Heat sink guidelines

Based on the results of your thermal testing you may find it is advisable or required to implement a method of dissipating excess heat. This section explains how to employ a heat sink on top of the XBee Smart Modem.

A bolt-down style heat sink on top of the XBee Smart Modem provides the best performance. An example part number is Advanced Thermal Solutions ATS-PCBT1084/ATS-PCB1084. You must use an electrically non-conductive thermal gasket on top of the XBee device under the area that will be covered by the heat sink. A thermal gasket such as Gap Pad® 2500S20 is suitable for this purpose. We recommend using a gasket with thickness of at least 0.080 in to ensure that components on top of the XBee device do not tear through the material when pressure is applied to the heat sink.

Install the SIM card prior to placement of the heat sink. Position the thermal gasket and heat sink assembly on the top of the device so that it covers the entire section shown inside the blue box below (covering the RF shield with thermal gasket is optional because the RF shield is allowed to directly contact the heat sink). Do not cover the U.FL connectors.

When attaching to the host PCB, tighten the mounting hardware until the gasket is compressed at least 25% and the heat sink is snug against the RF shield. Avoid overtightening. To prevent shorting, check that the surface of the heat sink does not directly contact any of the XBee device components.

You may also use an adhesive style heat sink with the XBee Smart Modem, such as Wakefield-Vette 658-45ABT3. Follow the instructions above, except do not cover the area under the RF shield with thermal gasket; connect this surface directly to the heat sink for mechanical strength. The thermal gasket must be used on top of all other components under the heat sink to prevent shorting and provide effective thermal transfer to the heat sink.

The following table provides a list of typical scenarios and the maximum ambient temperature at which the XBee Smart Modem can be safely operated under that condition. These are provided only as guidelines as your results will vary based on application. We recommend that you perform sufficient testing, as explained in Heat considerations and testing, to ensure that the XBee Smart Modem does not exceed temperature specifications.

Scenario

 

Average current consumption (VCC = 3.3V)

Example application

Peak data consumed (MB/hr)

Maximum safe ambient temperature

No heat sink

Heat sink

Heat sink and fan

Maximum transmission duty cycle

950 mA

Running video camera

500 to 2000

N/A

40 C

60 C

50% duty cycle

475 mA

Running low resolution video camera

200 to 400

42 C

65 C

75 C

20% duty cycle

200 mA

Sending high resolution photo less than once per minute

50 to 150

64 C

74 C

78 C

Device awake, limited transmissions

170 mA

Updating traffic sign

1 to 10

66 C

75 C

80 C

Device primarily asleep, very limited transmissions

20 mA

Small data transmission/receptions which occur once per hour

Less than 0.1

80 C

80 C

80 C