PCBA Tech - How to measure the temperature curve of SMT connection furnace

How to measure the temperature curve of SMT connection furnace

Generally, the temperature displayed on the operating 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 heating element surface or the resistance wire, but the temperature of the hot air. To be able to set the furnace temperature, you must understand two basic heat transfer laws.

1. At a given point in the furnace, if the temperature of the chip 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 lower than the furnace temperature If the temperature is equal, there will be no heat exchange.

2. The greater the difference between the furnace temperature and the temperature of the SMT chip circuit board, the faster the temperature of the circuit board changes.

To set the furnace temperature, generally determine the conveying speed of the furnace chain first, and then start the temperature setting. The chain speed is slow and the furnace temperature can be lower, because the heat balance can be reached in a longer time; on the contrary, the furnace temperature can be increased. If the components on the PCB are dense and the large components need to be balanced and require more heat, the furnace temperature must be increased; on the contrary, the furnace temperature will be 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 chip 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. The production capacity is relatively sufficient.

The setting of the furnace temperature is a process of setting, measuring and adjusting the temperature, the core of which is the measurement of the temperature curve. The measurement methods are divided into two categories: contact type and non-contact type. Thermocouples are generally used for temperature measurement in SMT reflow soldering, 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 used for high temperature measurement.

The temperature measurement of a contact instrument is to measure the temperature through 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. It is not limited by the upper limit of temperature measurement and will not damage the temperature field of the measured object., The reaction speed is generally faster; but the measurement error is relatively large due to the influence of external factors such as the emissivity of the object, the measurement distance, smoke, and water vapor.

This article describes two basic heat transfer laws that must be understood about setting the furnace temperature for SMT reflow soldering.