Introducing
The Testability Director Version 3.2
 

The main purpose of environmental test is to determine whether our products will be able to hold up reliably for a given length of time – at least as long as the warranty period lasts!

The best way to select the right test(s) is to start thinking about your end product.  You know it better than anyone else.  Think about the environments it will see.  The three main environments to think about are usage, storage, and transportation.

Product Questions

Here is a list of questions you can ask about your product, keeping all three of these environments in mind:

• What temperature will it see?
• Will humidity be a concern?
• Will it be exposed to vibration?  (Almost always the case during transportation)
• Will there be temperature swings?
• Will it be exposed to salt air or corrosion?
• Will it be in a high vacuum or high pressure situation?
• Do I need combined environments?
• Does it need to be powered up?
• What is the worst case scenario and what safety margin do I want beyond that?


Test Questions

Next, think about the purpose of the test:

• Is it a simple pass/fail test?  
• Are you doing it to satisfy a customer’s specifications?
• Are you following a standard?
• Are you trying to determine the product’s lifetime?
• Are you purposely taking it to a failure to find out which failures would be most likely to occur in the field?  (Like HALT testing)
• Is this pre- or post-production testing?
• Are you trying to replicate field failures?

Maybe you can see that there is no simple way of choosing the best test(s), without considering your product and your needs.

Chamber Choice

There are times that one chamber can serve several types of test.  For instance, according to MIL-STD 810F, a thermal change rate of 25C or more per minute is equivalent to thermal shock.  If you have an environmental chamber that can change temperature that quickly, you could have the option of running thermal shock, standard thermal tests, humidity, and possibly vibration all in the same chamber.  This cuts down on the floor space necessary if you were to do each test separately.  (Of course, throughput should be considered.  If you have tests that need to be vibration only and none with combined environments, it would make more sense to keep the vibration system separate.)

Products are exposed in real life to a combination of environments.  Temperature tests are seldom ignored, but sometimes people forget about humidity (or lack of it) and vibration – even if it is just from heavy trucks driving by outside the building.  A lot of electronics products create their own vibration – for instance office equipment like copy machines and printers will create vibrations by virtue of their own operations.

Measuring and Monitoring

When doing your testing, decide what to measure.  In standard ESS testing, temperature is usually measured on air.  However, there could be a huge difference in what is happening to a single PC board compared to a full computer system.  You should make your measurements on the product during test.

One company that I worked with used to bench test each unit before putting it in the chamber, then bench test it again after the chamber test.  They were having a much larger field failure rate than they expected.  They started to monitor the product during the environmental testing and found that 50% of the failures were intermittent.  If they wouldn’t have been checking during the test, they would have missed half of the failures.

The Value of Knowing Your Product

Knowledge is power.  Knowing your product well is the first step.  Knowing why you are doing the testing is the next.  Understand the equipment needed and how best to use it.

Knowing someone that you can trust to help you if you have questions is very helpful, whether they are a colleague, someone that helps to write specifications, or a chamber manufacturer.  I have seen a growing network of people willing to help each other out.  Stay as informed as you can about changes in your industry and in testing.  

I try to keep up to date with testing techniques and network with other experts through the IEEE and through IEST.  I am a working group chair for the ISO for vibration, working group member for the IEC for test conditions, a member of IEEE and IEST, and used to help chair the Accelerated Stress Test Workshop each year.  

Through these organizations or through your direct emails to me I look forward to any questions you may have on this subject.

 David Jung, Marketing Manager, ESPEC North America

Unfortunately ‘thermal shock tests’ are something that many people ask for without knowing that there are several different interpretations of what that is. And there are folks talking about ‘thermal cycling tests’ that actually mean ‘thermal shock’. A clear definition is in order:

Three Possible Definitions of ‘Thermal Shock’:

1. Alternately dipping the product in hot and cold liquids.  More precisely, this should be referred to as “liquid-to-liquid thermal shock”. Because this is not a common test, this possible definition is often disregarded.

2. Changing the air temperature as quickly as possible in a single chamber.  This should be called “thermal cycling” or “stress screening”. Because of the limited temperature change possible in a single chamber, this is not truly a shock condition.

3. Moving the product from a hot to a cold chamber or other methods for sudden change of the air temperature.  This is “air-to-air thermal shock” or “two-zone thermal shock”, and is what is most likely to be assumed what you are talking about ‘Thermal Shock’.

With thermal shock tests, there are also several ways to define the performance.  Thermal shock tests focus on “recovery time”, or how long it takes to stabilize after the switch. 

Three definitions of ‘recovery time’:

1. The time it takes for the product to move from zone to zone.  This should really be called: “transfer time”, and does not measure performance at all.  

2. The time for the air temperature to recover in the new zone.  This can be measured in the air stream before or after the test load.  This can be called “upstream recovery time” or “downstream recovery time”.

3. The time for the physical product to recover, called “product recovery”.  This time is dependent on where the sensor is placed on the load.  Mil-Std 883 method 1010.7 defines their requirements as “worst-case” product recovery, which is for a sensor embedded in a sample buried among other samples.

What a thermal shock chamber is:

A thermal shock chamber is essentially two connected test chambers, usually sitting on top of each other. While the hot-zone chamber is nothing more than a convection oven, the cold-zone is more elaborate.

The cold-zone is cooled by cascade refrigeration, which drives the temperature to extremes as low as -75°C. Because it stays cold all the time, it must be designed to keep moisture out. Even so, additional steps such as a nitrogen or dry-air purge may be necessary to limit the number of defrost cycles. (A defrost cycle holds the test load in the hot-zone while the cold-zone is warmed to defrost.)

During the hot-zone exposure, the cold-zone may be run to an even lower temperature than the setpoint. This allows the system to ‘store’ additional cooling capability by chilling aluminum plates which are included in the cold-zone—we call this a pre-chill function. Some systems may use a liquid nitrogen (LN2) ‘boost’ instead of a pre-chill, so watch for utility requirements of LN2.

The product carrier is moved from zone to zone by way of an elevator system. The mechanism used by the manufacturer should be of special interest to the buyer, as this is where most problems are seen with thermal shock chambers. We recommend direct-drive lifts and avoiding cables & pulleys.

Finally, how is the system controlled?

Older designs are operated as two separate chambers, each with their own controller, which is very limiting. Modern systems control both zones and the lift system in one integrated controller to maximize the performance and functionality of the system. Diagnostic alarms are also a significant aid in such a complex system.

A great example of comprehensive air-to-air thermal shock chamber is our TSE-11 (www.espec.com/digest/tse10.htm). It is meets strict Mil-Std requirements with only 208V power (no cooling water or LN2 needed).

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