SMT007 Magazine

SMT-July2016

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40 SMT Magazine • July 2016 mance, and variability across chamber manu- facturers can be expected. A new approach has been developed where the test board is mounted on a substrate whose temperature can be independently controlled without changing the ambient condition. Thus, the temperature of the test board can be de- pressed below ambient to any desired point and hence, produce condensation at different lev- els. It is then, therefore, straightforward to cycle between condensing and non-condensing con- ditions on the test board in a constant ambient environment. The technique has been demon- strated to be repeatable and controllable, with the user able to select a temperature differential that matches their worst in-use conditions, or to understand the performance of their system under a range of condensing conditions. The data for a range of conformal coatings are presented, and correlated back to the con- formal coating material type, and coverage and thickness by cross-sectioning. Introduction Condensation is the change of physical state of matter from the gas phase to the liq- uid phase. It is most often caused when gas- eous matter is in contact with a substrate that is at a lower temperature. The temperature at which condensation will form is known as the dew point. The dew point is the temperature at which the water vapor in a sample of air at con- stant barometric pressure condenses into liquid water at the same rate at which it evaporates. At temperatures below the dew point, water will leave the air. The condensed water is common- ly called dew when it forms on a solid surface. All things being equal, as the temperature falls, the relative humidity rises, reaching 100% at the dew point, at least at substrate level. The dew point temperature is never greater than the ambient temperature, since the relative humid- ity cannot exceed 100%. The dew point then, is the saturation tem- perature for water in air, and the dew point will be dependent on the relative humidity. A high relative humidity implies that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. When the moisture content remains constant and tem- perature increases, relative humidity decreases. Conformal coatings are thin, polymeric coatings, intended to preserve the condition of a printed circuit board (PCB) under a wide range of operating environments, including high humidity, corrosive gases, dust and other contaminants. Conformal coatings are often evaluated in high humidity environments, typically 40°C/93% RH, 65°C/ 95% RH or 85°C and 85% RH. Until recently, little attention has been paid to the characteristics of conformal coatings un- der condensing environments. Table 1 shows the dew point at several conditions, including those of the common evaluation test methods. The results in Figure 12 show that the two polyurethane coatings behave in a very similar mode as shown in Figure 10. The nano coat- CONDENSATION TESTING—A NEW APPROACH Table 1.

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