Climatic Testing

Those items fail to meet requirements all the time, yet the REASONS those items fail are limited to just six causes:

a) Wrong application – not for this material, not for this temperature, not for this atmosphere

b) Out of date – date codes – these things have limited shelf life.

c) Bad mix – precise measurements are required

d) Bad surface preparation – mechanical or chemical

e) Exceeded Working time

f) Bad application technique – spray, dip, brush

If anyone has ever seen a paint, coating, adhesive or potting compound fail for any OTHER reason, please post here.

Hello Blog readers – I recently received an e-mail asking me a question about Humidity testing, and I thought the question and the response might be relevant to you. Any comments would be appreciated.

Hi Steve,

I attended your MIL-STD-810G course in the spring.

I have a question on humidity testing.

I have an analysis to do on an electronic assembly and do not have the means to test it in a chamber.

The power supply is made up of a chassis with a cover. There is an o-ring seal between the cover and the chassis so I am not worried about that. In several places around the machined chassis we have counter sink screws that hold electrical components onto the chassis. These screws are not seal screws but they do have Loctite applied to them. In your experience would condensation go through the countersink into the unit? I have the same question about salt fog. All the hardware is Stainless steal so I am not worried about corrosion.

My response:

The question becomes – is the enclosure airtight, or not?

Keep in mind that the partial pressure of the water vapor will be higher then the dry air pressure, so EVEN if the internal and external pressure is the same, if you have high humidity air outside the enclosure, it may migrate in.

Then the question of temperature change comes in. Temperature cycling will cause a pressure differential. If the enclosure is not airtight, and there is a high moisture content in the external air, it will (again) migrate in.

Also – your O-ring seal – many o-ring seals only hold pressure in one direction. If your O-rings are designed to prevent air from coming in – then after it goes out, and there is a negative internal pressure, the air will try to get in around the screws.

I would definitely try to test.

Regards,

Steve

Most commercial and consumer products are tested and verified in non-condensing humidity environments, which verifies the ability to withstand operating in ambient atmosphere with a high moisture content.

Military and Aerospace products however, are often required to operate in a climatic testing environment where moisture will condense in liquid form directly on the product. This is NOT a test you want to immediately run with out doing some pre-test work.

For one thing, a product has to be designed to withstand this very severe environment. (How to do this could be the subject of another post.) If you “think” your design is good, now go on to some “pre-checks”.

Start by examining all of your conformal coated electronics, in detail under a UV light. Look for small voids. If you have them, you need to revisit your coating process. You would have failed your humidity reliability test.

Next, go the garden supply store and buy a plant mister.

Plant Mister

This device will put out a very fine mist, and when applied to a surface, it will produce a similar effect to condensing humidity. Use plastic sheets and tape to mask off parts of your system, and apply the mist to each circuit board individually, while operating. This will help find vulnerabilities with out having to actually use a humidity chamber, and you will know immediately which sub-assembly needs to be changed to get through this test.

Good Luck.

After 25 years in the reliability field I am still a bit mystified by the humidity testing.  Generally the environmental tests can be divided into two major categories: Durability tests, where some form of wear-out mechanism causes products to fail.  The most common examples of durability tests are vibration and thermal cycling.  The second group is the capability tests, often referred as overstress tests with the goal of determining how well the product can resist certain conditions such as high voltage, accidental drop, dust, or others. 

It appears that humidity tests belong somewhere in between the two categories.  On one hand there are electro-migration, corrosion, dendritic growth, and other failure mechanisms following the pattern of wear-out processes. Those failure mechanisms are indeed accelerated by the combined effect of temperature and humidity and the most commonly used acceleration models used to calculate the test durations are Peck’s and Eyring.  Both models have rather limited applications and varying accuracy, but currently the best what reliability science can offer to calculate the field to test ratios.  On the other hand humidity often causes the change of mechanical properties of the materials which often makes them more susceptible to failures.  For example, modules of elasticity of some materials go down after absorbing moisture and some plastics become more prone to developing cracks as a result of humidity exposure.  Those types of failure mechanisms can not be described by any known algebraic acceleration models.  Despite that engineers often mistakenly apply them for test time calculation. The desire to use algebraic acceleration models is very strong due to their relative simplicity and ease of comprehension.  The alternative to the use of acceleration models is the use of predetermined tests, like for example 164 hours of 85% relative humidity at 85 degree C.  Those tests are often based on some historical data rather than on test rationale or good understanding of failure mechanisms.

Therefore, before writing a product validation plan involving any humidity testing it is important to answer the following questions:

What are the expected humidity-triggered failure mechanisms for my product?

Are any forms or electro-migration involved?

Will the humidity affect any of the material properties and will it make my product more prone to failures?

What type of humidity test is most appropriate for my products? Steady-state, cyclic, both?

Will any of the possible failure mechanisms be accelerated by higher humidity/higher temperature combination or they would remain neutral to it?

Do any of the known acceleration models apply in my case and if not, how do we determine the test duration?

What exactly will my test represents for the life of the product? 

 Answering those and some other questions is critical to a successful validation program involving humidity testing.

What kind of test is a climatics test? Well the most common would be temperature testing, but a more complete list would look like this:

High Temperature
Low Temperature
Temperature Shock
Humidity – Condensing and non-condensing
Altitude
Temperature/Altitude
Rapid Decompression/Explosive Decomp
Combined Environments
Solar Radiation – actinic and thermal effects
Salt Fog
Sand & Dust
Rain
Immersion
Explosive Atmosphere
Icing
Fungus
Acceleration
Space Simulation
Exotics
Wind blast
Launch tube
Bird strike

Any questions about these tests? Our experts stand by to help out.

Steve Brenner, Consultant and Climatics Testing Blogmeister