Printed circuit board history stretches back to the early 1900s, with real promise shown in the industry after World War II. Through the 1940s, 1950s, and 1960s, PCB construction really started to progress when fiberboard and wood were replaced with resins and laminates, and rivets replaced early plated through-holes.
As the industry grew, IPC, the worldwide printed circuit board trade association, had its first meeting in the late 1950s. It was around this same time that the idea of testing printed circuit boards became a real discussion topic, and one of the first tests explored for helping to determine the robustness of a PCB’s construction was thermal shock.
The premise is quite simple: apply stress and strain to the PCB via exposure to hot and cold temperature extremes. One of the first methods developed for this type of testing, but geared more towards any type of test sample, was MILSTD-202, method 107—Thermal Shock. The method’s purpose statement provides a perfect depiction of what the test was designed to do: “This test is conducted for the purpose of determining the resistance of a part to exposures at extremes of high and low temperatures, and to the shock of alternate exposures to these extremes, such as would be experienced when equipment or parts are transferred to and from heated shelters in arctic areas.”
Further inspection of the test document describes thermal shock testing with the use of both environmental chambers as well as liquid baths. For the latter topic, the method even provides some guidance as to what type of fluid can be used, as water is obviously not a suitable fluid for all the temperature test conditions that are listed. Also in the document is a table which provides some knowledge about dwell times.
The dwell time is the duration that the test specimen is exposed to a given temperature extreme and should be sufficient in length to ensure that the test specimen reaches the desired extreme temperature.
The table itself provides some guidance on this topic relating the dwell time to the test sample’s weight. I would highly recommend perusing this table to educate yourself on the industry-accepted durations.
To read the full version of this column which appeared in the April 2017 issue of The PCB Magazine, click here.