Discussion on inspection method of PCB thermal design

- Sep 20, 2017-

The practical application of thermoelectric phenomena, of course, is the use of thermocouples to measure temperature. The complex relationship between electron energy and scattering leads to different thermoelectric potentials of different metals. Since the thermocouple is such a device, the thermoelectric potential difference between two electrodes it is the temperature difference between the hot end and the cold end of the thermocouple thermoelectric potential indication, if all metals and alloys are not the same, it is not possible to use a thermocouple to measure the temperature. Both the contact and the free end are in a region of uniform temperature, while both conductors undergo the same temperature gradient. In order to be able to measure the thermoelectric difference between A and B at the free end, a pair of conductor C of the same material is connected to the conductor A and B at temperature to, respectively, and a detector with a temperature of T1 is received.

Thermocouple uses a wide range of temperature measurement, and the problems encountered are varied. Therefore, this chapter can only deal with some important aspects of thermocouple thermometry. Thermocouples are still one of the major methods of temperature measurement in many industries, especially in the steel and petrochemical industries. However, with the development of electronics, the application of resistance thermometer in industry is more and more widespread. Thermocouple is no longer the only industrial thermometer.

Compared to thermocouples and thermocouples (resistance measurements and thermal potential measurements), the advantage is that the fundamental differences between the two elements work. A resistance thermometer indicates the temperature in the region of the resistor element, independent of the lead and the temperature gradient along the lead. However, the thermocouple is used to measure the temperature difference between the cold end and the hot end by measuring the potential difference between the two electrodes at the cold end. For an ideal thermocouple, the potential difference is only related to the temperature difference between the two ends. However, for an actual thermocouple, at the temperature gradient, some inhomogeneity of the wire leads to a change in the potential difference, which remains a factor limiting the accuracy of the thermocouple.

For thermal designs, we must validate them in subsequent work to determine that the operating temperatures of the chips are within the normal range.

Generally, the chips and components with larger heating capacity are selected to test the maximum working temperature, that is to say, the working temperature of the full load for a long time. The chips and components with high calorific value are determined by the designer before testing, and the highest temperature point of the chip is also required.

Temperature measurement using thermocouple line, line length is generally about 2m, the thread of the connection point placed at the location of the measurement point, and fixed with tape. At the same time, we must pay attention to the line can not be folded, otherwise it will affect the testing accuracy.

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