Condensation Testing—A New Approach

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The reasons for this are easy enough to understand. Conformal coatings are typically applied in the liquid state and dry by a variety of mechanisms. During the drying process, the materials are subject to gravity and capillary forces, making it difficult to ensure uniform and even thickness of the applied coating. When the surface is covered by a continuous water layer, any uncoated, partially coated or defectively coated areas will immediately be exposed to liquid water. Whilst liquid water is not a very good conductor of electricity (5.5 x 10-8 S/m), the presence of ionic species greatly increases the conductivity of water. In today’s no-clean chemistry dominated production process, the presence of ionic species is impossible to avoid.

The combination of lack of coating coverage (or sufficient thickness), the presence of ionic species and the presence of even nano-layers of moisture can have a devastating effect on electronics, resulting in short-circuits or even permanent corrosion.

Given the proximity of the dew point to ambient conditions in a variety of common working environments, the impact of condensation on the reliable operation of conformal coated PCBs requires more thorough examination.

There are existing methods for generating condensing conditions within humidity chambers, utilizing differing approaches. The biggest challenge to creating condensation in humidity chambers is related to the chamber hardware itself. It is designed to produce a stable, condensation free environment. The most simplistic method, and one that will struggle to generate condensation, is to ramp the temperature and humidity condition as quickly as possible causing a condensation event. However, chamber design is continually improving, and striving to avoid any such condensation event. A second approach is to introduce moisture by a secondary moisture source. This can be either by an independent heated water tray (as in test K15 from GS 95024-3-1)[1] or direct jetting of atomized water droplets. A third approach uses a multi-chamber approach where the working area climatic environment is rapidly changed by injecting an alternative environment from a reservoir chamber.

There are a number of standards that are based around the above approaches and some of these are given here:

• IEC 60068-2-30, Environmental testing - Part 2-30: Tests - Test Db: Damp heat, cyclic (12 h + 12 h cycle)

• IEC 60068-2-38, Environmental testing - Part 2-38: Tests - Test Z/AD: Composite temperature/humidity cyclic test

• ISO 6270-2, Paints and varnishes--Determination of resistance to humidity—Part 2: Procedure for exposing test specimens in condensation-water atmospheres

• ISO 16750-4, Road vehicles—Environmental conditions and testing for electrical and electronic equipment—Part 4: Climatic load.

• ASTM D1735, Standard Practice for Testing Water Resistance of Coatings Using Water Fog Apparatus

• ASTM D2247, Standard Practice for Testing Water Resistance of Coatings in 100 % Relative Humidity

• ASTM D4585 – Standard Practice for Testing Water Resistance of Coatings Using Controlled Condensation

• JIS 5600-7-2—moisture resistance (continuous condensation method) utilizing aerosol water injection

• JASO D 001-94 (Japanese Automobile Standard): General rules of environmental testing methods for automotive electronic equipment

While these methods can demonstrate controlled condensation the control and ease by which they generate condensation remains challenging. Almost all of these approaches generate condensation in humidity chambers, and this leads to a challenge, since the chamber systems are developed to minimize condensation. In these cases, chambers are run very close to 100%RH, inducing condensation. However, the uniformity of this condensation within the chamber is unknown. Hence, reproducibility of the given approaches will be variable between different chambers from different manufacturers, and furthermore with time developments in chambers will make them less prone to suffering from condensation events. The approach developed overcomes many of these difficulties.

None of these approaches control the sample temperature condition, and hence the uniformity of the condensed water layer is unknown and the stability of the condensed water film with time may not be constant, as the chamber control system attempt to compensate for the variance in the nominal conditions set for the chamber.

The approach adopted for achieving condensation in this study is one that can be realized in any humidity chamber and shows good correlation with the calculated dew point.

Experimental Setup

In this approach the temperature of the circuit board under test is lowered, and hence within a high humidity environment condensation will occur. To achieve this condition, the test boards are mounted on a platen whose temperature can be independently controlled. Hence by lowering the platen temperature below the chamber ambient, condensation will occur on the platen, and by the thermal coupling of the test board to the platen, condensation will occur on that as well. There is an assumption that the circuit board under test has a flat unimpeded under side so that it can make good contact with the platen. In Figure 1, we show four PCBs mounted on the platen.

NPL-2Aug16-Figure 1.jpg

Figure 1: Test boards on platen.

The mounting technique is very simple: the boards are held in position on the platen surface which is at 45° by magnets. The PCBs under test can be connected by the common technique of using connectors, since these are off the platen. In future PCB designs the tabs to the connectors will be extended away from the PCB area in contact with the platen to minimize further any tendency to cause condensation on the connectors themselves. In Figure 2 the ability of the system to control the test board temperature is shown.


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