Selecting Surface Mount Packaging Formats for Digital Environmental Sensors
Selecting surface mount sensor packages requires balancing land-pattern mechanical stress and reflow limits against bus integration effort and unit yield cost.
Siloxane Poisoning
Sensor Degradation
Chemical reactions between silicon-based vapors and heated surfaces result in the formation of a solid silica layer that inhibits the function of gas sensors. This siloxane poisoning occurs when volatile compounds found in sealants or lubricants decompose on the sensing element. The resulting deposit blocks the access of target gases to the active catalyst or electrode.
Metal oxide and catalytic bead sensors are particularly vulnerable to this type of permanent damage.
Environmental Source
Common household and industrial products release the precursors that lead to sensor failure. When siloxane poisoning is detected, the source is often traced back to silicone oils, cleaning agents or certain cosmetic products used in the vicinity of the device. These molecules are highly mobile and can migrate through air filters to reach the internal sensor chamber.
Industrial environments often mandate the use of silicone-free materials to protect sensitive monitoring equipment.
Failure Mechanism
The heat generated during the operation of the sensor provides the energy needed to break the chemical bonds of the siloxane molecules. During siloxane poisoning, the silicon atoms combine with oxygen to form a glass like coating over the sensor surface. This layer reduces the sensitivity of the device and increases the response time to the target gas.
Because the silica layer is chemically inert, it cannot be removed through standard cleaning or calibration procedures. Observation of a gradual drift in the baseline signal and a decrease in the span calibration indicates the progress of the poisoning over time. This loss of performance eventually leads to a complete failure of the sensor to respond even to high concentrations of the target gas.
Protection Method
Industrial applications where siloxane poisoning is a known risk typically utilize sensor designs with enhanced catalytic surface area to prolong operational life. Specialized filters containing activated carbon or specific chemical scavengers can trap these vapors before they reach the sensor.