ams Thermal Design Guideline for Wearables AS62xx Instructions

June 6, 2024
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ams Thermal Design Guideline for Wearables AS62xx

ams Thermal Design Guideline for Wearables

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

  1. Figure 1:
    Flex Mounted Sensor

Sensing Skin Temperature

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) 

Sensing Skin Temperature-2

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 )

Sensing Skin Temperature-3

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 CutoutSensing Skin Temperature-4

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

Sensing Environmental Temperature

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.

Legal Information

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Applications that are described herein are for illustrative purposes only. ams AG makes no representation or warranty that such applications will be appropriate for the specified use without further testing or modification. ams AG takes no responsibility for the design, operation and testing of the applications and end-products as well as assistance with the applications or end-product designs when using ams AG products. ams AG is not liable for the suitability and fit of ams AG products in applications and end-products planned.
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