The main reasons for uneven heating of components in the SMT lead-free reflow soldering process are: lead-free reflow soldering product load, conveyor belt or heater edge influence, and differences in heat capacity or heat absorption of lead-free reflow soldering components.
①The impact of different product loading volumes. The adjustment of the temperature curve of lead-free reflow soldering should consider obtaining good repeatability under no-load, load and different load factors. The load factor is defined as: LF=L/(L+S); where L=the length of the assembled substrate and S=the spacing between the assembled substrates.
②In the lead-free reflow oven, the conveyor belt also becomes a heat dissipation system while repeatedly transporting products for lead-free reflow soldering. In addition, the heat dissipation conditions are different at the edge and center of the heating part, and the temperature at the edge is generally lower. In addition to the different temperature requirements of each temperature zone in the furnace, the temperature on the same load surface is also different.
③ Generally, PLCC and QFP have a larger heat capacity than a discrete chip component, and it is more difficult to weld large-area components than small components.
To obtain repeatable results in the lead-free reflow soldering process, the larger the load factor, the more difficult it becomes. Usually the maximum load factor of lead-free reflow ovens ranges from 0.5-0.9. This depends on product conditions (component soldering density, different substrates) and different models of reflow furnaces. To obtain good welding results and repeatability, practical experience is important.
Post time: Nov-21-2023