ams Thermal Design Guideline for Wearables AS62xx Instructions
- June 6, 2024
- ams
Table of Contents
ams Thermal Design Guideline for Wearables AS62xx
Introduction
The AS62xx Product Family is a digital temperature sensor family for
applications that require small form factors, ultra-low power consumption and
high accuracy.
Apart from numerous other applications, wearables are a perfect fit for this
sensor. Especially for wearable devices, designing a housing for a good skin
and environmental temperature measurement could be challenging. In this
document, some design guidelines are listed to support you in your product
design.
Ordering Information
Ordering Code| Package| Marking| Delivery Form|
Delivery Quantity
---|---|---|---|---
AS6221-AWLT-S| WLCSP| AS6221| Tape & Reel| 500 pcs/reel
AS6221-AWLT-L| WLCSP| AS6221| Tape & Reel| 5000 pcs/reel
AS6212-AWLT-S| WLCSP| AS6212| Tape & Reel| 500 pcs/reel
AS6212-AWLT-L| WLCSP| AS6212| Tape & Reel| 5000 pcs/reel
AS6214-AWLT-S| WLCSP| AS6214| Tape & Reel| 500 pcs/reel
AS6214-AWLT-L| WLCSP| AS6214| Tape & Reel| 5000 pcs/reel
AS6218-AWLT-S| WLCSP| AS6218| Tape & Reel| 500 pcs/reel
AS6218-AWLT-L| WLCSP| AS6218| Tape & Reel| 5000 pcs/reel
AS6204-AWLM-S| WLCSP| AS6204| Tape & Reel| 500 pcs/reel
AS6204-AWLT-L| WLCSP| AS6204| Tape & Reel| 5000 pcs/reel
AS6200C-AWLM-S| WLCSP| AS6MCC| Tape & Reel| 500 pcs/reel
AS6200C-AWLT-L| WLCSP| AS6MCC| Tape & Reel| 5000 pcs/reel
AS6200-AWLT-S| WLCSP| AS6200| Tape & Reel| 500 pcs/reel
AS6200-AWLT-L| WLCSP| AS6200| Tape & Reel| 5000 pcs/reel
Sensing Skin Temperature
To measure a proband’s skin temperature with the AS62xx temperature sensor, it
is mandatory to have a good thermal connection between the sensor and the
skin. This can be achieved by placing a conductive pin in the housing of the
device. The pin consists of a thermal conductive material like metal. In case
of a metal housing, the pin should be isolated from the housing to reduce the
thermal capacity of the temperature sensing.
The AS62xx temperature sensor is thermally connected to the pin with thermal
adhesive or thermal paste and electrically with a flex PCB as shown in Figure
- Figure 1:
Flex Mounted Sensor
This design has the advantage that it enables a higher degree of freedom for
placing the sensor where it is convenient for the product design, but could
increase the costs due to the more expensive flex PCB.
In case it is preferred to have the sensor mounted on the mainboard, it is
possible to either use a contact spring or a thermal pad to establish a
thermal connection. In the first case the sensors exposed pad (or GND pad) and
the contact spring have to either be connected with a circuit path (top side
mounted) or a thermal via (bottom side mounted) as shown in Figure 2. This
design allows to have a cost effective design that can cover a relatively long
distance between the PCB and the housing.
Figure 2:
PCB Mounted Sensor (Contact Spring)
Connecting the sensor via thermal pad is probably the easiest and cleanest way
of establishing a thermal connection between the sensor and the outside of the
wearable. Therefor a thermal pad of the size of the sensor is fitted between
the device and the contact pin as seen in Figure 3
On the down side, it only allows a relatively small distance between the
device and the contact pin. A pad with high thermal conductivity is the key
factor to a fast and accurate temperature readout.
Figure 3:
PCB Mounted Sensor (Thermal Pad )
Temperature Response Time
To further improve the response time of the sensor, it is recommended to
reduce the thermal capacity of the PCB. One important measure is to keep the
area around the sensor free of any copper planes. In addition to that, a
cutout as can be seen Figure 4 should be applied.
Figure 4:
PCB Cutout
Sensing Environmental Temperature
Sensing the environmental temperature is more challenging, as the housing is
strongly influenced by the skin temperature of the proband. To avoid this, it
is recommended to use two sensors. One sensing the skin temperature and the
other to sense the environmental temperature. Developing a thermal model of
the device and a temperature prediction algorithm provides the best results in
determining this parameter.
Figure 5:
Environmental Sensing
The higher the thermal resistance between the environmental sensor and the skin sensor is, the more precise results are possible. This can be achieved by using low thermal conductive materials for the housing of the wearable device and deploying a good isolation between the sensor and the housing.
Revision Information
Changes from previous version to current revision v4-00
Updated Ordering Information table with AS6221
- Page and figure numbers for the previous version may differ from page and figure numbers in the current revision.
- Correction of typographical errors is not explicitly mentioned.
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