Do you think the inaccurate sensor reading is accidental? It might be those "invisible" VOC gases on the production line causing the trouble!
In the field of precision electronic manufacturing, temperature and humidity sensors are hailed as the "nerve endings" of environmental perception. However, many engineers have found during production tests that the originally high-precision Sensirion SHT series humidity sensors, after going through certain production processes, have shown unexplained deviations in their readings.
The "culprit" hidden behind this is often not the quality problem of the sensor itself, but the volatile organic compounds (VOCs) that are ubiquitous in the production process. Today, let's uncover this seemingly "mysterious" yet crucial technical truth.
No.1 Why do VOC cause humidity sensors to "distort"?
The core principle of Sensirion capacitive humidity sensor is to measure humidity by sensing the change in capacitance produced when the polymer layer absorbs water molecules. However, this highly sensitive characteristic also brings an inherent technical challenge, it cannot distinguish water molecules from other foreign molecules with similar dielectric properties
When sensors are exposed to an environment with VOC gases, two main pollution effects occur：
1. Surface contamination
Pollutants form a barrier on the surface of the sensor sensing device, hindering the normal entry and exit of water molecules. This is usually caused by particulate matter, conformal coating, paint or surface film. The good news is that this kind of pollution can be easily avoided through correct operation methods and the use of protective films.
2. Bulk phase contamination-It's a true invisible killer
This is the most common and most intractable problem. VOC gas molecules will diffuse into the interior of the sensing polymer and combine with it. These foreign molecules will have two negative effects.
·Altering the dielectric constant: The dielectric constant of the pollutant itself is different from that of water, directly interfering with the capacitance reading.
·Preoccupation of binding sites: Pollutant molecules occupy the binding sites that originally belong to water molecules, causing the sensor to be unable to absorb sufficient water molecules, thereby underestimating or overestimating humidity.
it follows that:
·In a low-humidity environment: Due to the superposition of dielectric constants of pollutants, the readings are often higher (mistakenly believing that water molecules are present)
·In a high-humidity environment: Due to the occupation of binding sites, water molecules cannot enter, and the reading is often lower.
·Extreme case: If all the sites are occupied, the sensor will become a "blind sensor", and the reading will no longer change with humidity, completely failing.
No.2 "High-risk moments" in production and manufacturing
Many engineers may ask: "Our workshop is very clean. Where does the VOC come from?" In fact, in the production and manufacturing process of electronic products, the sources of VOCs are far more abundant than imagined and are very easy to be overlooked.
1.Residue of cleaning agent and solvent
During the PCBA assembly process, commonly used cleaning agents (such as isopropyl alcohol, ethanol, acetone, etc.) are all highly polar volatile molecules. If the drying is not thorough after cleaning or the cleaning solution comes into direct contact with the sensor opening, the remaining solvent molecules will rapidly diffuse into the sensor polymer.
2.Conformal coating and glue
To protect circuit boards, conformal coating is often sprayed on the production line or adhesives are used to fix components. These materials will release a large amount of volatile organic compounds during the curing process. If the sensors are not protected during installation or the airflow channels are not considered in the design, these gases will rush in.
3.Plastic parts and packaging materials
Some plastic casings, connectors and even packaging foams will continuously release volatile substances such as plasticizers after production. Slow "chronic poisoning" can also occur if sensors are stored for a long time in sealed packaging containing these materials before assembly.
4.Welding and high-temperature processes
lthough high temperatures themselves are not VOCs, they can accelerate their diffusion. According to Fick's law of diffusion, the higher the temperature, the faster the molecules move. If reflow soldering or high-temperature aging tests are conducted in an environment with VOCs present, the speed and depth at which pollutants enter the sensor will increase exponentially.
No.3 How to build an "anti-virus" production line?
In the face of the threat of VOC, passive response is not as good as active defense. Sensirion proposed the SENSE framework (Selection, Evasion, Normalization, Setting), providing a complete mitigation strategy for production and manufacturing.
Step 1: Selection - Control from the source
·Selection protection: Choose sensor models with removable protective films or IP67 filter films. The protective film should be removed only after all potential contaminating processes (such as dispensing and coating) have been completed.
·Material review: Strictly examine all materials of contact sensors on the production line (cleaning agents, adhesives, plastic housings), and avoid using known high-risk volatile substances.
Step 2: Evasion - process control
·Control concentration: At the workstation where the sensor is exposed, ensure good fresh air circulation to prevent the accumulation of local VOC concentration.
·Shorten the time: Try to minimize the time that the sensor is exposed to a potentially contaminated environment without protection.
·Temperature management: In processes with VOC risks, try to maintain low temperatures. Conversely, if pollutants need to be removed, heating should be carried out in clean air.
Step 3: Normalization - It's never too late to mend.
If there is a slight contamination unfortunately, do not rush to scrap the sensor. You can try to restore it:
·Natural recovery: Place the sensor in a clean, dry and well-ventilated environment to allow contaminants to disperse naturally. However, this has limited effect on pollutants with strong binding force.
·Heating decontamination: Utilize the built-in heater of the sensor (such as the SHT4x series) to briefly heat up in clean air, accelerating the volatilization of pollutants. This is the most efficient online recovery method.
·Reconditioning: As a last resort, the sensor can be baked at 100-105°C for 10 hours (humidity <5%), and then rehydrated at 25°C/75%RH for 12 hours to restore it to the calibrated state. Note: This process must ensure that the environment is absolutely clean.
Step 4: Setting - Long-term Guarantee
·Structural design: During the product design stage, ensure that there are sufficient air flow channels around the sensor to avoid air accumulation in dead corners.
·Location optimization: The sensor should be kept away from heat sources and potential pollution sources (such as batteries, speakers, and glue injection points).
Conclution:
The impact of VOCs on humidity sensors is not "mysticism", but a scientific fact based on physical diffusion and dielectric principles. During the manufacturing process, any minor oversight - a bottle of unsealed cleaning agent, an unverified glue, or an improper exposure to high temperatures - can lead to a permanent deviation in the sensor's accuracy.
Only by thoroughly understanding the pollution mechanism and implementing strict prevention and control measures throughout the entire process of design, material selection, production and testing can we ensure that every sensor leaving the factory can achieve its due high-precision performance. After all, in the era of the Internet of Everything, the accuracy of data is the cornerstone of our trust.