What is a thermal shock test

舟游大地

Unfortunately many people use the same name to refer to different tests.
Thermal shock is one of those. Let me elaborate:

***Definitions:

1. Alternately dipping the product in hot and cold liquids. More
precisely, this should be referred to "liquid-to-liquid thermal shock".

2. Changing the air temperature as quickly as possible in a single
chamber. This we would recommend calling "thermal cycling" or "stress
screening" and isn't truly thermal shock.

3. Moving the product from a hot to a cold chamber or other sudden change
of the air temperature. This is "air-to-air thermal shock" or "two-zone
thermal shock".

***
Sometimes during transfer as described in 3, there is an intermediate step
of exposure to room temperature. This is done to allow manual transfer of
the product. This is called a "three-zone thermal shock". Mil-Std 202F
method 107G allows such a step, for example.

So, from the above list, you can see the variations in what people
commonly call "thermal shock". It should also be noted that some
specifications call for "thermal cycling" which is actually "air-to-air
thermal shock".

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

***Recovery time can also be defined several ways:

1. Time it takes for the product to move from zone to zone. True name:

"transfer time"

2. 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. 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 requirement as "worst-case" product
recovery, which is for a sensor embedded in a sample buried among other
samples.

***
Of course, some test specifications only make the confusion between
thermal shock and thermal cycling worse. One example is Telcordia
(Bellcore) GR-468 that states "Method 1010 of Mil-Std 883C with a minimum
temperature ramp rate of 10C/min." Many people assume that a *cycling*
chamber that does 10C/min. is sufficient, although the Mil-Std clearly
defines the transfer time and recover time, which, if you have been paying
attention a *thermal shock* test.

So when you are asked to do a thermal shock test, be sure that you have a
clear definition of the requirements, otherwise it is easy to do
incorrectly, or buy the wrong equipment.