The main factors affecting the uneven heating of reflow soldering
The main reasons for the uneven heating of the components in the SMT reflow soldering process are: the difference in the heat capacity or absorbed heat of the reflow soldering components, the influence of the conveyor belt or the edge of the heater, and the load of the reflow soldering product.
① Generally, PLCC and QFP have larger heat capacity than a discrete chip component. It is more difficult to solder large-area components than small components.
② In the reflow oven, the conveyor belt reflows the conveyed products and also becomes a heat dissipation system. In addition, the heat dissipation conditions of the edge and the center of the heating part are different, and the temperature of the edge is generally low, except for the temperature in the furnace. In addition to different requirements, the temperature of the same load surface is also different.
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③ The influence of different product loads. The adjustment of the temperature curve of reflow soldering should take into account that good repeatability can be obtained under no-load, load and different load factors. The load factor is defined as LF = L/(L + S) where: L is the length of the assembled substrate, and S is the interval of the assembled substrate.
For reflow soldering process to obtain reproducible results, the larger the load factor, the more difficult it is. Generally, the maximum load factor of the reflow soldering furnace ranges from 0.5 to 0.9. This depends on the product situation (component soldering density, different substrates) and different models of reflow soldering furnaces. To get good welding results and repeatability, practical experience is very important.
