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PCB-Apr2017

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60 The PCB Magazine • April 2017 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 re- ally 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 print- ed 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 expo- sure to hot and cold tempera- ture extremes. One of the first methods d e v e l o p e d for this type of testing, but geared more towards any type of test sample, was MIL- STD-202, meth- od 107—Thermal Shock [1]. The method's purpose statement pro- vides 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 tem- peratures, and to the shock of alternate expo- sures to these extremes, such as would be expe- rienced when equipment or parts are transferred to and from heated shelters in arctic areas." Further inspection of the test document de- scribes 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 tem- perature extreme and should be sufficient in length to ensure that the test specimen reaches the desired ex- treme temperature. The table itself pro- vides some guid- ance on this top- ic relating the dwell time to the test sam- ple's weight. I would highly r e c o m m e n d perusing this table to edu- cate yourself on the indus- try-accepted du- rations. Over time, other thermal shock test methods were de- veloped, but ultimately, given the uncomplicated nature of the test, they all ended up being incredibly similar. Circling back to IPC, interested committee vol- unteers also took it upon themselves to develop some thermal shock test methods that are di- rectly related to PCB construction. Two of the by Keith M. Sellers NTS-BALTIMORE You're in for a (Thermal) Shock! LET'S TALK TESTING

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