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.
Outgassing Contamination
Chemical Transfer
Volatile emission levels from non metallic materials determine the rate at which gaseous compounds condense onto sensitive electronic or optical surfaces. This outgassing contamination results from the release of volatile organic compounds found in epoxies, plastics and adhesives. The resulting film can degrade the performance of lasers or cause electrical leakage on high impedance circuit nodes.
Materials used in aerospace or medical devices are screened to ensure they meet low emission requirements.
Material Selection
Thermal stability is the primary factor in determining the rate at which a substance releases trapped gases. The presence of outgassing contamination often points to an incomplete curing cycle during the manufacturing of the pcb or its protective coatings. Selection of materials with a low total mass loss and low collected volatile condensable material is necessary for high reliability applications.
NASA and ESA maintain databases of materials that pass these specific vacuum stability tests. Engineers evaluate the compatibility of solder masks and potting compounds under extreme thermal cycling to prevent the fogging of optical lenses or the corruption of sensor data.
Qualification Test
Standardized procedures involve heating a sample in a vacuum chamber and measuring the weight change of a nearby collector plate. If outgassing contamination is suspected, technicians use infrared spectroscopy to identify the chemical signature of the deposit. The test specifies the temperature of the source material and the temperature of the cold plate to simulate real world conditions.
Results are expressed as a percentage of the initial mass.
Mitigation Strategy
Baking components in a vacuum oven before assembly helps to drive off volatiles and reduces the risk of later deposits. While outgassing contamination cannot be entirely eliminated, it can be managed by choosing inorganic materials or high grade polymers. Ventilation and the use of getters also help to capture these molecules before they reach critical surfaces.