stress, a
large collection samples of TMP117DRV were first
attached to a 32-mil thick FR4 board without the
thermal pad being attached. The accuracy of these
devices was then measured in an oil bath, using 8
sample averaging in a 1-Hz conversion cycle. The
devices were then resoldered, this time including the
thermal pad, and the process was repeated. Figure 2
shows the results of the shift in accuracy from this
experiment.
Figure 1. Cross Section of WSON (TMP117DRV)
Package Showing Heat Transfer
These tests only provide a good expectation of how
going through a single process may affect one kind of
sensor. There are many different factors, such as
solder temperature or material, that may affect the final
accuracy shift results. The primary takeaway from this
experiment should be that the shift in error from
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SNIA027 – November 2018
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Effects of Soldering on High Precision IC Temperature Sensors
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soldering can be significant enough to cause issues in
high-accuracy applications, and that the nature of this
shift is not easily predictable. For this reason, TI
recommends avoiding soldering the thermal pad of the
TMP117 whenever possible.
Thermal Response
In applications where the shift in accuracy can be
tolerated, attaching the thermal pad of the device can
often provide faster thermal response and better
mechanical stability. In terms of thermal response,
there is no significant change in response time when
the sensor is in a fluid medium, regardless of whether
or not the thermal pad is soldered. However, the
response time in moving or still air was noticeably
improved after attaching the thermal pad. The
determination of this is discussed in detail in the
Wearable Temperature Sensing Layout Considerations
Optimized for Thermal Response (SNIA021).
Conclusion
Figure 2. Test Results for Soldering TMP117DRV to
a Rigid Board. The Magnitude of Solder Shift was
Around 32m°C on Average.
Solder Shift on Flex PCBs
The primary cause of solder shift is mechanical stress
from the board to the die, therefore the use of flexible
boards might be expected to reduce the magnitude of
stress on the device. Figure 3 shows the results of a
comparison done on both 6-mil and 12-mil flex boards
for the TMP117. Unlike the previous test, the initial
accuracy of the part was first evaluated in an IC
socket, then the parts were attached to the flex PCB
either with or without the thermal pad soldered.
For both 6-mil and 12-mil flex PCBs, the distribution of
the accuracy shift are similar with or without the DAP
soldered. The average shift is also significantly lower,
leading to the conclusion that soldering of the thermal
pad does not have a significant effect on device
accuracy for flexible boards under 12 mil of thickness.
This data should be taken with a grain of salt however,
as sample size for each board type was not large
enough to be conclusive, and the details of the
soldering process are also important factors.
As IC temperature sensor accuracy begins to enter the
range of 10's of m°C, many previously negligible
effects must be considered. This tech note discussed
the phenomenon of solder-shift. The mechanical stress
on the die from the hardening of solder on the thermal
pad often manifests as a shift in error for temperature
sensors. The accuracy testing on the TMP117
measured both before and after the thermal pad was
soldered onto the device found that the magnitude of
temperature shift from soldering is non-negligible. In
flex board applications with thickness under 12 mils,
however, initial testing shows that solder shift caused
by the connecting the DAP appears to be less
significant. In general, TI recommends not soldering
the DAP of high-precision temperature sensors, for
conductivity purposes thermally conductive underfills
can aid in reducing response time.
Table 1. Device Recommendations
Device
Description
TMP117
±0.1°C Accurate Digital Temperature
Sensor with Integrated NV Memory , Digital
16-bit output via I2C and SMBus™
LMT70
±0.1°C Precision Analog Temperature
Sensor, low-power, fast response
TMP112
1.4V-Capable ±0.5°C Accuracy Digital
Temperature Sensor
Table 2. Related Documentation
Document Type
Description
Application Report
Wearable Temperature Sensing Layout
Considerations Optimized for Thermal
Response
Application Report
Temperature Sensors: PCB Guidelines for
Surface Mount Devices
Application Report
Precise Temperature Measurements With
TMP116
Figure 3. Solder Shift on 12-mil and 6-mil Flex
Boards When Compared to Readings Within an IC
Socket.
2
Effects of Soldering on High Precision IC Temperature Sensors
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SNIA027 – November 2018
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