Factors affecting uneven heating in lead-free reflow soldering

The main reasons for uneven heating of components caused by the SMT lead-free reflow soldering process are: differences in thermal capacity or absorption of heat in lead-free reflow soldering components, the influence of conveyor belts or heater edges, and the load of lead-free reflow soldering products.

1、 Usually, PLCC and QFP have a larger heat capacity compared to a discrete sheet element, making it more difficult to weld large area components than small ones.

2、 In lead-free reflow soldering furnaces, the conveyor belt becomes a heat dissipation system while repeatedly conveying products for lead-free reflow soldering. In addition, the heat dissipation conditions at the edge and center of the heating section are different, and the temperature at the edge is generally low. In addition to different temperature requirements for each temperature zone in the furnace, there are also differences in the temperature of the same load surface.

3、 The impact of different product loadings. The adjustment of the temperature curve for lead-free reflow soldering should consider achieving good repeatability under no-load, load, and different load factors. The load factor is defined as: LF=L/(L+S); Where L=length of the assembled substrate, S=spacing between assembled substrates.

The higher the load factor, the more difficult it is to obtain reproducible results in lead-free reflow soldering process. The maximum load factor range for lead-free reflow soldering furnaces is usually 0.5-0.9. This depends on the product situation (component welding density, different substrates) and the different models of the reflow furnace. To achieve good welding results and repeatability, practical experience is crucial.