Meaning
Environmental testing protocols subject electronic assemblies to extreme temperature cycles and high humidity to identify potential mechanical failures in solder joints and material interfaces. This testing is performed in a controlled chamber that can rapidly change the air temperature from as low as -40 degrees Celsius to as high as 125 degrees Celsius or more. By forcing the materials to expand and contract repeatedly, the test accelerates the aging process and reveals weaknesses that might not appear for years in normal use.
Thermal chamber stress is a standard part of the qualification process for any product intended for industrial, automotive or outdoor applications. It ensures that the product can survive the harsh environments it will encounter in the field.
Environmental Cycle
Duration and intensity of the temperature swings are defined by industry standards such as MIL-STD-810 or JEDEC JESD22. A typical test might involve hundreds of cycles where the temperature is held at each extreme for a specific soak time. This soak time ensures that the entire assembly, including the internal components, reaches the target temperature.
The rate of change between the temperatures, known as the ramp rate, is also carefully controlled to simulate different types of thermal shock. Some tests also include high humidity to check for corrosion and moisture ingress into the component packages. The combination of heat and moisture is particularly effective at finding delamination in the printed circuit board layers.
Joint Reliability
Mechanical stress caused by the different coefficients of thermal expansion of the various materials is the primary cause of failure during this test. The silicon die, the plastic package, the copper traces and the solder all expand at different rates as they heat up. This puts a huge amount of strain on the solder joints that connect the components to the board.
Over many cycles, this strain can lead to fatigue and the formation of micro cracks in the solder. Eventually, these cracks can grow until the electrical connection is broken, leading to an intermittent or permanent failure. Thermal chamber stress is used to verify that the choice of solder and the board finish are appropriate for the expected life of the product.
It is a critical test for the adoption of new lead free solder alloys.
Failure Acceleration
Finding defects before the product is released to the market saves the company from the high costs of field returns and warranty claims. This type of testing is known as accelerated life testing because it compresses years of real world stress into a few weeks in the lab. If a design fails during the test, engineers can analyze the failure mode and make improvements to the board layout or component selection.
For example, they might add thermal reliefs to large copper planes or choose components with more robust packaging. The data from the thermal chamber stress test is used to calculate the predicted reliability and mean time between failures for the product. It provides the evidence needed to sign off on the production of a new design.
