Handling and Assembly Instruction
For Sensirion’s STC Thermal Conductivity Sensor
Preface
The Sensirion STC thermal conductivity sensor is designed
for high volume applications. It is therefore compatible withstandard assembly and soldering processes. Nevertheless,these sensors are non-standard electronic components.
Some care must be taken to ensure proper and reliable1

operation of the sensors after assembly. This document
contains information on the handling and assembly ofSensirion STC thermal conductivity sensors in a production
environment.

Applicability

This document is applicable to all Sensirion STC thermal
conductivity sensors.

2ESD protection

The sensor shall be protected from ESD (ElectrostaticDischarge) and only be handled in ESD protected areas(EPA) under protected and controlled conditions (ground all
personnel with wrist-straps, ground all non-insulating andconductive objects, exclude insulating materials from theEPA, operate only in grounded conductive floor, etc.).
Protect sensors outside the EPA using ESD protective
packaging.
Figure 2 Area of the sensor suitable for application of force duringmounting process is indicated with dashed lines. All dimensions
are in mm.

4Figure 1

3

Water & dust protection membrane

The STC sensor is equipped with a water and dust
protection membrane that is directly attached to the sensor.
The membrane completely covers the sensor opening (seeFigure 3) and thus acts as a shield against pollution fromspray water and dust. The membrane is completely
permeable to all target gases the STC is intended tomeasure. Therefore, the water and dust protection
membrane must not be removed, damaged or altered in any
way to ensure reliable operation of the STC sensor.

Protection against ESD is mandatory.

Pick & place processStandard pick & place equipment for QFN packages may
be used for handling. The upper part of the sensor, where
the membrane-covered cavity resides, is very sensitive tomechanical impact and any contact must be prevented.
Direct contact between the nozzle and sensor membranemust be avoided. The lower 1.6 mm of the sensor surface
is intended for pick & place (see Figure 2). The tool surface
touching the module in the pick area should be flat andsmooth. Pick & place forces should not exceed 4N.

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Figure 3 The dust protection membrane completely covers theSTC sensor surface.

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56Soldering instructions

Standard reflow soldering ovens may be used for soldering.
The sensors are designed to withstand a soldering profile
according to IPC/JEDEC J-STD-020 with a recommended
peak temperatures of 260 °C up to 30 seconds for Pb-free
assembly in IR/Convection reflow ovens (see Figure 4). In
addition, we also recommend a maximum ramp-down rateof <4 °C/s.

Temperature

TPConformal coatingLow viscose conformal coatings or potting materials canflow onto the sensor, cover the sensor element, and thus
make the gas sensor inoperative. Ensure that nothing is(even partly) covering the sensor’s dust protection
membrane. Do not protect/cover the top surface with tapeas this may harm the sensor’s dust protection membrane.

tP

TL

tL

TS (max)

preheatingliquid phase

TimeFigure 4 Soldering profile according to JEDEC standard.
Recommended TP =260 °C and tP ≤ 30 sec for Pb-free assembly.
TL < 220 °C and tL < 150 sec. Ramp-up rate < 3 °C and rampdown rate < 4 °C/sec.

Figure 5

Do not put tape on sensor.

It is recommended not to use vapor phase soldering to
avoid potential contamination of the sensor.
The use of “no clean” type 3 solder paste is recommended.
An appropriate amount of solder paste shall be used toachieve a stand-off height (clearance between the package
body and any part of the substrate) of 50 – 75 μm. Please
consult the appropriate sensor datasheet for device specificinformation on metal land patterns and recommendations
on solder paste printing stencils.
Board wash is to be avoided for Sensirion STC sensors andit is therefore recommended to use a “no-clean” solder
paste. In addition, we recommend not to use ultrasoniccleaning since it could result in damage of the sensor. If aboard wash is a requirement for the final product orapplication, the corresponding wash process needs to beproperly qualified to be proven compatible with the STCsensor.

Figure 6 If conformal coating is applied, the top surface of thesensor must remain free of coating.

We recommend not to manually solder the Sensirion STC
sensor since corresponding process parameters may not
be controlled well and it could result in a damage of thesensor. In particular, hot air with an air temperature above
260°C on the device surface should be avoided. Since thesensing element has a very low thermal mass, it heats up
to the air temperature very quickly.
It is important to note that the side faces of the I/O pads may
oxidise over time. Therefore, a solder fillet may or may notform.

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Figure 7

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Do not spray onto sensor.

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Specifically, in applications where a form of coating isnecessary the following approach can be used.

7Use a PCB with plugged vias. Cover traces and vias withsolder mask. For most applications the solder mask will
already protect the PCB traces and the vias from corrosion.
The solder joints can be protected with an underfill material.
The underfill must precisely coat the solder joints of the STC
sensor without coating the top surface. This can beachieved with a jetting procedure. This procedure is easier
to control with a low viscosity coating.

Sensirion STC thermal conductivity sensors are nonstandard electronic components and need to be handledwith care.

As an example, we can recommend the 2-component
epoxy Epotec Epoxy 301-2fl. Due to the low viscosity of thecoating, it can be jetted at an appropriate distance to theSTC sensor while still covering all soldering joints.

Wear clean gloves or finger cots while handling the sensor
and avoid touching the sensor’s dust protection membrane.

Do not cure the coating at high temperatures and ensurethat temperature ramps are slow, since deformation due tostress in the material could damage the Sensor.
Also follow the guidelines for handling and soldering theSHT4x sensor in the Handling Instructions for SHT Sensors
documentation.

Storage conditions
instructions

andhandlingSensors in SMT packages, such as Sensirion STC sensors,
are classified as Moisture Sensitivity Level 1 (IPC/JEDEC
J-STD-020). It is recommended to process the sensors
within 1 year after delivery.

Ensure recommendations in this application note are
equally considered during repair and rework of assemblies
containing the STC sensor.
Sensor storage is recommended to be under ambient
humidity conditions.
For more detailed information please contact Sensirion.

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