/Reflow soldering furnace temperature curve design considerations
During the manufacturing process of the circuit board, the temperature setting of the reflow oven determines the temperature resistance and soldering effect of the components. Sometimes it is not possible to debug as request when debugging the temperature curve. This is because for a specific package, its heat capacity, heat receiving area and thermal conductivity have been determined, and it takes a certain time to heat to a certain temperature, as shown in the figure. At the same time, from the point of view of the pcb manufacturing process, the heating rate cannot exceed 3 ° C / s, that is, the difference between the hot air temperature and the peak temperature of the design should not be too large. Thus, the maximum peak temperature that can be achieved is limited under certain time conditions. For example, the welding time is 20s. For a large-size BGA, the maximum peak temperature of the welding cannot exceed 230 °C.
If the heat capacity is large, even if there is no temperature increase rate limit, there must be sufficient time to reach a certain peak temperature. The most typical example is the soldering of the copper substrate.
It is also necessary to understand that the rate of temperature rise reflects the temperature change of the measuring point. If the difference between the furnace temperature and the target peak temperature of the PCB is relatively large, even if the heating rate reflected by the test curve meets the requirements, the temperature difference inside and outside the component package cannot be guaranteed to meet the requirements. Therefore, it is not advisable to increase the temperature to accelerate the temperature rise. But if you are doing experiments and want to get a big temperature difference, this method can be used.
At present, the thermometers used have the function of analog temperature measurement. Since there is a model in the software design, after we test it once, it can automatically extract the relevant parameters of the model according to the temperature curve of the test board, and perform virtual setting and debugging, which can greatly improve the setting efficiency. If the design curve does not match the heat capacity of the test board, it will not be designed.
If a lead solder paste is used to solder a lead-free BGA, we hope to raise the peak temperature of the solder to 220 °C within 20 seconds, but this is generally difficult to achieve on existing reflow ovens. Because the furnace has been designed with the proper temperature range and chain speed range, it can’t be set arbitrarily.
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