5.3 Environmental Influences, Temperature, and Humidity

The most relevant environmental influences are temperature, humidity, and airborne contaminants. These environmental influences contribute to the general breakdown and introduction of electrochemical migration (ECM) on the PCB. These influences can also introduce leakage paths in the crystal oscillator circuit. And while the impact associated with ECM are generally seen after years of operation, the impact on the low power crystal-oscillator circuit can be immediately seen with the introduction of leakage paths. This impact may range from slow start up times to a complete inability to oscillate.

Temperature, specifically higher temperatures can have both positive effects and negative effects. A positive effect is that higher temperatures promote evaporation of weak organic acids found in flux residues and thus reduce leakage associated with the residue. A negative effect is that the higher temperature can reduce the drive strength of the output stage transistors thus, the safety margin of an oscillator setup decreases. As long as the safety factor test has shown good results, as classified in Table 2, and the crystal is being used in the standard industrial temperature range, the application should work safely.

In addition to high temperature, temperature cycles, especially fast temperature cycles combined with high ambient humidity, can result in condensed water on the PCB. Condensation and humidity in general contribute to the degradation of the PCB. And as already discussed, the humidity is drawn to the hygroscopic residue associated with no-clean flux, resulting in leakage paths and a higher ESR for the crystal.

Together with soldering residues and other board contaminations, dust (air-borne contaminants) can easily accumulate in applications with a lifetime of several years and non-air-proof housings. This debris can decrease the insulation of the oscillator signals towards each other and towards neighboring signals on the PCB. Thus, it is a good practice to introduce a protective coating of the crystal, the attached externals, and the MSP430 MCU oscillator pins. Examples of protective coatings are conformal coating, encapsulation, and potting.

Conformal coating is more popular, providing a transparent protective layer that allows visible inspection. Silicon coatings are typically superior to urethane and acrylic, but all three types can be used to preserve the parameters and performance of the oscillator over many years of operation in the field.

Potting and encapsulation materials offer similar benefits. Typically made of silicon or polyurethane, potting provides vibration dampening, heat dissipation, security (non-transparent), and mechanical protection. Working with the contract manufacturer will help avoid possible issues associated with potting materials coefficient of thermal expansion and the associated stress placed upon a surface mount crystal. High (>450V/mil) dielectric strength materials are recommended for coatings to ensure little or no performance difference is seen in the crystal-oscillator circuit. Table 4 lists examples of high-dielectric materials.

If no coating is provided, then a continuous degradation of the oscillator performance can occur and should be taken into account. While a very effective defense against environmental influences, the use of protective coatings brings back into focus the importance of board cleanliness. Protective coatings introduce reliability risks which are associated with the contamination trapped under the coating into which moisture can diffuse.