Generally, the temperature displayed on the operation interface of the SMT reflow soldering furnace is the temperature measured by the built-in thermocouple in the furnace. It is neither the temperature on the PCB, nor the temperature of the surface of the heating element or the resistance wire, but actually the temperature of the hot air. To be able to set furnace temperature, two basic laws of heat transfer must be understood.
1. At a given point in the furnace, if the temperature of the SMD processing circuit board is lower than the furnace temperature, then the circuit board will heat up; if the PCB temperature is higher than the furnace temperature, then the PCB temperature will drop; if the temperature of the circuit board is the same as the furnace temperature If the temperature is equal, there will be no heat exchange.
2. The greater the temperature difference between the furnace temperature and the SMT circuit board, the faster the temperature of the circuit board changes.
The setting of the furnace temperature generally first determines the transmission speed of the furnace chain, and then starts to set the temperature. The chain speed is slow and the furnace temperature can be lower, because the thermal equilibrium can be achieved in a longer time. Otherwise, the furnace temperature can be increased. If the components on the PCB are dense and the large components need to be balanced, more heat is required, and the furnace temperature is required to be increased; on the contrary, the furnace temperature is lowered. It should be emphasized that in general, the adjustment range of the chain speed is not very large, because the process time of SMT patch processing and welding and the total length of the temperature zone of the reflow soldering furnace are determined, unless the temperature zone of the reflow soldering furnace is more and longer, production capacity is relatively sufficient.
The setting of furnace temperature is a process of setting, measuring and adjusting, the core of which is the measurement of temperature curve. Measurement methods are divided into two categories: contact and non-contact. In reflow soldering, thermocouples are generally used for temperature measurement, which is a contact temperature measurement method. Contact temperature measuring instruments are relatively simple, reliable, and have high measurement accuracy. However, due to the need for sufficient heat exchange between the temperature measuring element and the measured medium, it takes a certain time to reach thermal equilibrium, so there is a delay in temperature measurement; at the same time, due to the limitation of high temperature resistant materials, it cannot be applied to very high temperature measurement.
The temperature measurement of a contact instrument is based on the principle of thermal radiation. The temperature measurement element does not need to be in contact with the measured medium. The temperature measurement range is wide, not limited by the upper limit of the temperature measurement, and will not destroy the temperature field of the measured object. , the response speed is generally faster; but due to the influence of external factors such as the emissivity of the object, the measurement distance, smoke and water vapor, the measurement error is large.
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